ML17083C249
| ML17083C249 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 04/02/1990 |
| From: | Shiffer J PACIFIC GAS & ELECTRIC CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| Shared Package | |
| ML17083C250 | List: |
| References | |
| DCL-90-090, DCL-90-90, NUDOCS 9004260163 | |
| Download: ML17083C249 (455) | |
Text
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REGULATORY INFORMATION DISTRIBUTION" SYSTEM (RIDS) t ACCESSION NBR:9004260163 DOC.DATE: 90/04/02 NOTARIZED: NO 'OCKET FACXL:50-275 Diablo Canyon Nuclear Power Plant, Unit 1, Pacific Ga 05000275 50-323 Diablo. Canyon Nuclear Power Plant, Unit 2, Pacific Ga =05000323 AUTH. NAME AUTHOR AFFILIATION SHIFFER,J.D. Pacific Gas & Electric Co.
RECIP.NAME RECIPIENT AFFILIATION Document Control Branch (Document Co trol Desk),,
I
SUBJECT:
Forwards response to questions on geology/seismology/
geophysics/tectonics xn long. term seismic prog final rept. D g
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77 Beale Street Dames D. Shiffer San Francisco, CA 94106 Senior Vice President and 415I972 7000 General Hanager Nuclear Power Generation 415/973-4684 Apri 1 2, 1990 PG&E Letter No. DCL-90-090 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C. 20555 Re: Docket No. 50-275, OL-DPR-80 Docket No. 50-323, OL-DPR-82 Diablo Canyon Units 1 and 2 Response to NRC Staff Questions on Geology/Seismology/Geophysics/Tectonics Long Term Seismic Program Final Report Gentlemen:
In letters to PG&E dated August 1 and October 13, 1989, and February 23, 1990, the NRC Staff requested additional information to support its review of geoscience topics presented in the Long Term Seismic Program (LTSP) Final Report. Enclosed are PG&E's responses to these requests.
The August 1, 1989, letter summarized a June 12-16, 1989, LTSP public meeting and included 16 NRC Staff comments and questions regarding geology/seismology/geophysics (GSG) . PG&E responses to GSG Questions 2, 3, 4, 5, 7, 8, 9, 10, 12, 13, 14, and the 29 montages that accompany Question GSG 1 are provided herewith. The responses to GSG Questions 1, 12, and 13 contain propri etary geophysical data that are exempt from public disclosure in accordance with 10 CFR 2.790(a)(9). The proprietary information is appropriately marked and has been transmitted only to the NRC (2 copies) and to NRC consultants (1 copy to R. D. Brown and 1 copy to D. B. Slemmons). It is requested that this material be handled in accordance with NRC procedures for proprietary material.
The October 13, 1989, letter contained six questions on seismic source characterization (SSC). PG&E's responses to SSC Questions 1, 2, 3 and 4 are enclosed.
Responses to the remaining questions from both letters currently are in the review process. We anticipate that these responses will be lO transmitted to the NRC in the coming week.
NU.
DNA .With regard to the letter of February 23, 1990, which contained 13 OOQ CO questions requesting additional data and analysis, Questions 2, 4, OO OIO OO 6, and 9 have been addressed in our responses to other questions.
Namely, Questions 2, 4, and 6 wi 11 be addressed in our current AU response to Question GSG 1; Question 9 was addressed in the response wO OQ to ground motions Question 5 that was submitted to the NRC in August
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Document Control Desk Apri 1 2, 1990 PGE E Letter No. DCL-90-090 Questions 10, ll, and 12 are related to the Lorna Pri eta earthquake. He have recently received abundant preliminary results of some investigations of the Lorna Pri eta earthquake, and expect the results of other studies shortly. He will be prepared to address these questions during the NRC/PGhE Seismic Source Characterization meeting scheduled for April 17-20, 1990, in San Francisco.
He plan to devote more than one full day to the Lorna Prieta earthquake, including a field trip to the epicentral region. In addition to addressing these questions orally at the meeting, we wi 1 1 follow up with written summaries of our presentations.
The remaining questions in the letter of February 23, 1990, Questions 1, 3, 5, 7, 8, and 13, will be addressed orally at either the Seismic Source Characterization meeting or the NRC/PGhE Ground Hotions meeting scheduled for April 30 and Hay 1, 1990, in San Francisco. In addition, we will submit a written summary of these presentations.
Kindly acknowledge receipt of this material on the enclosed copy of this letter and return 'i,t in the enclosed addressed envelope.
pl h I incerely,
. D.'Shi,f cc w/encs.: R. D. Brown, 3r.-'.
Clark
- 3. B. Hartin H. Rood (2)
D. B. Slemmons cc w/o encs.: A. P. Hodgdon H. H. Hendonca P. P. Narbut B. Norton CPUC Diablo Distribution Enclosures 3122S/0081K/DNO/1587
pa Ir
RESPONSE TO QUESTIONS SSC 1, 2, 3, and 4 March 1990 This volume responds to 4 of 6 Seismic Source Characterization questions asked of PG&E by the Nuclear Regulatory Commission (NRC) on October 13, 1989. Responses to the remaining 2 questions will be submitted later. These responses provide data requested to augment or clarify information presented in the Final Report of the Long Term Seismic Program, submitted by PG&E to the NRC on July 31, 1988, and presented in responses to December 13, 1988, questions, submitted by PG&E to the NRC in January and February of 1989.
/DOC.( OI45 Diablo Canyon Power Plant Pacific Gas and Electrfc Company Long Term Seismic Program
ueti n 1 Mrch 1 P el QUESTION SSC I Provide additional support for the choice of logic tree parameters and weights used in the seismic source characterization analyses, including a description of the solicitation process of the PGBE panel from which the weighting of the various source characteristics were derived and applied in the program.
The logic trees developed for the probabilistic seismic hazard analysis are presented in detail in Chapter 3 of the Final Report (PG&E, 1988, p. 3-19 to 3-32). In that discussion, the geology/seismology/geophysics (GSG) conclusions are directly used to arrive at the values or hypotheses assigned to the branches of the logic trees and the relative weight associated with each branch. In response to Question 40, January 1989, we explained how the logic tree elements and weights are directly related to GSG data, using the specific data developed for the Hosgri fault as an example. The response to Question 40, January 1989, further explains the rationale for the use of logic trees and the support of this approach in the context of uncertainty treatment and probability theory. We interpret Question SSC 1 to request clarification of the solicitation process by which the project team arrived at the values given in the logic trees.
The values and associated probabilities given in the logic trees were developed by a consensus of the project team. The project team consisted of PG&E Geosciences staff, GSG consultants, and appropriate members of the Board of Consultants who also served as technical advisors to the project.
Individuals participating in the process included:
Dr. Clarence Allen CalTech Dr. Bruce Bolt UC Berkeley Mr. Lloyd Cluff PG&E Dr. Kevin Coppersmith Geomatrix Consultants Dr. N. Timothy Hall Earth Sciences Associates Dr. Douglas Hamilton Earth Sciences Associates Ms. Kathryn Hanson Geomatrix Consultants Dr. William Lettis Geomatrix Consultants Ms. Marcia McClaren PG&E Mr. Cole McClure Bechtel Dr. Jan Reitman Consultant Dr. William Savage PG&E Dr. Paul Somerville Woodward-Clyde Consultants Dr. Yi-Ben Tsai PG&E All the above individuals were directly involved in various aspects of the GSG activities or in the review of these activities. The goal of this aspect of the Long Term Seismic Program was to arrive at a consensus assessment of the logic tree elements by relying heavily on the GSG data developed during the Program. In this sense, the project team provided the basis for technically supported assessments and for properly representing the spectrum of uncertainty given the available data. The contribution that any particular individual on the team made to the assessments was related to his experience in the GSG activities. The project team did not serve as a "panel of experts" whose individual assessments were subsequently aggregated into a final assessment. Rather, the project team, a~sa r u, evaluated pertinent data sets and arrived at the logic tree assessments.
In terms of the procedures involved in soliciting the team's assessments, meetings were periodically and regularly held to discuss GSG findings and review alternative interpretations. In some cases, field visits were made for direct observation of field geologic relationships. To arrive at the individual parameters and hypotheses that are given on the branches of the logic tree, the team developed alternatives that spanned the range of credible alternatives, based on the available data.
The relative credibility of each alternative was then evaluated by critically reviewing the level of support that each individual had in the data. Often, this was done by having members of the team present to the group the applicable data sets and "defend" the alternative values. For example, if the Diablo Canyon Power Plant Pacific Gas and Electric Company long Term Seismic Program
i n 1 Mrchl Pa e2 rupture length of a fault was the parameter of interest and alternatives of 20, 45, and 60 kilometers had been developed as credible values, detailed discussion would focus on, say the 20-kilometer length. Arguments were presented describing the basis in the GSG data for particular segmentation points, including fault behavioral, geometric, and structural geologic interpretations. In many cases, a group consensus was reached by polling individual team members, combining the results of the poll, and reviewing the aggregate result. In the course of the discussion, the arguments for and against various alternatives as well as justification for the weights were documented for later inclusion in the Final Report (PG&E, 1988). To provide the team with a clear view of the implications of their assessments, the subsequent analyses of, for example, maximum magnitudes and earthquake recurrence rates, were presented to them.
When appropriate, sensitivity studies were also conducted, such as those presented in response to Question 45, January 1989, to observe the relative impact of various assessments with and without the relative weights assigned to each logic tree branch. Examples of these sensitivity studies were presented at the NRC/PG&E Seismic Source Characterization Meeting in August, 1989. In that meeting, we considered the sensitivity of the sequencing of the elements of the logic tree and whether the results of the logic tree analysis would change if a different sequence of the elements had been used. Specifically, we explored the impact of reversing the sequence of "sense of slip" followed by "fault dip" on the Hosgri fault zone. As discussed in Chapter 3 of the Final Report (PG&E, 19S8, p. 3-3 to 3-S), the logic tree elements were sequenced to describe logically the characteristics of a particular seismic source. For example, sense of slip was placed first because subsequent seismic source characteristics, such as fault dip, are dependent on the particular slip-type model being considered. For sensitivity purposes, we reversed the sequence to evaluate the effect on the relative weights for various fault dips. This was accomplished by calculating the conditional probabilities of various dips, which are the products of the probabilities of each slip type and dip combination on the Hosgri fault tree (PG&E, 198S, Figure 3-5, p. 3-21). The result is the following dips and associated relative weights:
~Pr a~ilit 900 0.48 60 -
70'45'.46 0.06 This shows that the assessments on the logic tree for the Hosgri fault zone, aggregated over three possible slip-types, result in an equal likelihood of the fault having a near-vertical dip or a 60- to 70-degree dip. In this way, we were able to clearly see the assessments of fault dip as though they were the first element of the logic tree. Sensitivity studies of this kind provided insights into the logic tree assessments being made, and served to further define the evaluations made by the project team.
REFERENCES Pacific Gas and Electric Company, 19S8, Final report of the Diablo Canyon Power Plant Long Term Seismic Program: U. S. Nuclear Regulatory Commission Docket Nos. 50-275 and 50-323.
Diablo Canyon Power Plant Pacific Gas and Electric Company Lang Term Seismic Program
ue tion S 2 March l 0 Pa el QUESTION SSC 2 The sense of slip PG&E derives for the Hosgri fault is based on evidence of strike-slip observed on the San Simeon fault 50 kilometers north of the Diablo Canyon site.
Geologic and geophysical data along the Hosgri fault appear to offer more direct evidence of a thrust or reverse component of slip. This evidence includes:
- a. A thick upper Pliocene and Quaternary marine section west of the fault that is juxtaposed against a basement surface, or a thin cover of upper Quaternary sediment on basement, east of the fault.
- b. Angular unconformities (top Miocene and mid-Pliocene) that are deformed as they approach the fault from the west; southwest dips on the top-Miocene unconformity range up to 40 degrees, those on the mid-Pliocene unconformity up to 20 degrees. These relations indicate that the fault has had a vertical component of slip during the Pliocene and probably well into the Quaternary.
The wedging-out against the western face of the Hosgri (or over its blirrd western branches) of unconformity-bound packets of Pliocene and younger strata. The wedging, together with the deformation described above, indicates a growth fault that has been active since early Pliocene time. Moreover, it demonstrates that the time values assigned to the two uncon formities may beconre unreliable near the fault and in the block to the northeast of it; in these regions each unconformity represents a much greater depositional, or time, gap than it does to the west, in the offshore Santa Maria basin.
- d. A warped and faulted present-day sea floor and a similarly deformed late lYisconsinan (18 Ka) sur face follow the same trend, and i>rdicate the same sense of vertical slip, as that defined by the late Pliocene and Quaternary structures along the Hosgri.
The broad Hosgri fault zone now separates a subsiding depositional basin on the southwest from a stable or rising wave-cut plat form on the northeast; the present boundary and stratigraphic relations across the fault closely mimic those that prevailed during the earlier history of the Hosgri.
In light of these factors. justify the interpretation of the Hosgri fault as being predominantly strike-slip in character.
Question SSC 2 inaccurately implies that we based our assessment of the Hosgri fault zone solely on evidence of strike-slip displacement on the San Simeon fault zone, 50 kilometers north of Diablo Canyon. Although evidence of strike slip on the San Simeon fault zone along trend with the Hosgri fault zone is important, our assessment of the Hosgri fault zone is based on an integrated analysis of geologic, seismologic, and geophysical data along the entire length of the Hosgri fault zone, combined with tectonic and kinematic analyses of the San Gregorio/San Simeon fault system, the offshore Santa Maria Basin, and the onshore Los Osos/Santa Maria domain. Question SSC 2 also provides a brief summary of the evidence for a vertical component of slip on the Hosgri fault zone. Although these lines of evidence are cited as "direct evidence" of a thrust or reverse component of slip, it is important to recognize that all the data and lines of reasoning presented in the question are evidence of vertical separation. They do not in any instance provide direct evidence of dip-slip displacement on the Hosgri fault zone; that is, the true amount of vertical slip of a particular unit, measured in the fault plane. Our assessment of the Hosgri fault zone incorporates all the data, analyses, and interpretations presented in the question as "direct evidence of a thrust or reverse component of slip" on the Hosgri fault zone.
Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
uestion S 2 March 19 0 Pa e2 In this response, we summarize the data sets, observations, and lines of reasoning used in our analysis that justify our characterization of the Hosgri fault zone as a strike-slip fault. Important to this characterization is the classification criteria for distinguishing strike-slip, oblique-slip, and dip-slip (thrust or reverse) faults. We describe our definitions of the three fault classes and reconcile the evidence of vertical separation cited in the question with our assessment of the Hosgri fault zone as a strike-slip fault.
DEFINITION OF STYLE OF FAULTING Critical to the classification of style of faulting on the Hosgri fault zone is a clear, well-understood definition of the various classes of fault types. In our assessment of style of faulting, we used three classifications of fault types: strike slip, oblique slip, and reverse or dip slip. These fault types are defined by the "rake," or direction of displacement in the plane of the fault. The use of fault rake, therefore, is a measure of the direction of slip and is independent of the attitude of the fault plane.
We define a strike-slip fault as one in which the rake of fault displacement is less than 30 degrees, an oblique-slip fault as one in which the rake is 30 to 60 degrees, and a reverse or dip-slip fault as one in which the rake is greater than 60 degrees.
These definitions, which allow for a continuum of vertical and lateral components of slip from pure dip slip to pure strike slip, are consistent with definitions used by other workers (Bonilla and Buchanon, 1970; Slemmons, 1977) to categorize historical surface ruptures for the purpose of evaluating empirical relationships between fault magnitude and various fault parameters (Figure SSC Q2-1). As such, they have gained acceptance in the profession not only to classify style of faulting on a particular fault zone, but also to characterize earthquake behavior.
Direct observation of striae in the plane of a fault, from which fault rake can be determined, is commonly not possible. A case in point is the Hosgri fault zone. The fault zone lies entirely offshore, where direct observations of the fault plane cannot be made. To resolve this difficulty, we translated the amount of strike slip and dip slip implied by the rake of striae into a ratio of apparent horizontal (H) to apparent vertical (V) separation. Figure SSC Q2-1 provides the ratios for each fault class. In this manner, we can use estimates of vertical and horizontal slip components to classify fault type. For faults dipping 60 degrees or more, an H:V ratio greater than 2:1 indicates the fault is a strike-slip fault according to the definition given above. For a vertical fault, an H:V ratio of greater than 1.7:1 indicates the fault is a strike slip fault.
CHARACTERIZATIONOF SENSE OF SLIP ON THE HOSGRI FAULT ZONE Characterization of the Hosgri fault zone as a seismic source requires an evaluation of its present sense of slip. This evaluation is hampered by two important factors: (1) its offshore location precludes the use of conventional geologic mapping and paleoseismic investigations; and (2) geologic evolution of the Hosgri fault zone from the Miocene to the present spans a significant, well-documented change in plate motion and, thus, tectonic setting. To assess sense of slip, we have evaluated the ratios of horizontal to vertical components of slip along the entire length of the Hosgri fault zone. The analytical approach and values of horizontal and vertical rates of slip are provided in Responses to Questions GSG 3 and GSG 4. The rake angles of faulting defined by the ratio of horizontal to vertical slip clearly meet the definition of a strike-slip fault. In the contemporary tectonic setting, the Hosgri fault zone is primarily a strike-slip fault having a subordinate component of dip slip that varies both in cumulative amount and sense (east versus west side up) along strike.
Because the fault is entirely offshore, where direct observations cannot be made, we have also used indirect data and several lines of geologic reasoning to assess the sense of slip on the Hosgri fault zone. These data and lines of reasoning include:
Diablo Canyon Power Plant Paclflc Gas and Electric Company Long Term Seismic Program
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ue i n 2 March l 0 Pa e Sense Of Slip Definitions A
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30 O 30 oo CO odg~+ec@ o, CO pP pe 9c o CO Of d n 60 60 90 Reverse-slip B From Bonilla and Buchanon, 1970 Rake angle g, SS/DS Fault Type degrees (cotan g) HN (dip 90') HN (dip 60')
Strike-slip Reverse Oblique
<<30'1.732
<<60'o 30'0.577 to 1.732
>1.732
>0.577 to 1.732
>2
>0.67 to 2 Reverse 90'o 60' to 0.577 0 to 0.577 0 to 0.67 g Rake of striae; inclination from the horizontal measured in the plane of the fault For a 90'-dipping fault plane: For a 60'Nipping fault plane with rake of 304:
ss a 1.732 30'ake 60'ap V apparent vertical separation DS V 1 sin 60'/DS DS 1 0.866 ~ V/1 cotan p strike-slip (SS)/dip-slip(DS) H/V a 1.732/0.866 a 2 Figure SSC Q2-I Sense of slip definitions.
Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
uestion SC 2 March 1 0 Pa e4 Assessment of down-dip geometry from geophysical high-resolution and common-depth-point (CDP) seismic data, retrodeformable structural modeling (see Attachment GSG Ql-A for discussion), and seismologic data. These data indicate a high-angle (60 degrees or greater) down-dip geometry for the primary traces of the Hosgri fault zone.
Assessment of geomorphic expression from analysis of bathymetry,'side-scan sonar data, and high-resolution seismic data. There is a well-documented absence of laterally continuous sea-floor inflections, slope breaks, and scarps along the fault zone.
Persistent dip-slip displacement would produce a sea-floor reflection, slope break, and/or scarp along the fault. The Hosgri fault zone is clearly not coincident with a laterally continuous slope break (see Attachment GSG Q4-A), as implied by point "d" of the question. In addition, high resolution seismic data indicate there is no abrupt or even subtle change in thickness of post-late-Wisconsinan sediment across the fault zone. A laterally continuous change in thickness of up to tens of meters would be produced by persistent dip-slip displacement of 1 millimeter per year along the fault zone. There is no evidence to support a high rate of dip slip on the Hosgri fault zone during the late Quaternary.
The Hosgri fault zone is comparable in style of deformation to other strike-slip faults in the world (see Response to Question SSC 4):
Reversals in the sense of vertical separation both down dip and along strike of the fault indicate lateral offset and juxtapositioning of stratigraphic units at different structural levels.
The presence of local intra-fault-zone pull-apart basins at right-releasing stepovers in the fault trace near Point Sal and Point San Luis indicate right-slip displacement (see Attachment GSG Ql-A, Point Sal reach and San Luis/Pismo reach montages; Response to Question GSG 15).
Compressional and extensional flower structures having associated intra-fault-zone anticlinal and synclinal folding are common along the Hosgri fault zone. These features commonly occur along and are diagnostic of strike-slip faults (Christie-Blick and Biddle, 1985; Lowell, 1985; Harding, 1983, 1985; see Response to Question SSC 4).
The fault zone and individual fault strands within the zone are linear at regional scale (greater than 20 kilometers) and curvilinear to linear at local scale (less than 20 kilometers). Fault-trace sinuosity is lower than 1.1 and is similar to other known strike-slip faults (such as the San Andreas and North Anatolian fault zones) and independently indicates a high-angle fault dip. Fault-trace sinuosity is not similar to other known reverse or thrust faults (such as the Pleito and San Fernando faults),
where fault-trace sinuosity typically is greater than 1.2.
Regional tectonic association and alignment with the well-documented strike-slip San Gregorio/San Simeon fault system. Numerous studies demonstrate that the San Gregorio and San Simeon fault zones are strike-slip faults having slip rates on the order of millimeters per year. The Hosgri fault zone aligns with and is nearly identical in structural style and fault zone complexity to these faults. Tectonic and kinematic analyses that treat slip-rate budgets along south-central coastal California virtually require that lateral slip continues southward onto the Hosgri fault zone. There are no other candidate structures or tectonic explanations that can account. for an abrupt termination of lateral slip at the southern end of the San Simeon fault zone. Interpretations that the Hosgri fault zone is predominantly a reverse or oblique-slip fault have severe slip-rate-budget discrepancies and are not consistent with the tectonic setting of south-central coastal California.
Diablo Canyon Power Plant Paclflc Gas and Electric Company Long Term Seismic Program
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ueti nS C2 March 1 Pa e5
~ The northern Hosgri fault zone is related to the southern San Simeon fault zone via the San Simeon/Hosgri pull-apart basin. The dimensions and age of the basin indicate that a minimum of 1 to 4 millimeters per year of lateral slip is transferred between the southern San Simeon fault zone and northern Hosgri fault zone (see Response to Question GSG 12 for discussion). The presence and location of this subsiding basin are geologic observations and are not model-driven interpretations. The presence of a subsiding basin between two large fault systems, one of which (the San Simeon fault zone) is a well-documented strike-slip fault, provides strong tectonic and kinematic evidence that the other bounding fault (the Hosgri fault zone) is a strike-slip fault.
~ Tectonic, kinematic, and regional stress data indicate that the Hosgri fault zone decouples regional crustal shortening of differing orientations within the Los Osos/Santa Maria domain and the offshore Santa Maria Basin, requiring lateral slip on the Hosgri fault zone (see PG&E, 1988, p. 2-34 through 2-37; Lettis and others, 1989).
~ Subparallel strike-slip and dip-slip faults are consistent with strike-slip displacement along the Hosgri fault zone given the concept of strain partitioning (see Response to Question GSG 2).
~ Seismicity DatL Microearthquakes having strike-slip focal mechanisms have occurred along the Hosgri fault zone near Point Buchon, suggesting a vertical fault to a depth of at least 7 kilometers.
The 1980 Point Sal earthquakes and aftershock sequence constrain the down-dip geometry of Hosgri fault zone to be near-vertical to a depth of 8 to 10 kilometers (PG8r,E, 1988).
When taken collectively, the weight of evidence from these data sets and lines of reasoning strongly indicate that the Hosgri fault zone is a strike-slip fault with a variable component of dip slip. When taken individually, however, many of these data sets, or parts of data sets, illustrate only (or primarily) the vertical sense of separation on the Hosgri fault zone. A case in point is the geophysical seismic reflection data set. The CDP seismic reflection data provide two-dimensional profiles of the upper crust (roughly 1 to 3 seconds two-way travel time). They do not image the lower, seismogenic part of the crust, nor do individual lines illustrate out-of-the-plane lateral offset.
The seismic data are excellent for providing direct observational evidence of vertical separation across a fault; however, they cannot provide direct evidence of lateral separation across a fault.
Thus, seismic reflection information taken by itself cannot be expected to provide convincing direct evidence of lateral offset, nor can the absence of such direct evidence on the seismic data be used to conclude that lateral offset has not occurred. Seismic data do provide indirect evidence of lateral offset across a fault in the form of secondary deformation such as flower structures or reversals in the sense of vertical separation, both within a single section and along strike. A description of these secondary phenomena, supported by examples of strike-slip faults worldwide, is presented in Response to Question SSC 4.
Each of the issues and analytical approaches presented in Question SSC 2 reflect interpretations drawn principally from the seismic reflection data along selected parts of the Hosgri fault zone. We agree with many of these interpretations and an assessment that there has been vertical separation of variable amounts along the Hosgri fault zone. This vertical separation is included in our assessment of the vertical component of slip, which is discussed in detail in Response to Question GSG 3. The Question GSG 3 response also addresses, in detail, points a, b, c, and e listed in this question. The apparent northeast-side-up sense of vertical separation along much of the Hosgri fault zone, combined with the presence of positive flower structures, which we document in Response to Questions GSG 1 and SSC 4, indicate there is some component of convergence along the Hosgri fault zone. However, this component of vertical separation does not, in any way, preclude the occurrence of lateral slip. An assessment of the style of deformation of any fault must address both the vertical Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
u in 2 Marchl 0 and lateral components of slip. In Response to Questions GSG 3, GSG 4, and GSG 12, we provide documentation of both the lateral and vertical components of slip along the Hosgri fault zone. Our analyses indicate. that the ratio of lateral to vertical slip ranges from at least 2.3:1 to 30:1. As described earlier, this ratio of slip meets our definition and published definitions of a strike-slip fault.
REFERENCES Bonilla, M. G., and Buchanon, J. M., 1970, Interim report on worldwide historic surface faulting:
U. S. Geological Survey Open-File Report, 32 p.
Christie-Blick, N., and Biddle, K. T., 1985, Deformation and basin formation along strike-slip faults; in Biddle K. T., and Christie-Blick, N., eds., Strike-slip Deformation, Basin Formation, and Sedimentation: Society of Economic Paleontologists and Mineralogists Special Publication 37, p. 1-34.
Lettis, W. R., Hall, N. T., and Hamilton, D. H., 1989, Quaternary tectonics of south-central coastal California (abs.): 28th International Geological Congress, v. 2, p. 2-285 to 2-286.
Lowell, J. D., 1985, Structural styles in petroleum exploration: Oil and Gas Consultants International Inc., Tulsa, Oklahoma, 477 p.
Harding, T. P., 1983, Divergent wrench fault and negative flower structure, Andaman Sea; in Bally, A. W., ed., Seismic Expression of Structural Styles - A Picture and Work Atlas: American Association of Petroleum Geologists Studies in Geology, Series 15, v. 3, p. 4.2-1 to 4.2-8.
Harding, T. P., 1985, Seismic characteristics and identification of negative flower structures, positive flower structures, and positive structural inversion: American Association of Petroleum Geologists Bulletin, v. 69, p. 582-600.
Pacific Gas and Electric Company, 1988, Final report of the Diablo Canyon Long Term Seismic Program, U. S. Nuclear Regulatory Commission Docket Nos. 50-275 and 50-323.
Slemmons, D. B, 1977, Faults and earthquake magnitude: U. S. Army Corps of Engineers, Waterways Experimental Station, Miscellaneous Papers S-73-1, Report 6, p. 1-129.
Diablo Canyon Power Plant Pacific Gas and Electric Company Lang Term Seismic Program
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uestion SSC 3 March 1 0 P el QUESTION SSC 3 Provide a detailed discussion of how and why the average displacement estimated for the San Simeon fault was applied to the Hosgri fault as a maximum displacement.
Question SSC 3 appears to represent some confusion regarding our use of average and maximum displacements per event for the San Simeon and Hosgri fault zones. The average displacements estimated for the San Simeon fault were not applied to the Hosgri fault zone as maximum displacements. As discussed in Chapter 3 of the Final Report (PG&E, 1988, p. 3-21, 3-24), we estimated the average amount of displacement per event along the San Simeon fault zone to be either one, two, or three meters. Because the data leading to these estimates were gathered at specific localities along the fault, we assumed that the estimates are average rather than maximum values (PG&E, 1988, p. 3-24). This is because typical displacement distributions along fault ruptures indicate that the maximum displacement usually occurs along a limited reach of the fault (for example, Thatcher and Bonilla, 1989; Zhang and others, in preparation); any random point along a rupture is more likely to be a location of average displacement, rather than maximum displacement.
We used the one-, two-, or three-meter average displacements in our assessment of maximum magnitude for the Hosgri fault zone. We did not estimate a maximum displacement per event for the Hosgri fault zone, nor did we use maximum displacement per event to assess maximum magnitude along the fault zone (p. 3-21, 3-24).
REFERENCES Pacific Gas and Electric Company, 1988, Final report of the Diablo Canyon Power Plant Long Term Seismic Program: U. S. Nuclear Regulatory Commission Docket Nos. 50-275 and 50-323.
Thatcher, W., and Bonilla, M. G., 1989, Earthquake fault slip estimation from geologic, geodetic and seismological observations: implications for earthquake mechanisms and fault segmentation:
Proceedings of the Conference on Fault Segmentation and Contacts of Rupture Initiation and Termination; U. S. Geological Survey Open-File Report 89-315, p. 386-399.
Zhang, X., Slemmons, D. B., Wells, D. L., and Coppersmith, K. J., in preparation, Worldwide data base of maximum and average displacements.
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ue tion C 4 March 1 0 P el QUESTION SSC 4 The characterization of the Hosgri fault as a predominantly strike-slip fault has been questioned because of the presence of low dip angle reverse faults in the zone. It has been suggested that other faults which are known to be strike-slip may have low dip angle reverse or thrust faults associated with them in a manner similar to that seen along the Hosgri. If there is geophysical evidence for such a situation provide it. The San Gregorio fault in the vicinity of Monterey Bay has been mentioned as a possible candidate.
We have characterized the Hosgri fault zone as a convergent (or transpressional) strike-slip fault based on an integrated analysis of a wide variety of data. These data include shallow and deep-penetration seismic reflection data along the entire length of the fault zone; geologic and geomorphic data from the San Simeon fault zone, the San Simeon/Hosgri pull-apart basin, and the Hosgri fault zone; the distribution and nature of seismicity; consideration of regional tectonic kinematics; and comparison of the Hosgri fault zone with worldwide analogs.
On seismic data, the Hosgri fault zone is imaged within the upper 1 to 3 kilometers of the crust as a complex system of high- and low-angle fault strands. Below a depth of 1 to 3 kilometers, the fault zone is difficult to image due to structural complexity and data degradation within basement. We have stated that the presence of such a complex system of faults is not only compatible with an interpretation that the Hosgri fault zone is a strike-slip fault, but is one of the diagnostic criteria used to distinguish strike-slip faults on seismic data from other types of fault zones. The presence of low-dip reverse faults within and subparallel to a strike-slip fault zone is observed in outcrop (for example, Sylvester and Smith, 1976; Weber and Lajoie, 1979a, 1979b; Wilcox and others, 1973), and is well documented in industry seismic reflection profiles and drilling data (for example, Harding, 1983, 1985; Harding and others, 1983; Lemiszki and Brown, 1988; Crouch and Bachman, 1989).
In this response, we provide criteria for identifying strike-slip faults on seismic data published by a variety of researchers who use seismic data to assess structural deformation. We then describe published and unpublished examples of seismic data across well-known strike-slip fault systems.
These examples illustrate the utility of the published criteria for interpreting strike-slip fault deformation on seismic data, as well as the similarity of these strike-slip faults with the Hosgri fault zone. We conclude this response with a summary of the Hosgri fault zone and the pertinent observations from seismic data that, based on the criteria published by others, clearly indicate that strike-slip is the predominant sense of displacement on the fault.
CRITERIA FOR DISTINGUISHING STRIKE-SLIP FAULTS Criteria for identifying strike-slip faults are summarized by Christie-Blick and Biddle (1985), Stone (1986), Zalhn (1987), Withjack and others (1987), and Harding (1989). These criteria are schematically illustrated on Figures SSC Q4-1 and Q4-2 and include:
Map Characteristics (Figure SSC Q4-1)
A linear or curvilinear, long, laterally continuous, solitary fault zone A narrow zone of highly varied structures (faults and folds) with en echelon arrangement The principal displacement zone is associated with a narrow, laterally persistent antiform or synform bounded by downward merging faults Structural trends adjacent to and on either side of the principal displacement zone are incompatible Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
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uestion SS 4 March I P e2 Detail of Principal displacement zone Synthetic (R) shear Hors etail splay
~Secondary synthetic (P) shear Antithetic (R shear
~ Principal displacement zone Releasing bend e de Principal displacement zone Restraining bend EXPLANATION Normal-separation fault Reverse-separation fault Fault, arrows indicate sense of slip
+- Fold Overturned fold o~p Areas of subsidence and sediment accumulation (From Christie-Blick and Biddle, 1985)
SSC Q4-1 Plan view of typical strike-slip fault showing linear or curvilinear principal displacement zone.
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ue tion 4 M rch l Pa e MAJOR CHARACTERISTICS
~ Basement involved
~ Zone sub-vertical at depth
~ Upward<lverglng and rejoining splays JUXTAPOSED ROCKS
~ Contrasting rock type
~ Abrupt variations In thickness and facies In a single stratigraphic unit
. SEPARATION IN ONE PROFILE
~ Normal- and reverse-separation faults In same profile
~ Variable magnitude and sense of separation for different horizons displaced by the same fault N Apparent normal SUCCESSIVE PROFILES R Apparent reverse
~ Inconsistent dip on a single fault
~ Variable magnitude and sense of separation for a given horizon on single fault
~ Variable proportions of apparent
/(r/4//1/r~ normal- and reverse-separation faults I / i(/) r/r qrl w
pi~ / l/
/ ///~
i
// i/ ~i/
$M //$ /l q //
1~/~ tl~ / </r %/ ~l~ Time-stratigraphic unit with variable
/% I) /<) ~<a (t r l~
/I I g r~ ~rl~ / / sedimentary facies l~
i+'lhw/ %p> lg I l~w ( (I I/ w>~/ ~l>l Crystalline basement r/ r Principal dtspiacement zone (From Christie-Blick and Biddle, 1985)
SSC Q4-2 Schematic section of an idealized strike-slip fault showing common stratigraphic and structural characteristics.
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I usi n 4 rchl P 4
~ The principal displacement zone is associated with anomalous concentrations of compressional and extensional structures at restraining and releasing bends/stepovers in an. otherwise linear zone (that is, presence of local pull-apart basins or compressional structures along the fault)
Profile Characteristics (Figure SSC Q4-2)
~ Basement separation along the principal displacement zone
~ Upward-diverging splays from the principal displacement zone, with either reverse or normal displacement (positive and negative flower structures, respectively; also called parafolds, and first-order thrust faults)
~ The thickness of juxtaposed sedimentary sections changes across the fault
~ Adjacent fault strands in a single profile have a different sense of vertical separation o The sense of vertical separation changes along trend from profile to profile (that is, upthrown block switches sides along strike)
~ The sense and magnitude of vertical separation in a single profile on a fault strand changes up-section within a single profile (that is, apparent reversal in fault throw with depth)
~ Merging fault strands at depth have different apparent vertical separation
~ Change in the amount and/or direction of dip of the principal displacement zone along strike
~ Abrupt change in the nature of seismic facies or signature across the fault
~ Abrupt change in the style and/or intensity of deformation along the fault
~ Upward-widening zones of reflection terminations These criteria have been developed by numerous researchers in academia and the petroleum industry over the past two decades. Interpretations of strike-slip fault deformation based on these criteria have been tested by independent geologic field data and drill core data. These criteria, therefore, provide a valuable framework for assessing the presence of strike-slip deformation on a suite of seismic data. The most commonly cited and diagnostic of these criteria for identifying strike-slip deformation are the linear or curvilinear principal displacement zone, the upward-diverging fault splays (negative and positive flower structures) with associated subparallel anticlines or synclines, and the presence of local extensional or compressional structural features at releasing or restraining fault geometries.
Upward-diverging braided splays that originate from a narrow, steeply dipping principal displacement zone are commonly observed on industry seismic reflection profiles (for example, Harding, 1983, 1985; Harding and others, 1983), and where those faults crop out (Sylvester and Smith, 1976; Wilcox and others, 1973). This upward-diverging geometry is referred to as a "flower structure" (attributed to R. F. Gregory by Harding and Lowell, 1979), or less commonly, as a "palm tree or tulip structure" (Sylvester, 1984; Naylor and others, 1986), or a parafold (Withjack and others, 1987).
Flower structures develop along both convergent strike-slip faults, where they are associated with prominent antiforms and known as positive flower structures (Allen, 1957, 1965; Wilcox and others, 1973; Sylvester and Smith, 1976; Harding and Lowell, 1979; Harding and others, 1983; Harding, 1985), and along strike-slip divergent faults, where they are associated with synforms and termed Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Selsmlc Program
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u i n 4 arch l Pa e negative flower structures (D'Onfro and Glagola, 1983; Harding, 1983, 1985; Harding and others, 1985). The flower structures may be symmetric ("two-sided") or asymmetric ("one-sided"). Harding and others (1983,.p. 4.2-13), for example, state, "the fault architecture is not always symmetric. In some areas, the subsidiary segments may diverge toward only one flank of the structure. Positive flower structures in these instances can have profile characteristics indistinguishable from compressive fault blocks." This observation is especially noteworthy for interpreting the many one-sided flower structures along the Hosgri fault zone.
We have used many of these criteria to identify and document strike-slip deformation on the Hosgri fault zone from the suite of seismic data. For example, the Hosgri is a long, linear fault zone with associated folding in a narrow, elongate zone; flower structures (negative and positive) are common along the fault zone; local extensional and compressional features are present at releasing and restraining bends or steps in the fault zone; and the sense and magnitude of vertical separation varies both along trend and within a single profile. These and other, less prominent, features identified on the seismic data distinguish the Hosgri fault zone as a strike-slip fault. These features are not diagnostic of dip-slip faults and have not been, to our knowledge, cited in the literature as occurring along prominent thrust, reverse, or oblique-slip faults.
EXAMPLES OF STRIKE-SLIP FAULT DEFORMATION In recent years, numerous geological and geophysical studies have addressed the geometry and kinematics of strike-slip fault zones. Sylvester (1984, 1988), for example, provides a comprehensive review of strike-slip faults based on examples of transform and transcurrent faults from around the world. In addition, many examples of shallow structural geometries across strike-slip fault zones are available from industry seismic reflection profiles and drill data (Harding, 1983, 1984, 1985; Harding and others, 1983; Lowell, 1985), and Lemiszki and Brown (1988) provide a review of the variable down-dip geometry and crustal structure of strike-slip fault zones from around the world as observed on deep seismic reflection profiles.
These studies show that high-angle strike-slip faults are virtually impossible to directly detect or demonstrate based on seismic data. Lynn and Deregowski (1981) show that steeply dipping faults are difficult to image on seismic reflection profiles, but that they are frequently detectable by indirect means such as a lateral change in reflection character. Lemiszki and Brown (1988, p. 665) state that "the complex geology often associated with strike-slip fault zones might also result in severe seismic velocity variations and resultant ray-path distortion, as well as high seismic energy attenuation, in such a manner as to obscure or mask structural characteristics associated with the deeper portions of such faults." In this respect, one should not expect to image on seismic data the down-dip geometry of high-angle strike-slip faults in the mid to lower crust.
Despite these limitations and data processing difficulties, Christie-Blick and Biddle (1985) report that analyses of geophysical data have led to a "growing appreciation for the wide range of structural styles that exist along strike-slip faults, both on the continent and in the ocean basins, and for the processes by which those styles arise." These analyses indicate that most prominent strike-slip faults involve basement and, in map view, are characterized by a linear or curvilinear principal displacement zone with a variety of folds and secondary branching associated faults in a narrow, elongate zone (Figure SSC Q4-1).
Seismic reflection data acquired by industry and academic institutions in areas affected by strike-slip deformation provide analogs that can be used to assess the nature of deformation observed in seismic records across the Hosgri fault zone. Below, we describe several examples of reflection profiles across faults known to have strike-slip displacement on the basis of map relations, tectonic relations,focal mechanisms, and/or exposed kinematic indicators on fault planes.
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ueti n S 4 March 1 Washita Valley Fault Zone, Ardmore Basin Figure SSC Q4-3.is a seismic profile across a strike-slip fault within the Washita frontal fault zone in the Ardmore basin of southern Oklahoma (Harding and Lowell, 1979). This fault is part of a 500-kilometer-long fault system that extends from the Arbuckle Mountains in southern Oklahoma to the Amarillo uplift in the Texas panhandle. Both subparallel strike-slip and reverse faulting occurred along this fault system during the late Paleozoic Ouachita orogeny (Wielchowsky and Gilbert, 1982).
Displaced stratigraphic markers indicate from 100 to 120 kilometers of cumulative left-lateral displacement (Tanner, 1967). Structural analyses performed by Donovan (1985) indicate at least two periods of convergent strike-slip faulting. Thus, the nature and style of deformation along this fault is very comparable to our interpretation of the Hosgri fault zone in that both faults are prominent strike-slip faults involving basement, and have undergone several tens of kilometers or more of convergent lateral slip.
The seismic profile (Figure SSC Q4-3) across the Washita Valley fault illustrates a positive flower structure and demonstrates important criteria for identifying convergent strike-slip faults with seismic control (Harding and others, 1983). Positive flower structures are characterized by the abrupt upturn of bedding in a band parallel to and on both sides of, or only one side of, a throughgoing strike-slip fault. The strike-slip fault is characterized by a subvertical central strand and upward-diverging, inward-dipping subsidiary faults having dominantly reverse separation. These subsidiary faults have profiles ranging from steep to shallow-dipping, and they root within the principal strike-slip displacement zone. In addition to having these characteristics, this profile also illustrates basement displacement across the primary displacement zone. The recognition of basement displacement is another important criterion that can be used to distinguish flower structures along strike-slip faults from detached folds, such as delta structures and box folds, which contain thrust faults at their cores and typically do not involve basement (Harding and others, 1983).
The Ardmore basin profile shares many characteristics with those observed in seismic profiles across the Hosgri fault zone. Both substantial vertical separation of basement across a subvertical principal fault and subsidiary upward-diverging reverse faults are typical of much of the Hosgri fault zone between Point Buchon and Point Sal (see Attachment GSG Ql-A, San Luis/Pismo, San Luis Obispo Bay, and Point Sal montages).
Newport-Inglewood Fault Zone, South-Coastal California The Newport-Inglewood fault is a near-coastal fault in southern California. Onshore geologic studies and focal mechanism solutions for the 1933 Long Beach earthquake indicate the fault is an active zone of strike-slipe deformation. To the southeast, the fault extends offshore subparallel to the coastline along the eastern margin of the San Pedro Channel. A thick sequence of Tertiary and Quaternary sediment is present in the channel. A review of proprietary, seismic data by Crouch and Bachman (1989) indicates that wrench-related fold and flower structures are present in a 2- to 3-mile-wide zone along the offshore Newport-Inglewood fault zone. These authors also report that thrust-related structures have formed parallel to, and seaward of, the offshore Newport-Inglewood fault zone within the San Pedro Channel, and appear to be related to fault-normal compressional forces.
The general style and distribution of deformation along the Newport-Inglewood fault zone and bordering San Pedro Channel is nearly identical to that observed along the Hosgri fault zone and bordering offshore Santa Maria Basin. The Hosgri fault zone is characterized by an elongate, narrow zone of high- and low-angle faults, folds, and flower structures, and thrust-related structures have formed parallel to, and seaward of, the fault zone in the southern offshore Santa Maria Basin (for example, the Lompoc, Purisima, and Queenie structures). Similar to the fault-normal compressional structures west of the Newport-Inglewood fault zone cited by Crouch and Bachman (1989), we interpret the fold and thrust faults within the southern offshore Santa Maria Basin to be the result of compressional forces normal to the Hosgri fault zone.
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ue tion 4 March I P 7 SW 0
~ ~ ~
O
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~
k~ l>>
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r a
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EXPLANATION M Mississippian T Displacement toward viewer 0 Ordovician A Displacement away from viewer SSC g4-3 Interpreted migrated seismic profile across a strike-slip fault of the Washita frontal Ardmore Basin of Oklahoma. fault zone in the Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
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uesti n C 4 March 1 0 Pa e8 West Andaman Fault, Andaman Sea Negative flower structures are observed along divergent strike-slip faults. Examples of negative flower structures are shown on seismic profiles across the West Andaman fault (Figures SSC Q4-4, Q4-5, and Q4-6). This fault trends north-south across the Andaman Sea, a marginal basin bounded on the west by the Andaman-Nicobar Ridge and Sunda subduction zone, and on the east by the magmatic arc terranes of the Malay Peninsula (Figure SSC Q4-4). It is, in part, the northern extension of the Great Sumatran fault in Sumatra and, together with tectonic relationships and seismicity, is interpreted to be a prominent right-slip fault zone. Points A and A'nd B and on Figure SSC Q4-4 identify correlative folds interpreted to be offset by the fault in a right- B'hown lateral sense. On both seismic profiles, strands of the strike-slip fault diverge upward and bound a relatively downdropped block within the fault zone. The down-dropped block or negative flower structure is indicative of divergent lateral offset. On Profile 1 (Figure SSC Q4-5) the reversal up-section in sense of vertical separation also distinguishes the zone as a divergent strike-slip rather than a normal fault. On Profile 2 (Figure SSC Q4-6) the presence of both normal and reverse separations within the same fault zone is consistent with a prominent strike-slip fault zone and is the feature that most clearly distinguishes the zone from an extensional dip-slip fault. Also noteworthy is the pattern of associated deformation in plan view along the fault zone (Figure SSC Q4-4). The fault trace is complex, marked by associated, subparallel anticlines occur along the fault in a zone about 3 to 5 kilometers wide.
Locally, the Hosgri fault zone has many structural features that are similar to those observed along the West Andaman fault. On seismic lines PGE-3 and GSI-101 (Attachment GSG Ql-A, Point Sal reach montage) a reversal in the apparent sense of vertical separation occurs up-section along a single strand of the Hosgri fault zone. In addition, these profiles clearly show that the sense of apparent vertical separation of basement and the top of Miocene and mid-Pliocene unconformities is different on different fault strands within the fault zone and the bordering, upward-diverging reverse-fault splays. Local extensional deformation or negative flower structures also are common along the fault as shown on seismic profiles Comap-117, GSI-100, and CSLC D-29 (Attachment GSG Ql-A). In plan view, the Hosgri fault zone is locally complex, with multiple, sometimes sinuous, fault traces. Regionally, however, the fault trace is linear over tens of kilometers. Numerous subparallel anticlines and synclines are also present along and to the west of the Point Sal and Southern reaches of the fault zone, similar to the associated deformation along the West Andaman fault.
Bering Sea Fault Zone A seismic profile across a major fault zone in the Bering Sea (Figure SSC Q4-7) provides another example of structural characteristics comparable to the Andaman Sea fault. In this case, the area has undergone several episodes of deformation having different structural styles (W. A. Spindler, pers.
comm., 1984, cited in Harding and others, 1985). Divergent strike-slip characteristics are most apparent in the younger Tertiary sediments above 2 seconds two-way travel time (Harding and others, 1985). Upwarping and folding of older Tertiary sediments within a 2- to 4-kilometer-wide zone adjacent to the primary strike-slip fault is very similar to folding observed along the Hosgri fault zone between Point Buchon and Point Sal (see Attachment GSG Ql-A.4, San Luis/Pismo, San Luis Obispo Bay, and Point Sal montages). Like the Hosgri fault zone, much of the apparent vertical separation of the basement and folding in the older Tertiary sediments is inherited deformation from the earlier periods of deformation along the fault zone.
Lake Basin Fault Zone, Montana The Lake Basin fault zone in Montana is interpreted to be a left-slip fault whose principal period of activity is dated as post-Paleocene and possibly post-Eocene (Harding and others, 1985). Figure SSC Q4-8 is a seismic profile across the west end of the Lake Basin fault zone interpreted by G. H.
Weisser (pers. comm., 1983, cited in Harding and others, 1985). The zone includes faults with normal separation and faults with reverse separation. The faults bracket a down-dropped block and are interpreted to converge downward into a single principal displacement zone. The down-dropped Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
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uesti n C 4 March l Pa e A'rench fault with normal separation; U= up, D= down Normal fault profile Reverse fault profile Crest of anticline ao Ug~ / "'. 1 Possible correlative folds B-B't Selsmh profile (Figure SSC Q4-5)
O2 Seismh profile (Figure SSC Q4-6)
U D
- e
+19'pproximate, area shown 9'+ +F 9V 9$ ' summa'. 91~
U D (From Harding and others, 1985)
SSC Q4-4 Tectonic map of structures in the Andaman Sea mapped on a lower Miocene horizon. Locations of seismic profiles presented in Figures SSC Q4-5 and SSC Q4-6 are shown as l and 2, respectively.
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uestion SS 4 March 1 Pa e 10 West . East.
io 0.0' h
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~
See Figure SSG Q4-4 for location of profile (From Harding and others, 1985)
EXPLANATION T Displacement toward viewer A Displacement away from viewer SSC Q4-5 Interpreted seismic profile 1 across the Andaman Sea fault.
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uestion SS 4 M rch I 0 P 11 unE O2 West
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EXPLANATION T Displacement toward viewer A Displacement away from viewer SSC 04-6 Interpreted seismic profile 2 across the Andaman Sea fault.
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uestion 4 M rch I 0 Pa e 12
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SSC Q4-7 Interpreted seismic profile across a major strike-slip fault zone in the Bering Sea.
Olablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
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uesti n S 4 March 1 Pa e 13 108' 460N 0 12 ml 8 gag r 0 20 km m Billings (After Dobfn and Erdmann, 1S55)
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Surface map of Lake Basin fault zone (a) and interpreted seismic profile across western end of Lake Basin fault zone (b).
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I ueti n 4 Mrchl Pa e 14 block or negative flower structure and different sense of displacement on individual strands within the fault zone are characteristic features on seismic data of.a strike-slip fault zone.
San Gregorio Fault Zone, Central Coastal California Published examples of geophysical data across the San Gregorio fault zone, the northernmost component of the San Gregorio/Hosgri fault system, are limited and of poorer quality than the preceding examples. Interpretations of both high- and intermediate-resolution seismic profiles across the San Gregorio fault zone in the northern part of Monterey Bay near Santa Cruz (Figures SSC Q4-9 and SSC Q4-10) are presented by Greene (1973). Approximately 25 kilometers north of the offshore region in which these profiles are located, the San Gregorio fault zone is exposed onshore at Aiio Nuevo and Seal Cove (Figure SSC Q4-11). Detailed geologic investigations of this onshore reach show that the San Gregorio fault is a prominent right-slip fault zone having an estimated late Quaternary slip rate of about 10 millimeters per year (Weber and Lajoie, 1977, 1979a, 1979b).
Like much of the central part of the Hosgri fault zone, the San Gregorio fault zone consists of at least two primary fault traces that are interpreted to be subvertical within at least the upper 1 second (Figure SSC Q4-9). These primary traces occur in a zone from 1 to 5 kilometers wide, and their existence is confirmed in onshore exposures at Ano Nuevo where the Frijoles, Coastways, and Greyhound Rock strands have been mapped as throughgoing prominent strike-slip fault traces (Weber, 1979; Weber and Lajoie, 1979a, 1979b). In addition to these traces, Weber and Lajoie (1979) map the Aiio Nuevo thrust fault about 1.5 kilometers west of the San Gregorio fault zone, and an unnamed thrust fault about one-half kilometer west of the Frijoles fault strand. These faults trend subparallel to and dip toward the high-angle San Gregorio fault zone. The Ano Nuevo fault displaces, and locally folds, marine terrace deposits near Point Aiio Nuevo, clearly indicating late Quaternary thrust faulting and folding contemporaneous with and subparallel to the strike-slip San Gregorio fault zone.
In the offshore, the high-angle fault strands of the San Gregorio fault zone cut sediment overlying a buried erosional unconformity of late Miocene age and, in places, closely approach the ocean floor (Greene and others, 1973; Greene, 1977). Fold deformation is observed in near-surface sediments west of the fault zone (Figure SSC Q4-10). Note, however, that folds in the deeper part of the section (Figure SSC Q4-9) are overlain by subhorizontal reflectors and appear to be related to underlying faults that are presently inactive. Although Greene and others (1973) and Greene (1977) did not map reverse faults beneath the folds along the San Gregorio fault from this high-resolution seismic data, current concepts and modeling techniques in structural geology would suggest that these folds may be produced by underlying dip-slip faults. Similar folds are interpreted from seismic data to occur along the Hosgri fault zone and to be underlain by reverse faults that diverge upward from the Hosgri fault zone.
To further assess the down-dip geometry, structural character, and associated deformation along the San Gregorio fault, we have acquired and interpreted additional unpublished geophysical data across the fault zone north of Ano Nuevo between Seal Cove and Bolinas Lagoon (Figure SSC Q4-12).
These data include two seismic reflection lines shown on Figures SSC Q4-13 and Q4-14).
Comparable seismic reflection data are not available across the San Gregorio fault zone in Monterey Bay, where Greene (1977) conducted his study. The San Gregorio fault is imaged as a steeply northeast dipping plane that typically juxtaposes basement up on the east against a thick Tertiary sequence on the west. Folds and underlying reverse faults that diverge upward from the steeply dipping San Gregorio fault deform the Tertiary strata west of the fault zone. These relations are also presented on a regional structural cross section across the California borderland by Saleeby (1984)
(Figure SSC Q4-15).
The down-dip geometry, sense of basement displacement, and associated contractional deformation along the San Gregorio fault are nearly identical to those observed on seismic data along many reaches of the Hosgri fault zone. The San Gregorio fault branches from the San Andreas fault north of Bolinas Lagoon in an analogous pattern to that of the right-slip Calaveras fault branching from the San Andreas fault south of Hollister. Less than 5 kilometers to the south, the San Gregorio fault Diablo Canyon Power Plant Pacific Gas and Electric Company long Term Seismic Program
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uestin 4 Mrhl of seismic profile 122'15'ocation 122'00'21'45'anta Cruz 37400'y iL o( Monterey Moss correction Bay Vertical exaggeration 14:1 Landing 36O45'fp High resolution Monterey 36'30'fp correction 10 Vertical exaggeration 6:1 Intermediate resolutfon Nautical miles Section D resolution D-D'ntermediate D'
0.5 2250 1.0 1.5 0 4500 1375 2 mt Seconds Feet Meters 0 3.6 km SSC Q4-9 (Froln Greene, 1973)
Location map and interpretation of seismic profile D-D'cross the San Gregorio fault zone in Monterey Bay.
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SSC Q4-1y Locations of seismic reflection data across the San Gregorio fault zone between Seal Cove and Bolinas Lagoon.
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uesti n 4 rch I P 2 Outer Sallnia Santa Cruz Granitold Basin Basement 0
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SSC Q4-14 Regional section across central coastal California and bordering continental shelf.
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ue i n 4 Mrchl Pa e 21 comes onshore at Seal Cove, and farther south at Aiio Nuevo, its late Quaternary behavior as a right-slip fault is well-documented by displaced marine and fluvial terrace deposits (Weber and Lajoie, 1979a, 1979b). The nature of deformation imaged on seismic data along the San Gregorio fault zone in Monterey Bay by Greene (1977) and north of Seal Cove during this study bracket the known strike-slip deformation observed onshore at Seal Cove and Ano Nuevo. We conclude that the nature of deformation observed on the seismic data is compatible with and, in fact, is probably produced by strike-slip deformation. The similarity of this deformation with that observed on seismic data along the Hosgri fault zone and along the strike continuity of the two fault systems strongly indicates that the Hosgri fault zone is a strike-slip fault.
DISCUSSION As described in the preceding sections and examples, the Hosgri fault zone displays many of the characteristics of strike-slip fault deformation. The fault zone is linear and is associated with a narrow complex zone of reverse faulting, subparallel folding, and localized extensional deformation.
The presence of reverse faulting and folding in a narrow, elongate zone is indicative of convergent or transpressional strike-slip faulting.
Regional plate motion, in the present tectonic setting, imposes a component of convergence along the margin of south-central California (Harbert and Cox, 19S9; DeMets and others, 1987).
Characteristics typically observed along convergent strike-slip faults worldwide, such as abundant reverse faults and low-angle thrust faults, as well as folds arranged both en echelon and parallel to the principal displacement zone (Lowell, 1972; Wilcox and others, 1973; Sylvester and Smith, 1976; Lewis, 1980; Anadon and others, 1985; Mann and others, 1985; Sengor and others, 1985; Steel and others, 1985), are observed along the Hosgri fault zone. As noted by Wilcox and others (1973) and Harding and others (1985), the structural style is affected by even a small component of extension or shortening across the principal displacement zone, circumstances that favor fault-parallel folds rather than en echelon folds (Christie-Blick and Biddle, 1985).
In addition to extensional and contractional deformation on a regional scale, a strike-slip fault may also experience small-scale extensional and contractional deformation along parts of the fault zone as a result of local fault geometry. For example, curvature or abrupt bends along a strike-slip fault, braiding of fault traces within a strike-slip fault system, or en echelon fault traces willproduce local areas of extension or compression along the fault (Crowell, 1974; Woodcock and Fischer, 1986; Reading, 1980). Those local structures occur in restraining and releasing bends (Kadinsky-Cade and Barka, 1989) or en echelon stepovers (Crowell, 1974; Barka and Kadinsky-Cade, 1989; Brown and Sibson, 1984), where compression and extension, respectively, would be predicted by kinematic models of a strike-slip fault system (for example, Reading, 1980; Sibson, 19S5, 1986; King and Nabelek, 1985; King, 19S6; Bilham and King, 1989). These phenomena are common along the Hosgri fault zone where small extensional zones characterized by pull-apart structures and/or negative flower structures alternate with reaches along which positive flower structures and fault-parallel folds indicate a component of convergence.
The history or structural evolution of a fault system is also important in the recognition of strike-slip behavior in the contemporary tectonic setting. Sylvester (1988) and Christie-Blick and Biddle (1985), for example, stress that in the analysis of fault behavior, one must separate structures resulting from deformation of different ages, especially to distinguish structures due to strike-slip deformation from those of preexisting or subsequent contractional deformation.
In our assessment of the contemporary behavior and style of deformation along the Hosgri fault zone, we evaluated the entire deformational history of the fault, and identified those structures and stratigraphic displacements that are wholly or in part the result of earlier deformational episodes that are not currently operable in the present tectonic setting (PG&E, 198S). Some of the folding adjacent to and west of the vertical strands of the Hosgri fault zone, for example, is related to reverse or thrust faults that were active in the late Miocene and early Pliocene but are no longer active in the present tectonic setting (for example, see Response to Question GSG 3 and Attachment Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term SeismIc Program
I ueti n S 4 March 1 0 Pa e 22 GSG Ql-A, Point Sal montages, Sections W and WW'). In addition, much of the apparent up-on-the-east vertical separation of basement across the Hosgri fault zone is inherited from a mid-Miocene episode of extensional or transtensional deformation (Response to Question GSG 3; PG&E, 1988,
- p. 2-29, 2-105, 2-106). This episode of extensional or transtensional deformation and basin formation is widely recognized over most of the offshore Santa Maria Basin (McCulloch, 1987).
Recognition of these earlier episodes and styles of deformation along the Hosgri fault zone and bordering offshore Santa Maria Basin has led to the assessment of the contemporary behavior of the Hosgri fault zone as a prominent right-slip fault.
CONCLUSIONS The diverse styles of faulting along and within the Hosgri fault zone imaged on seismic reflection data are typical of strike-slip fault zones, based on examples of several other well-documented strike-slip fault systems worldwide. Indeed, the presence of varied styles of faulting subparallel to and within large strike-slip fault zones is to be expected for several important and commonly occurring reasons:
Many strike-slip fault systems, principally transcurrent faults (Sylvester, 1988; Lemiszki and Brown, 1988) such as the Hosgri fault zone, are oriented oblique to the direction of plate motion, and thus have a component of tensional or compressional stress in addition to the resolved shear stress along the fault zone. The tensional or compressional stress will produce regional extensional or contractional deformation, respectively, along the fault. Where strain partitioning (see Response to Question GSG 2) is operating, subparallel strike-slip and dip-slip faults will occur.
- 2. Local bends, en echelon steps or other changes in local fault geometry will produce localized extensional or contractional deformation along strike-slip faults (for example, Crowell, 1974; Woodcock and Fischer, 1986; Barka and Kadinsky-Cade, 1989; Kadinsky-Cade and Barka, 1989; Brown and Sibson, 1989; Bilham and King, 1989; King, 1986).
- 3. The structural evolution of many strike-slip faults involves the cumulative effects of more than one deformational episode. The imprint of inherited structures and displacements will produce complex, difficult-to-interpret fault geometries, patterns, and sense of displacement (Sylvester, 1988; Christie-Blick and Biddle, 1985). The effects of each prior episode of displacement must be discerned to properly assess the contemporary style of deformation.
Each of these phenomena, changing plate motions that have triggered distinct deformational episodes strain partitioning, and variations in local fault geometries, have played an important role in producing the deformation that we observe along the Hosgri fault zone today. Given these considerations, we interpret the Hosgri fault zone to be a right-slip fault with variable rates of dip slip that depend on local fault geometry, orientation of the fault zone with respect to plate motion, local associated deformation, and structural/stratigraphic features inherited from preexisting deformation.
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Allen, C. R., 1965, Transcurrent faults in continental areas; in A Symposium on Continental Drift:
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uestion SSC 4 March I 0 Pa e2 Anadon, P., Cabrera, L., Guimera, J., and Santanach, P., 1985, Paleogene strike-slip deformation and sedimentation along the southeastern margin of the Ebro Basin; in Biddle, K. R., and Christie-Blick, N., eds., Strike-slip Deformation, Basin Formation, and Sedimentation: Society of Economic Paleontologists and Mineralogists Special Publication No. 37, p. 303-318.
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Brown, ¹ N., and Sibson, R. H., 1989, Structural geology of the Ocotillo badlands antidilational fault jog, southern California; in Schwartz, D. P., and Sibson, R. H., eds., Fault Segmentation and Controls of Rupture Initiation and Termination: U. S. Geological Survey Open File Report 89-315,
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Donovan, R. N., 1985, The Meers faults as a hinge to the Wichita frontal fault zone (abs.):
Earthquake Notes, v. 55, p. 1.
Greene, H. G., 1977, Geology of the Monterey Bay region: U.S. Geological Survey Open-File Report 77-718, 347 p.
Greene, H. G., Lee, W. H. K., McCulloch, D. S., and Brabb, E. E., 1973, Faults and earthquakes in the Monterey Bay region, California: U.S. Geological Survey Miscellaneous Field Studies Map MF-518.
Harbert, W., and Cox, A., 1989, Late Neogene motion of the Pacific plate: Journal of Geophysical Research, v. 94, no. B3, p. 3052-3064.
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uesti n 4 March 1 0 Pa 24 Harding, T. P., 1983, Divergent wrench fault and negative flower structure, Andaman Sea; jn Bally, A. W., ed., Seismic Expression of Structural Styles - A Picture and Work Atlas: American Association of Petroleum Geologists Studies in Geology, Series 15, v. 3, p. 4.2 4.2-8.
Harding, T. P., 1984, Graben hydrocarbon occurrences and structural style: American Association of Petroleum Geologists Bulletin, v. 68, p. 333-362.
Harding, T. P., 1985, Seismic characteristics and identification of negative flower structures, positive flower structures, and positive structural inversion: American Association of Petroleum Geologists Bulletin, v. 69, p. 582-600.
Harding, T. P., 1989, Criteria and pitfalls in identification of wrench faults from exploration data (abs.): Program, 64th Annual Meeting, Pacific Section, AAPS-SEPM, p. 31.
Harding, T. P., and Lowell, J. D., 1979, Structural styles, their plate tectonic habitats, and hydrocarbon traps in petroleum provinces: American Association of Petroleum Geologists Bulletin,
- v. 63, p. 1016-1058.
Harding, T. P. Gregory, R. F., and Stephens, L. H., 1983, Convergent wrench fault and positive flower structure, Ardmore basin, Oklahoma; ~i Bally, A. W., ed., Seismic Expression of Structural Styles - A Picture and Work Atlas: American Association of Petroleum Geologists Studies in Geology, Series 15, v. 3, p. 4.2 4.2-17.
Harding, T. P., Vierbuchen, R. C., and Christie-Blick, N., 1985, Structural styles, plate-tectonic settings, and hydrocarbon traps of divergent (transtensional) wrench faults; in Biddle, K. R., and Christie-Blick, N., eds., Strike-slip Deformation, Basin Formation, and Sedimentation: Society of Economic Paleontologists and Mineralogists Special Publication No. 37, p. 51-77.
Kadinsky-Cade, K., and Barka, A. A., 1989, Effects of restaining bends on the rupture of strike-slip earthquakes; in Schwartz, D. P., and Sibson, R. H., eds., Fault Segmentation and Controls of Rupture Initiation and Termination: U. S. Geological Survey Open File Report 89-315, p. 181-192.
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Lemiszki, P. J., and Brown, L. D., 1988, Variable crustal structure of strike-slip fault zones as observed on deep seismic reflection profiles: Geological Society of America, v. 100, p. 665-676.
Lewis, K. B., 1980, Quaternary sedimentation on the Hikurangi oblique-subduction and transform margin, New Zealand; jn Ballance, P. F., and Reading, H. G., eds., Sedimentation in Oblique-slip Mobile Zones: International Association of Sedimentologists Special Publication No. 4, p. 171-189.
Lowell, J. D., 1972, Spitsbergen Tertiary orogenic belt and the Spitsbergen fracture zone: Geological Society of America Bulletin, v. 83, p. 3091-3102.
Lowell, J. D., 1985, Structural styles in petroleum exploration: Oil and Gas Consultants International, Inc., Tulsa, Oklahoma, 477 p.
Lynn, H. B., and Deregowski, S., 1981, Dip limitations on migrated section as function of line length recording time: Geophysics, v. 46, p. 1392-1397.
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es i n 4 M, rch I Pa e2 Mann, P., Draper, G., and Burke, K., 1985, Neotectonics of a strike-slip restraining bend system, Jamaica; in Biddle, K. R., and Christie-Blick, N., eds., Strike-slip Deformation, Basin Formation, and Sedimentation: Society of Economic Paleontologists and Mineralogists Special Publication No.
37, p. 211-226.
McCulloch, D. S., 1987, Regional geology and hydrocarbon potential of offshore Central California:
U.S. Geological Survey Professional Paper, 127 pp.
Naylor, M. A., Mandl, G., and Sijpesteijn, C. H. K., 1986, Fault geometries in basement- induced wrench faulting under different initial stress states: Journal of Structural Geology, v. 8, no. 7,
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Pacific Gas and Electric Company, 1988, Final report of the Diablo Canyon Power Plant Long Term Seismic Program: U. S. Nuclear Regulatory Commission Docket Nos. 50-275 and 50-325.
Reading, H. G., 1980, Characteristics and recognition of strike-slip fault systems: International Association of Sedimentologists Special Publication, v. 4, p. 7-26.
Saleeby, J. B., 1984, Continent/Ocean Transect 010, C-2 central California offshore to Colorado Plateau: Geological Society of America.
Sengor, A. M. C., Gorur, N., and Saroglu, F., 1985, Strike-slip faulting and related basin formation in zones of tectonic escape: Turkey as a case study; ~i Biddle, K. R., and Christie-Blick, N., eds.,
Strike-slip Deformation, Basin Formation, and Sedimentation: Society of Economic Paleontologists and Mineralogists Special Publication No. 37, p. 227-264.
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1985, The Tertiary strike-slip basins and orogenic belt of Spitsbergen; in Biddle, K.R., and Christie-Blick, N., eds., Strike-slip Deformation, Basin Formation, and Sedimentation: Society of Economic Paleontologists and Mineralogists Special Publication No. 37, p. 339-360.
Stone, D. S., 1986; Wrench faulting and Rocky Mountain tectonics: The Mountain Geologist, v. 6, no. 2, p. 67-79.
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ue i n 4 March I Ps e2 Weber, G. E., 1979, Vertical displacements of the first marine terrace near Greyhound Rock, Santa Cruz County, California, fault or landslide induced; in Weber, G. E., Lajoie, K. R., and Griggs, G.
B., eds., Coastal Tectonics and Coastal Geologic Hazards in Santa Cruz and San Mateo Counties, California: Field Trip Guide for Cordilleran Section of the Geological Society of America, 75th Annual Meeting, p. 81-91.
Weber, G. E, and Lajoie, K. R., 1977, Late Pleistocene and Holocene tectonics of the San Gregorio fault zone between Moss Beach and Point Aiio Nuevo, San Mateo County, California (abs.):
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RESPONSE TO QUESTIONS GSG 2, 3, 4, 5, 7, 8, 9, 10, 12, 13, and 14 March 1990 This volume responds to 11 of 16 Geology/Seismology/Geophysics questions asked of PG&E by the Nuclear Regulatory Commission (NRC) on August 1, 1989. Responses to the remaining 5 questions will be submitted later. These responses provide data requested to augment or clarify information presented in the Final Report of the Long Term Seismic Program, submitted by PG&E to the NRC on July 31, 1988, and presented in responses to December 13, 1988, questions, submitted by PG&E to the NRC in January and February of 1989.
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ueti n 2 March I 0 Pa el QUESTION GSG 2 D. B. Slemmons proposed that the Hosgri may be experiencing oblique fault motion at seismogenic depths and that this is being partitioned into strike slip arrd dip slip on the near surface faults. As stated by George Thompson, this region nray be responding similarly to the San Andreas region with the horizontal strike slip component of strain being accommodated on the Hosgri system and the compressional component being accommodated on the sub-parallel reverse faults and folds. Provide a discussion of these models their appropriateness and any implicatio>rs of these concepts to the LTSP.
Recently, several authors (Tapponnier and others, 1989; Richard and Cobbold, 1989; Lettis and others, in review) have indicated that oblique strain or crustal shortening in the lower crust or lithospheric mantle may be partitioned up-section into nearly pure tangential and normal components of strain in the mid to upper crust. This concept, termed "strain partitioning" or "fault bifurcation" (Tapponnier and others, 1989) is used to describe the occurrence of subparallel, coeval strike-slip and dip-slip (normal and reverse) faults in a variety of tectonic settings around the world (Richard and Cobbold, 1989) Theoretically, the partitioning produces decoupled transcurrent and compressive
~
deformation at the earth's surface, potentially operating simultaneously and independently (Mount and Suppe, 1987), and at a variety of scales ranging from regional (tens of kilometers) to local (intra-fault-zone) (also called "full crustal" and "basinal" scales, respectively, by Richard and Cobbold, 1989).
The concept of strain partitioning may be a useful approach for describing the distribution and nature of deformation in south-central coastal California. The Hosgri/San Simeon fault system has geologic evidence of both regional and local strain partitioning (Figure GSG Q2-1). Regional strain partitioning may be occurring between the San Simeon fault zone and Piedras Blancas antiform, and between the Hosgri fault zone and South Basin compressional domain (for example, Purisima and Lompoc structures). Local strain partitioning may be occurring along parts of the Hosgri fault zone where upward-diverging fault splays (for example, positive and negative flower structures) are imaged on seismic reflection data. An important question, therefore, for seismic hazard evaluation is whether oblique slip at seismogenic depths along the Hosgri fault zone partitions up-section into discrete tangential and normal strain components on the high- and low-angle components, respectively, of the Hosgri fault zone; ifthis occurs, how should the partitioning be incorporated into seismic source characterization?
In this response, we first provide worldwide observations of strain partitioning, followed by a description of the "strain partitioning" model, including proposed explanations for the mechanical and kinematic partitioning of strain. We then discuss implications of the model for assessing seismic source characteristics of the Hosgri fault zone. We conclude that (1) regional and local partitioning of strain may be occurring along several but not all reaches of the Hosgri fault zone; (2) faults produced by regional strain partitioning are best characterized as separate seismic sources; (3) our characterization of the Hosgri fault zone as a seismic source (PG&E, 1988) captures the distribution of local strain partitioning across the fault zone; and (4) quantitative comparison of the components of lateral and vertical slip on the Hosgri fault zone indicates that the Hosgri is best characterized as a high-angle, strike-slip fault for seismic hazard evaluation.
OBSERVATIONS OF STRAIN PARTITIONING There is abundant worldwide geologic evidence for occurrences of both regional and local partitioning of strain. By regional, we mean partitioning that occurs over lateral dimensions of ten to hundreds of kilometers; local partitioning occurs over dimensions of less than several kilometers.
Regional partitioning is particularly well expressed within many subduction zones, where oblique convergence is partitioned into a nearly pure dip-slip subduction zone, and nearly pure strike-slip faults in the back arc region often several tens of kilometers from the plate interface. Examples include the Andaman and Great Sumatran faults along the Sunda arc (Moore and others, 1980) and Dlahlo Canyon Power Plant Pacific Gas and Electric Company Lang Term Seismic Program
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u in 2 Mrchl P e3 the Unimak and Fairweather faults along the Alaskan arc. Other examples of regional strain partitioning occur in tectonic settings ranging from transpressional plate margins to continental collisions. For example, Richard and Cobbold (1989) cite the occurrence of regional, subparallel, coeval strike-slip and reverse faults in continental environments of Asia (Pakistan, northwestern edge of Tibetan Plateau; Tien Shan Ranges, Burma), South America (Cordillera Oriental of Colombia) and Europe (central Alps). In south-central coastal California, Crouch and Bachman (1989) cite the occurrence of regional thrust-related structures parallel to, and seaward of, the Newport-Inglewood strike-slip fault.
The San Andreas fault system in California, as mentioned by George Thompson during the GSG NRC workshop in June 1989, provides several excellent examples of regional strain partitioning that are supported by both geologic and seismologic data. In south-central California, the San Andreas is a prominent right-slip fault having a horizontal to vertical ratio of slip of 42:1 in the Carrizo Plain (Sich and Jahns, 1984), and is bordered along certain reaches by regions of crustal shortening nearly orthogonal to the fault trace. These regions of crustal shortening include the Coalinga thrust fault and related folds, the thrust fault beneath Wheeler Ridge in the southern San Joaquin Valley, and the thrust faults and related folds beneath the northern Los Angeles Basin (Figure GSG Q2-2). The crustal shortening occurs in elongate zones,subparallel to and about 15 to 40 kilometers from the San Andreas fault. Based on stress orientations and structural modeling, Mount and Suppe (1987) and Namson and Davis (1988), respectively, conclude that crustal strain is regionally decoupled between the San Andreas fault and Coalinga thrust fault. Coeval, regional strain partitioning between the San Andreas fault zone and subparallel fold and thrust belts is strongly supported by historical seismicity (Figure GSG Q2-2), where nearly pure strike-slip earthquakes have occurred along the fault (for example, the 1966 Parkfield event and the 1857 Fort Tejon event), and nearly pure dip-slip reverse earthquakes have occurred within the nearby fold and thrust belts (for example, the 1952 Kern County event, 1971 San Fernando event, 1983 Coalinga event, 1985 Kettleman Hills event, and the 1987 Whittier Narrows event).
Local intra-fault-zone or basinal strain partitioning is similarly well-expressed in a variety of tectonic settings. Examples of locally partitioned subparallel strike-slip and dip-slip faults are reported for the Nyainchengthanglea fault in Tibet (Tapponnier and others, 1989), the Red River fault in Yunnan (Allen and others, 1984), the San Gregorio fault at Aiio Nuevo (Figure GSG Q2-3)
(Weber and Lajoie, 1979), the Newport-Inglewood fault in south-coastal California (Crouch and Bachman, 1989), and the Altyn Tagh fault in Xinjiang (Molnar and others, 1987; Tapponnier and others, 1989). In addition, the presence of flower structures imaged on seismic reflection profiles across many known strike-slip faults worldwide provides evidence for local intra-fault-zone or basinal-scale strain partitioning. For example, flower structures are present along the West Andaman fault in the Andaman Sea (Figure GSG Q2-4, Harding and others, 1985), the Washita frontal fault system in the Ardmore Basin (Figure GSG Q2-5, Lowell, 1985), the Newport-Inglewood fault (Crouch and Bachman, 1989), the Unimak fault in the Gulf of Alaska (Steffy, 1989), and the San Gregorio fault in central coastal California (Figure GSG Q2-6, see also Response to Question SSC
- 4) The locally partitioned strike-slip and dip-slip faults observed both at the surface and within
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flower structures on seismic data are typically separated by less than 3 to 5 kilometers and merge downward into a principal displacement zone within a depth of 4 to 6 kilometers (Figures GSG Q2-3 through GSG Q2-5).
Although there is abundant geologic evidence to support the concept of strain partitioning, there are only a few reported cases of potential coseismic strain partitioning. Coseismic strain partitioning may have occurred in 1872 along the Owens Valley fault zone in eastern California (Beanland and Clark, 1987), in 1932 along the Chang Ma fault in Gansu, China (Peltzer and others, 1988; Meyer and others, 1989), in 1954 during the Rainbow Mountain earthquake in Nevada (dePolo and others, 1989), in 1957 along the Gobi-Altai fault (Florensov and Solonenko, 1965), and in 1974 along faults in the Pamir-Karakorum region (Ni and Guangwei, 1989). In each of these events, coseismic deformation was characterized locally by instances of clear separation of dip-slip and strike-slip surface displacement on subparallel fault traces within a zone typically less than 5 kilometers wide.
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EXPLANATION T Displacement toward viewer A Displacement away from viewer Figure GSG Q2-4 Seismic profile across the right-slip IVest Andaman fault in the Andaman Sea, north of Sumatra.
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uestion SG 2 March 1 0 Pa e9 In addition, the 1989 Lorna Prieta earthquake along the San Andreas fault appears to have aspects of coseismic strain partitioning. Oblique slip between depths of 6 and 18 kilometers produced the 7.0 Mw earthquake. Although the San Andreas fault is a nearly pure strike-slip fault along most of its length, the Santa Cruz Mountains segment occupies a large, 7- to 10-degree left-restraining bend in the fault where kinematic and plate motion constraints predict oblique slip. The oblique slip at depth, modeled by geodetic measures to be 1.9 + 0.2 meters of right slip and 1.3 + 0.4 meters of reverse slip (U. S. Geological Survey Staff, 1990), produced distributed strain at the surface.
Extensional and lateral offsets occurred within the Santa Cruz Mountains proximal to (but not on) the fault trace (Ponti, 1989; Cotton and others, 1990) and compressional deformation occurred in an elongate zone along the northeastern flank of the Santa Cruz Mountains (for example, from Guadalupe Creek south of Los Gatos to Page MillRoad in Palo Alto) (Haugerud and Ellen, 1989).
The zones of compressional deformation are located about 3 to 6 kilometers northeast of the San Andreas fault and are subparallel to or locally lie along previously mapped faults (for example, the Shannon, Monte Vista, and Berrocal faults). These zones of surface deformation may be the coseismic surface manifestation of displacement on deep-seated tectonic faults, or they might reflect secondary deformation related to distributed shear, folding, or localized amplification of ground motion. Whether these zones of surface deformation are the surface expression of faults that can also act as seismic sources temporally independent of slip on the San Andreas fault is unknown at present.
STRAIN PARTITIONING MODEL Preliminary models to explain the mechanical and kinematic process of partitioning of strain within the earth's crust have been proposed by several authors. These models principally build upon the observations that strain partitioning appears to occur in many regions of the world. Based on empirical studies of partitioned strain in various active tectonic environments, Tapponnier and others (1989) suggest that the partitioning occurs at a "bifurcation line along which an oblique-slip fault at depth splays into two faults" at shallower crustal levels. They suggest that the bifurcation occurs where the oblique fault intersects a near-horizontal interface, separating layers having different mechanical properties. The shallower layers are typically less cohesive and thus prone to dilatent behavior. The interface may be located at various depths in the crust or lithosphere; for example, between basement and overlying sediments, between high-grade metamorphic or igneous rocks and lower-grade metamorphic rocks, between mantle and crust, or at the base of the seismogenic part of the crust.
Richard and Cobbold (1989) suggest that partitioning of strain occurs in the upper crust when a ductile layer is present within the crust. They investigated the partitioning of fault motions at regional (crustal) scale and the generation of flower structures at local (basinal) scale, using experiments scaled to account for gravitational forces, Coulomb behavior of the brittle upper crust, and viscous behavior at depth. In their interpretation, a near-horizontal ductile layer within the crust reduces the amount of basal drag transmitted to the upper crust, producing principal stresses in the upper crust that are horizontal and vertical. The resulting stress field yields faults that are nearly pure strike-slip or dip-slip, but will not produce oblique-slip faults. By contrast, their model also suggests that oblique-slip faults with little partitioning of strain should be generated in the absence of ductile layers at depth.
The presence of a ductile layer or change in rock rheology within the crust has important implications for interpreting the character of surface deformation withrespect to the nature of coseismic slip at depth. The style of surface deformation associated with the 1989 Lorna Prieta earthquake, for example, may be explained by a change in rock rheology within the mid to upper crust. Based on a preliminary analysis of lithology and thermal gradients, Furlong and others (1989) interpret a ductile zone or change in crustal rheology at roughly a depth of 8 to 10 kilometers in the crust along the southern Santa Cruz Mountains segment of the San Andreas fault. Thus, oblique slip initiating at a depth of 18 kilometers may partition up-section across the ductile zone into tangential and normal components of strain. In addition, focal mechanisms of aftershocks indicate that both strike-slip and dip-slip faulting occurred. The distribution of aftershocks define a relatively narrow Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
l u i n 2 March 1 Pa lo fault plane from 18 to 6 kilometers depth, and an upward-broadening zone of deformation from about 6 to 2 kilometers depth, suggesting that if partitioning occurred, it occurred in the upper crust above 6 kilometers depth.
Mount and Suppe (1987) and Zoback and others (1987) have proposed a model of strain partitioning based on stress rotation in which the strength of the fault controls the orientation of the stress field.
In this model, low-strength faults may localize deformation and reorient tectonic stresses. The direction of the maximum horizontal compressive stress in the vicinity of an extremely weak fault must be either nearly perpendicular to or parallel to the fault to minimize resolved shear stresses on the fault plane. Based on observed patterns of regional maximum horizontal stress, Mount and Suppe (1987), for example, interpret the San Andreas fault to be a nearly frictionless interface, with the maximum horizontal compressive stress nearly normal to the fault. They conclude that the transpressive strain in central California is decoupled into transcurrent deformation along a low-strength San Andreas fault and contractile deformation on neighboring subparallel thrust faults and folds that are produced by a high deviatoric stress component oriented roughly normal to the San Andreas fault.
IMPLICATIONS OF STRAIN PARTITIONING FOR SEISMIC HAZARD EVALUATION The concept of strain partitioning affects the assessment of seismic hazard primarily with respect to the identification and characterization of seismic sources. Critical to this assessment is the scale of partitioning and whether structural features should be treated as individual seismic sources or collectively as a single seismic source. Figure GSG Q2-7 schematically illustrates the concept of strain partitioning in the upper lithosphere. Oblique strain in the lower lithosphere may partition upward in the brittle crust into nearly pure strike-slip and dip-slip deformation, with the dip-slip component expressed as reverse faults, folds or, most commonly, both reverse faults and folds.
Depending on the depth of partitioning, these partitioned structures may be independent regional sources of seismicity or they may be dependent local structures above a single seismic source at depth.
In western California, the brittle part of the lithosphere, within which elastic strain energy is stored and released during earthquakes, extends from near the earth's surface to a depth that locally varies from 10 to 20 kilometers. Although microseismicity is common throughout this vertical zone, the release of seismic moment in earthquakes is not uniform within the zone. Above a depth of about 5 to 7 kilometers, as seen in Figure 4-12 of the Final Report (PG&E, 1988), slip modeling of recent moderate to large earthquakes (magnitude 6 or greater) indicates that only a small amount of the total seismic moment of the event is released. Low seismic moment is due in part to lower stored elastic strain and in part to lower rock rigidity, or shear modulus. Seismogenic slip and released elastic strain increases significantly across the depth range of 5 to 7 kilometers into a region of high moment release, which extends to near the bottom of the seismogenic zone. Rock rigidity is higher within this zone and moderate to large earthquakes are observed to nucleate at depths greater than about 5 kilometers. On the basis of these observations, we divide the upper, seismogenic part of the lithosphere into a region of low moment release during large earthquakes above 5 kilometers depth, and a region of high moment release during earthquakes below 7 kilometers depth, separated by a transitional zone 1 to 2 kilometers wide (Figure GSG Q2-7). In this model, faults that extend through the low-moment-release zone and into the high-moment-release zone, or occur only within the high moment-release zone below a depth of 5 to 7 kilometers are independent sources of large earthquakes. Faults confined to the upper crust above a depth of 5 to 7 kilometers are not capable of releasing large earthquakes, only smaller events. Such shallow faults may be dependent upon independent faults at depth.
Based on worldwide empirical data that we have examined, the scale of strain partitioning is a continuum from regional to local. Strain partitioning at regional or full-crustal scale produces neighboring strike-slip and dip-slip faults that are independent sources of seismicity having specific source parameters. As schematically shown on Figure GSG Q2-7, to be treated as independent seismic sources, discrete faults should extend into the lower seismogenic crust where large-moment Diablo Canyon Power Plant Pacific Gas and Electric Company long Term Seismic Program
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u i n 2 March I P e ll Regional Local 23 to 6km s3to6km 6 km 3 0 0
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~SSSSSS~S EXPLANATION Q+ Displacement toward viewer Q Displacement away from viewer Figure GSG Q2-7 Idealized section of upper lithosphere, illustrating features of regional "full-crustal" and local "basinal" strain partitioning.
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ueti n 2 Mrchl P e 12 earthquakes occur. Each fault, therefore, is a potential seismic source at crustal scale and should be identified and characterized separately, such as would be the case with the San Andreas fault and Coalinga thrust fault in central California. Field observations suggest that spacing between faults at the surface is at least five and typically twenty or more kilometers. Alternatively, strain partitioning at a local, intra-fault-zone scale produces geologic deformation (faults and folds) in the upper crust that is not a separate source of seismicity. In this latter case, coseismic deformation from a single fault at depth partitions up-section in the upper crust into tangential and normal components of deformation. The partitioned surface and near-surface structures should be identified and charac-terized collectively to assess the seismic source structure at depth (for example, the Santa Cruz Mountains segment of the San Andreas fault). As diagrammed in Figure GSG Q2-7, these structures merge with the primary deep-seated fault or die out at shallow depths above the zone of large moment release. Field observations indicate that surface deformation typically is confined to a zone less than 3 to 6 kilometers wide.
IMPLICATIONS FOR CHARACTERIZATIONOF HOSGRI FAULT ZONE The Hosgri/San Simeon fault system has a style of deformation comparable in many respects to examples of strain partitioning observed elsewhere in California and throughout the world. Evidence of both local and regional strain partitioning is present along specific reaches of the San Simeon and Hosgri fault zones. Areas of partitioning are shown on Figure GSG Q2-1 and are described below.
Partitioning of strain may occur along certain reaches of the Hosgri fault zone, but clearly does not occur along the entire fault system (Figure GSG Q2-1). For example, neither the Northern reach nor the northern part of the San Luis/Pismo reach of the Hosgri fault zone have evidence of either regional or local strain partitioning in the contemporary tectonic setting (see Attachment GSG Ql-A). In both these areas, the Hosgri fault zone is characterized by two primary, high-angle strands, which accommodate both the vertical and lateral components of slip. Lower-angle fault strands along these areas generally do not deform the mid-Pliocene unconformity and are therefore inactive in these areas in the current tectonic setting. Seismic lines PG&E 1 and W-14 (Attachment GSG Ql-A), for example, show that the Hosgri fault zone is defined by one or two prominent high-angle strands and locally by one or more lower-angle strands that do not deform the mid-Pliocene unconformity.
Strain partitioning is confined to the west-southwest side of the Hosgri/San Simeon fault system (Figure GSG Q2-1). Crustal shortening within the Los Osos/Santa Maria domain on the east-northeast side of the Hosgri/San Simeon fault system is oriented oblique to and not normal to the trend of the fault system. The crustal shortening in this region is due, in part, to northward-directed, clockwise rotation of the western Transverse Ranges, and compression of the domain against the rigid Salinian block underlying the southern Coast Ranges. This crustal shortening is accommodated by reverse faulting, localized folding, and block uplift oblique to the Hosgri/San Simeon fault system. Faults in the Los Osos/Santa Maria domain do not merge with the Hosgri fault zone as subparallel upward-diverging splays. Rather, the Hosgri fault zone acts to decouple the deformation within the Los Osos/Santa Maria domain from the deformation occurring in the offshore Santa Maria Basin. Therefore, the observed crustal strain in the domain is not the result of simple partitioning of normal and tangential strain components in the upper lithosphere above a zone of oblique strain in the lower lithosphere. Similar "one-sided" strain partitioning is observed along the Newport-Inglewood fault in south-coastal California, where Crouch and Bachman (1989) cite the presence of regional, thrust-related structures parallel to, and seaward of, the fault.
Regional Strain Partitioning Regional strain partitioning appears to be occurring along the southern part of the San Simeon fault zone and within the offshore Santa Maria Basin along the Southern and Point Sal reaches of the Hosgri fault zone (Figure GSG Q2-1). In these areas, transcurrent strain is accommodated along the nearly pure strike-slip San Simeon and Hosgri fault zones, and contractile strain is accommodated Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
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ueti n 2 March I 0 P el by dip-slip faults and folds within the Piedras Blancas antiform and South Basin compressional domain, respectively.
San Simeon Fault Zone. The southern part of the San Simeon fault zone is bordered to the west by the Piedras Blancas antiform (Figure GSG Q2-8). The antiform is a basement-cored uplift flanked by Miocene and younger folds. Dip-slip focal mechanisms of small earthquakes beneath the antiform and uplift of late Quaternary marine terraces along the San Simeon coast indicate that the antiform is an actively deforming structure (Hanson and Lettis, in review; PG&E, 1988). Many of the fold axes within the antiform are subparallel to the trend of the San Simeon fault zone (although other fold axes are more westerly trending). An assessment of the style and rate of activity of the Piedras Blancas antiform is provided in Response to Question GSG 13. Structural modeling across portions of the antiform indicates that many of these folds are fault-propagation or fault-bend folds underlain by low-angle thrust faults. Faults beneath the western part of the antiform are east-vergent and probably are not directly related to the San Simeon fault zone. Faults beneath the eastern and northern part of the antiform, however, are southwest-vergent and probably intersect the San Simeon fault zone at depth. These faults and overlying folds are about 10 to 20 kilometers west-southwest of the San Simeon fault zone. Projection of the underlying thrust faults to the northeast suggests they would intersect or merge with the San Simeon fault zone within or beneath the lower seismogenic crust, where large-moment earthquakes typically occur. It can be argued, therefore, that structures within the Piedras Blancas antiform and the San Simeon fault zone are sufficiently separated to be independent sources of seismicity.
Hosgrl Fault Zone. Regional strain partitioning appears to be occurring along the Southern and Point Sal reaches of the Hosgri fault zone within the southern part of the offshore Santa Maria basin. This part of the basin lies within the South Basin compressional domain that is characterized by a series. of late Cenozoic folds and underlying thrust faults that strike subparallel to the Hosgri fault zone (Figure GSG Q2-1). The occurrence of seismicity having dip-slip focal mechanisms, deformation of the mid-Pliocene unconformity across the folds, local ponding of late Quaternary deposits adjacent to fold axes, and bathymetric expression of several folds on the sea floor strongly suggest continuing compressional fold deformation in the domain. Fold axes within the domain trend north-northwest, generally subparallel to and 5 to 30 kilometers west of the Hosgri fault zone, except locally along the southern reach of the Hosgri, where they trend oblique to the fault trace.
Prominent folds within the domain include the Queenie structure (Clark and others, in press), the Lompoc structure, and the Purisima structure. Structural modeling of these folds suggests they are underlain by southwest-vergent thrust faults.
Thrust faults underlying several of the folds probably merge with or intersect the Hosgri fault zone to the northeast at mid- to lower-crustal levels within the zone of large moment release (Figure GSG Q2-9). For example, seismic line GSI-112c (Attachment GSG Ql-A, Point Sal reach montage) illustrates the regional partitioning of strain between the Hosgri fault zone and the thrust fault producing the Purisima structure. The thrust fault probably intersects or merges with the Hosgri fault zone at a depth of 4 to 8 kilometers (Figure GSG Q2-9), and is separated from the Hosgri fault zone at the surface by about 5 to 7 kilometers. This separation decreases to 1 to 2 kilometers to the south. Therefore, the Hosgri fault zone and Purisima structure may be either dependent or independent seismic sources. Where they intersect within the region of low moment release, they are dependent structures reflecting a single seismic source at depth, and should be characterized collectively for seismic hazard evaluations. Where they intersect within the region of large moment release, they are potentially independent sources of seismicity and should be characterized separately for seismic hazard evaluation. Thrust faults underlying the Lompoc structure and other smaller folds west of the Purisima structure appear to intersect or merge with the Hosgri fault zone within the region of large moment release, and are separated from the Hosgri fault zone at the surface by more than 10 kilometers. Therefore, these structures are probably the result of regional strain partitioning and should be characterized as distinct, independent seismic sources for seismic hazard evaluations.
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Fault, relative sense of motion shown, dashed Hosgri where location not well constrained, dotted Fault Zone where fault does not deform late Pliocene strata Thrust fault, sawteeth on hanging wall, dashed where location not welt constrained, dotted where fault does not deform late Pliocene strata Anticlinal and synclinal fold axis, dashed where location not well constrained, dotted where structure does not deform late Pliocene strata Figure GSG Q2-8 Structural trends map of the Piedras Blancas antiform and San Simeon fault zone, south-central coastal California.
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ueti n 2 March I 0 P I Regional strain partitioning Local strain partitioning SW Purisima structure 0 Sea floor Mid-Pliocene Unconformit Mid-Pliocene O O+
Top of Miocene Unconformity Miocene Zone of low moment release during earthquakes Basement
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h Depth and nature of Zone of large interaction moment release not well during earthquakes constrained 0+
10 10 6 4 2 0 Distance from fault (km)
EXPLANATION S Displacement toward viewer e Displacement away from viewer Figure GSG Q2-9 Idealized section across Hosgri fault zone and Purisima structure illustrating probable interaction of faults within the brittle seismogenic crust.
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u in 2 Marchl 0 Pa ei Local Strain Partitioning Strain partitioning also may be occurring locally within the Hosgri fault zone. Partitioning of strain is suggested by the presence of upward-diverging fault splays and localized folding or warping of Tertiary strata in close proximity to the fault zone. The upward-diverging fault splays generally consist of one or more high-angle fault strands along which deformation is predominantly strike slip, and one or more lower-angle fault strands that underlie the folded Tertiary strata and are interpreted to have predominantly dip slip. These faults strands typically converge with the Hosgri fault zone at shallow crustal depths (2 to 4 kilometers), suggesting they are not independent seismic sources and thus should be characterized collectively, rather than individually, to assess the seismogenic character of the seismic source at depth. That is, the slip components on the seismogenic fault at depth should be represented by the sum of the slip components on the faults in the shallow section. Potential local strain partitioning is evident primarily along the San Luis Obispo Bay reach of the Hosgri fault zone.
For example, seismic line GSI-97 on the San Luis Obispo Bay reach montage (Attachment GSG Ql-A) shows upward-diverging fault splays that converge at shallow depth and are separated at the surface by generally less than 1 to 2 kilometers.
DISCUSSION The concept of "strain partitioning" is a useful model for describing crustal deformation. The available geologic, geophysical, and seismologic evidence support the interpretation that both regional and local strain partitioning is occurring along. specific reaches of the Hosgri/San Simeon fault system. We have incorporated this potential for strain partitioning directly and indirectly into our characterization of the Hosgri fault zone in several ways:
- 1. Slip-rate estimates of the Hosgri fault zone include cumulative lateral and vertical components of slip across the entire fault zone, including high and low-angle fault strands and structural relief due to folding proximal to the fault zone. (Slip rate computations are provided in response to Questions GSG 3 and GSG 4.) Thus, our estimates of slip encompass all coeval intra-fault-zone (basinal) strain partitioning. Even if strain partitioning is occurring, which implies oblique slip at seismogenic depths, the rate of vertical separation on low-angle components is much less than the rate of lateral separation on high-angle components. When combined at seismogenic depths, these estimates of slip yield ratios of lateral to vertical slip greater than 2:1, for a fault dipping 60 degrees or more. Thus the slip-rate estimates indicate that the Hosgri fault zone is predominantly a strike-slip fault.
- 2. Segmentation of the Hosgri fault zone is defined, in part, by the presence (or absence) and nature of associated deformation. For example, the Point Sal and Southern reaches of the fault zone are distinguished, in part, from the San Luis Obispo Bay reach by their regional association with the South Basin compressional domain and general absence of active (post-mid Pliocene) intra-fault-zone low-angle fault strands. The San Luis Obispo Bay reach is characterized, in part, by active intra-fault-zone low-angle fault strands.
To the north, the San Luis/Pismo and Northern reaches of the fault zone are characterized, in part, by the general absence of associated regional or intra-fault-zone contractional deformation.
- 3. Because regional crustal partitioning of strain produces structures that behave as independent sources of seismicity, we characterize the Point Sal and Southern reaches of the Hosgri fault zone as seismic sources that are contemporaneous with but independent from seismic sources within the South Basin compressional domain (for example, dip-slip faults that underlie the Lompoc and Queenie structures). An exception to this may be the thrust fault underlying the Purisima structure that may intersect the Hosgri fault zone within the upper crust and thus could be characterized collectively, at least at its southern end, with the Hosgri fault zone.
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- 4. Timing of deformation on components of the Hosgri fault zone and on structures in adjoining areas was used to assess the contemporary tectonic setting, describe orientations and rates of crustal shortening, and delineate tectonic domains or provinces within the region.
CONCLUSIONS The relatively new concept of strain partitioning appears to provide a useful way to describe and assess the complexity of crustal deformation along regional strike-slip faults. In our assessment of the Hosgri fault zone and related structures, we have incorporated this concept during our identification of potential seismic sources and during the characterization of their behavior, nature, and structural associations in order to provide an integrated interpretation of tectonic deformation in south-central coastal California.
REFERENCES Allen, C. R., Gillespie, A. R., Yuan, H., Sich, K. E., Buchun, Z., and Chengnan, Z., 1984, Red River and associated faults, Yunnan Province, China; Quaternary geology, slip rates, and seismic hazards: Geological Society of America Bulletin, v. 95, p. 686-700.
Beanland, S., and Clark, M. M., 1987, The Owens Valley fault zone, eastern California, and surface rupture associated with the 1872 earthquake (abs.): Seismological Research Letters, v. 58, no. 1,
- p. 32.
Clark, D. H., Hall, N. T., Hamilton, D. J., and Heck, R. G., in press, Structural analysis of late Neogene deformation in the central offshore Santa Maria Basin, California: Journal of Geophysical Research.
Cotton, W. R., Fowler, W. L., and Van Velsor, J. E., 1990, Coseismic bedding plane faulting associated with the Lorna Prieta earthquake of October 17, 1989: Seismological Society of America, Abstracts with Program, Santa Cruz (submitted) ~
Crouch, J. K., and Bachman, S. B., 1989, Exploration potential of offshore Newport-Inglewood fault zone (abs.): Program, 64th Annual Meeting, Pacific Section, AAPG-SEPM, p. 25-26.
dePolo, C. M., Clark, D. G., Slemmons, D. B., and Aymard, W. H., 1989, Historical basin and range province surface faulting and fault segmentation; in Schwartz, D. P., and Sibson, R. H., eds., Fault Segmentation and Controls of Rupture Initiation and Termination: U. S. Geological Survey Open-File Report 89-315, p. 131-162.
Florensov, N. A., and Solonenko, V. P., (eds.), 1965, The Gobi-Altai earthquake: Academy of Sciences of the USSR, translated from Russian by Israel Program for Scientific Translations, Jerusalem, 424 p.
Furlong, K. P., Langston, C. A., Ammon, C. J., Clouser, R. H., Vogfjord, K. S., and Wagner, G. S.,
1989, Seismic rupturing in the lower crust along the San Andreas? lessons from Lorna Prieta:
American Geophysical Union, Poster Session, December.
Greene, H. G., 1977, Geology of the Monterey Bay region: U. S. Geological Survey Open-File Report 77-718, 347 p.
Hanson, K. L., and Lettis, W. R., in review, Estimated Pleistocene slip rate for the San Simeon fault zone, south-central coastal California; in Alterman, I. B., ed., Seismotectonics of Central and Coastal California: Geological Society of America Special Paper.
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ue tion 2 March 1 P 1 Harding, T. P., 1983, Divergent wrench fault and negative flower structure, Andamen Sea; in Seismic expression of structural styles -- a picture and work atlas: American Association of Petroleum Geologists, Studies in Geology 15, v. 3, p. 3.3-13 to 3.3-18.
Harding, T. P., Vierbuchen, R. C., and Christie-Black, N., 1985, Structural styles, plate-tectonic settings, and hydrocarbon traps of divergent (transtensional) wrench faults; in Biddle, K. T., and Christie-Blick, N., eds., Strike-slip Deformation, Basin Formation and Sedimentation: Society of Economic Paleontologists and Mineralogists, Special Publication No. 37, p. 51-77.
Haugerud, R. A., and Ellen, S. D., 1989, Deformation along the northeast foot of the Santa Cruz Mountains during the Lorna Prieta earthquake: American Geophysical Union, Poster Session, December.
Lemiszki, P. J., and Brown, L. D., 1988, Variable crustal structure of strike-slip fault zones as observed on deep seismic reflection profiles: Geological Society of America Bulletin, v. 100,
- p. 665-676.
Lettis, W. R., Hanson, K. L., and Hall, N. T., in review, Strain partitioning: implications for seismic hazards assessment (abs.): Seismological Society of America, Abstracts with Program, Santa Cruz (submitted).
Lowell, J. D., 1985, Structural styles in petroleum exploration: Oil and Gas Consultants International, Inc., Tulsa, Oklahoma, 477 p.
Meyer, B., Tapponnier, P., Gaudemer, Y., Peltzer, G., Blusson, A., 1989, 1932, Chang Ma (M-7.6) earthquake surface breaks and neotectonics of northern Tibet-Quinghai Highlands (abs.): EOS Transactions, American Geophysical Union, v. 70, no. 43, p. 1350.
Molnar, P., Burchfiel, B. C., K'uangyi, L., and Ziyun, Z., 1987, Geomorphic evidence for active faulting in the Altyn Tagh and northern Tibet and qualitative estimates of its contribution to the convergence of India and Eurasia: Geology, v. 15, p. 249-253.
Moore, G. F., Curray, J. R., Moore, D. G., and Karig, D. E., 1980, Variations in geologic structure along the Sunda Forearc, northeastern Indian Ocean; in Hayes, D.E., ed., The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands: American Geophysical Union, Geophysical Monograph 23, p. 145-160.
Mount, V. S., and Suppe, J., 1987, State of stress near the San Andreas fault; implications for wrench tectonics: Geology, v. 15, p. 1143-1146.
Namson, J. S., and Davis, T. L., 1988, Seismically active fold and thrust belt in the San Joaquin Valley, central Californix Geological Society of American Bulletin, v. 100, p. 257-273.
Ni, J. F., and Guangwei, F., 1989, Fault plane solutions of earthquakes and active tectonics of the Pamir-Korakorum region (abs.): EOS Transactions, American Geophysical Union, v. 70, no. 43,
- p. 1226.
Pacific Gas and Electric Company, 1988, Final report of the Diablo Canyon Long Term Seismic Program: U. S. Nuclear Regulatory Commission Docket Nos. 50-275 and 50-323.
Peltzer, G., Tapponnier, P., Gaudemer, Y., Meyer, B., Shunmin, G., Kelun, Y., Zhitai, C., and Huagung, D., 1988, Offsets of late Quaternary morphology, rate of slip, and recurrence of large earthquakes on the Chang Ma fault (Gansu, China): Journal of Geophysical Research. v. 93, no. B7,
- p. 7793-7812.
Ponti, D., 1989, Surface effects of the 1989 Lorna Prieta earthquake: American Geophysical Union, Oral Presentation, December.
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i n SG2 March 1 0 Pa e 19 Richard, R., and Cobbold, P. R., 1989, Mechanical reasons for partitioning of fault motions in continental convergent wrench zones (abs.): International Workshop on Active and Recent Strike-Slip Tectonics, Florence, Italy.
Sich, K. E., and Jahns, R. H., III., 1984, Holocene activity of the San Andreas fault at Wallace Creek, California: Geological Society of America Bulletin, v. 95, no. 8, p. 883-896.
Steffy, D. A., 1989, Geology of the Unimak Forearc basin, western Gulf of Alaska: EOS, Transactions, American Geophysical Union, v. 70, no. 43, p. 1063.
Tapponnier, P., Armijo, R., and Lacassin, R., 1989, Fault bifurcation and partition of strike-slip and dip-slip of mechanical interfaces in the continental lithosphere (abs.): International Workshop on Active and Recent Strike-Slip Tectonics, Florence, Italy.
U. S. Geological Survey Staff, 1990, The Lorna Prieta, California, earthquake; an anticipated event:
Science, v. 247, p. 286-293.
Weber, G. E., and Lajoie, K. R., 1979, Late Pleistocene rates of movement along the San Gregorio fault zone, determined from offset of marine terrace shoreline angles; in Weber, G. E., Lajoie, K.
R., and Griggs, G. B., eds., Coastal Tectonics and Coastal Geologic Hazards in Santa Cruz and San Mateo Counties, California: Geological Society of America, Cordilleran Section 75th Annual Meeting, Field Trip Guide.
Zoback, M. D., Zoback, M. L., Mount, V. S., Suppe, J., Eaton, J. P., Healy, J. H., Oppenheimer, D.,
Reasenberry, P., Jones, L., Raleigh, C. B., Wong, I. G., Scotti, O., and Wentworth, C., 1987, New evidence on the state of stress of the San Andreas fault system: Science, v. 238, no. 4830,
- p. 1105-1111.
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u i n S March l 0 Ps e 1 QUESTION GSG 3 Provide the new information presented at the meeting on the uplift rates across the Hosgri fault zone, based on relative displacement of the basement, top of Miocene, the mid-Pliocene discontinuity and the post-IYisconsin low stand, including the uncertainties in the analysis. Also, clarify the apparent conflict regarding the elevation of the 18 thousand year, late wisconsin, low stand. This is given as -120 meters, in Table 3 arrd Plate 5 of the Response to Question 431, but as -140 and -160 meters in the discussion, at the meeting of June 14, of the north Estero Bay slope break. If the lower value is correct, provide the published source or other supporting data for this departure from globally established values. Ifevidence supports both the lower value to the west of the surface scarp of the Hosgri, and the mapped -120 meter level near the Hosgri scarp, discuss the rate of vertical fault slip thereby implied.
Geophysical and stratigraphic data indicate that vertical separation is common along the entire length of the Hosgri fault zone (see Response to Question GSG 1, Attachment Ql-A). The vertical separation is generally down on the west, although down-on-the-east separation occurs locally along several parts of the fault. The Hosgri fault zone developed as a major structure during the late Oligocene or early Miocene, and the vertical separation developed subsequent to this. The post-Oligocene history of deformation along this fault zone spans at least one well-documented change in plate motions, and thus a change in tectonic setting. Because this change in tectonic setting may have prnduced changes in the style and relative amounts of slip along the Hosgri fault zone, it is importa t to assess both the spatial and temporal variation of vertical separation on the fault. As describe l in this response, most, but not all, of the vertical separation on the Hosgri fault zone is inherited from deformation that occurred during earlier periods in the history of the fault zone, in a tecton'. setting that differs from the contemporary setting.
To asse the timing, amount, location, and sense of vertical slip that have occurred along the Hosgri fault zone, we have used geophysical and stratigraphic information to calculate vertical separation of three distinct geologic horizons across the Hosgri fault zone at numerous locations. These geologic marker horizons, which are interpreted to be unconformities at the top of basement, the top of Miocene sediments, and between lower and upper Pliocene sediments, can be identified as distinct seismic reflectors throughout much of the offshore Santa Maria Basin (Clark and others, in press).
In this sponse we discuss (1) the methods and assumptions involved in assessing the timing and rates of vertical separation; (2) the sensitivity of these results to velocity models used to convert geophysical time sections to depth sections, measurement errors, uncertainties in the age and time-transgressive nature of unconformities, and lateral displacement along the fault zone; and (3) the calculated rates of vertical deformation for three discrete time intervals across the Hosgri fault zone.
The strandline for the late Wisconsinan (1S,000-year-old) low sea-level stand does not cross the Hosgri fault zone anywhere along its length and was not used to evaluate rates of slip along the Hosgri fault zone for a variety of reasons. These reasons are described in Response to Question GSG 4, Attachment Q4-A. A discussion of low sea-level stands and elevations of submerged paleo-shorelines such as the north Estero Bay scarp, as requested in the question, is also provided in Response to Question GSG 4, Attachment Q4-A. As described in Attachment Q4-A, the north Estero Bay slope break in no instance crosses the Hosgri fault zone and thus cannot be used to assess vertical separation across the fault. For most of its length, the slope break does not coincide with the Hosgri fault zone or any other tectonic structure. It generally lies north or northwest of the northern end of the Hosgri fault zone. The slope break consists, in part, of two or more submerged shorelines locally modified by subsequent deposition, erosion, and slump failure. Uncertainty in the ages and elevations of the submerged shorelines, combined with the subsequent slope modification, limits the usefulness of the slope break for evaluating tectonic deformation.
By examining the rate of vertical separation during discrete time intervals, we have differentiated and quantified rates of slip during different periods of deformation in the history of the Hosgri fault zone. This, in turn, provides a basis for differentiating inherited deformation from the rate of Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
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uestion GSG 3 March 1 Pa e2 vertical separation presently occurring along the Hosgri fault zone. If these periods of earlier deformation along the Hosgri fault zone are not recognized and accounted for in slip-rate estimates, significant misinterpretations of both the deformational history and contemporary style of faulting along the fault zone can result. Assumptions that the Hosgri fault zone has maintained a constant rate and sense of displacement through time, during which a significant, well-documented change in plate motion occurred, are simply incorrect.
A comparison of the vertical rates over different time intervals provided in this response with lateral-slip-rate estimates along the Hosgri fault zone is presented in Response to Question GSG 4.
The ratio of horizontal to vertical rates of slip is essential for defining the style of faulting along the Hosgri fault zone. As described in Response to Question SSC 2, ratios of post-mid-Pliocene vertical and lateral slip indicate the Hosgri fault zone is a strike-slip fault in the contemporary tectonic setting.
METHOD Calculations of vertical separation of marker horizons across the Hosgri fault zone were based largely on the interpretation of offshore geophysical data, including common-depth-point and high-resolution seismic reflection surveys. Representative interpreted time and depth sections are presented in Attachment GSG Ql-A. The attachment provides a discussion of the loop-tie methods used to correlate stratigraphic horizons and the velocity models applied to the time-sections to produce depth sections showing true spatial relationships.
Using depth-corrected sections, we measured the vertical separation of three distinct seismic reflectors, interpreted to be unconformities, across the entire width of the Hosgri fault zone, including both the high- and low-angle components of the fault zone. These unconformities include the top of basement, top of Miocene, and mid-Pliocene. To estimate the vertical separation due to faulting and/or folding across the Hosgri fault zone, and to minimize nontectonic factors, we selected inflection points in the unconformities that most likely mark the outer boundaries of the zone of deformation associated with the fault zone. A representative profile showing the technique used to select these points is illustrated in Figure GSG Q3-1.
Compaction, dewatering and draping of younger sediments over the preexisting topography has probably resulted in some nontectonic vertical separation across the fault zone. The amount of vertical separation resulting from these phenomena have not been subtracted from the estimates of vertical separation in this study. Therefore, vertical slip rates based on the measured vertical separation are probably over-estimated and are considered to be conservative values.
Several other factors were considered in making these calculations. Many of the common-depth-point geophysical lines do not extend across the entire Hosgri fault zone. In these areas, high-resolution surveys, geologic mapping in onshore areas, and sampling and identification of sea-floor bottom samples of bedrock (see Plate GSG Q16-IA) were used to estimate the elevation of the unconformities.
In some areas east of the fault zone, uplift has been sufficient to expose basement and older Tertiary bedrock at the present sea floor. Where the younger two unconformities have been eroded, only minimum values of vertical separation can be estimated. In these areas, conservative maximum rates of vertical separation were estimated by directly adding the rate of coastal uplift, based on studies of elevated Pleistocene marine terraces (Response to Question 43, Attachment Q43i-2, January 1989; Hanson and others, in review), to the maximum values of subsidence based on the elevation of the mid-Pliocene unconformity west of the fault zone.
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ue in March I 0 Pa e3 Hosgrl Purisima Fault Zone Structure West trace East trace Shotpoints~ 550 500 450 400 350 300 250 0.0 dP Mid -190
-881 Mid P -690 '?? o-452 > 7 1.0 Top M
~ 2.0 -1238 Top M -1023
-1476
~ Top B
-2286 Top B Coastal uplift rate 3.0 -2428 L based on elevated marine terraces 4.0 CI 5.0 6.0 HOSGRI FAULT ZONE Maximum Vertical Vertical Separation Vertical Slip Rate Coastal Uplift Rate Slip Rate (meters) (mm/yr) (mm/yr) (mm/yr)
Mid P (2.8 Ma) 690-190 = 500 0.18 0.00 0.18 Top M (5.3 Ma) 1023 0.19 0.00 0.19 Top B (-17 Ma) 2428-452 = 1976 HOSGRI FAULT ZONE AND PURISIMA STRUCTURE Mid P (2.8 Ma) [691] [0.25] 0.00 [0.25]
Top M (5.3 Ma) [1238] [0.23] 0.00 [0.23]
Top B (-17 Ma) 2286452 [1834]
EXPLANATION Mid P Middle Pliocene unconformity Top M Top of Miocene unconformity Top B Top of basement unconformity cr Depth in meters from seafloor to unconformity Figure GSG Q3-1 Schematic profile showing technique used to calculate vertical separation.
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u i n March l 0 Pa e4 CHARACTERISTICS AND AGES OF UNCONFORMITIES Top of Basement Unconformity The 'unconformity at the top of basement is generally a well-defined seismic reflector in most areas of the central offshore Santa Maria Basin west of the Hosgri fault zone (Clark and others, in press).
This unconformity is less distinct and locally difficult to map in elevated areas east of the Hosgri fault zone. Where Tertiary rocks lie directly on basement, there is a considerable hiatus. Erosion of the Franciscan Complex rocks probably occurred during the extensive, regional, late Oligocene low sea-level stand, which occurred approximately 22 to 25 million years ago (McCulloch, 1987).
The unconformity is overlain in many areas by volcanic rocks and volcanogenic sediments that are correlative with the early Miocene, Saucesian Stage, Tranquillon volcanic rocks described by Crain and others (1987) in the offshore Arguello-Conception area, and with the Obispo Formation, as described by Hall (1981), in the onshore Santa Maria Basin (Clark and others, in press). Potassium-argon ages for the onshore Obispo Formation range from 15.3 to 16.5 million years old, and Tranquillon rocks have been dated at 17.0 million years old (Turner, 1970). The age of the unconformity, therefore, is considered to be as young as 17 to 22 million years old.
Top of Miocene Unconformity The intermediate-aged unconformity used in this investigation lies within the Sisquoc Formation and marks the boundary between late Miocene and early Pliocene strata. This unconformity can be traced on seismic data over much of the offshore Santa Maria Basin to Point Arguello, where it has been identified and illustrated on seismic lines by Crain and others (1987) and Fischer (1987)
(Clark and others, in press). Based on recent paleontological investigations in the onshore Santa Maria Basin (Barron, 1986; Dumont, 1984, 1989; Dumont and others, 1986; Dumont and Madrid, 1987), the Miocene/Pliocene boundary is estimated to be 5.3 million years old. This is in close agreement with the chronology of the AAPG Correlation of Stratigraphic Units of North America for the Santa Maria Basin (Bishop and Davis, 1984), with Berggren and others (1985), and with Barron (1986).
Mid-Pliocene Unconformity The upper unconformity is within Pliocene strata and is represented by a seismic reflector that is mappable throughout most of the offshore Santa Maria Basin west of the Hosgri fault zone. To the
'ast of the Hosgri fault zone, where well data for age control are not available, seismic character and stratigraphic position are the primary criteria used to identify the mid-Pliocene unconformity and to correlate it with the mid-Pliocene unconformity west of the Hosgri fault zone.
Onlap relationships locally observed in the sedimentary section above the unconformity (for example, seismic line GSI-86, Attachment GSG Ql-A) indicate that the unconformity may be time-transgressive. Therefore, it is likely that the mid-Pliocene unconformity east of the Hosgri fault zone represents a longer period of time of nondeposition or erosion than the unconformity in the deeper part of the basin west of the Hosgri fault.
Because of the time-transgressive nature of the unconformity, we examined its lateral continuity and correlation with known dated locations to provide a conservative minimum age estimate.
Correlation of the unconformity to offshore well data from the Point Sal region (Figure GSG Q3-
- 2) indicates that it coincides with the top of the Sisquoc Formation. The rock unit that stratigraphically lies above the unconformity in this region is informally called the "Foxen" unit.
Sediments assigned to this provincial unit are not strictly correlative in time or lithology with the formally defined Foxen Formation of the onshore Santa Maria Basin.
Based on unpublished paleontological data, the mid-Pliocene unconformity represents the depositional hiatus between the early Pliocene (Repettian and lower Venturian) and the late Pliocene (lower Wheelerian) in the offshore basin. Recent paleontological work in the offshore Santa Maria Basin provides an estimate of 4.0 million years for the "Foxen"/ Sisquoc boundary (R. Boettcher, pers.
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u i n M rch I Seismic Line GSI 80-100 S.P. 450 S.P.500 SW Typhat Onttor Seaagrapht c Column illllIilRIEIIRIII Santa Maria Ar Wel PM06 et T.D. 0
.8 s If lrttll I IIV P. Pi 0
5755'amma O 0'o Aay AoahtNity rc era
- SH333 aSISPa 4':4'
'Foxen and youngei P ne Ptt ~
14
'Upper X O
Shquoc tt V u
>>oaee Iln tutotomr Upper Shquoc
)t Lower Shquoc IXI '
Lower'hquoc 4
44
')I I
,l 99 4
lf4 ,' 44
. 6000
~ It t
~
~
'r
~
,pyle't 4, rtt,. ~
I ~
i I 1 I
~
~PP Pratt IL'Pi ttAI
'.h' ab 8000 fr0
~ 1 I
9000 (From Cia rk an do there, inp ress )
Figure GSG Q3-2 Onshore-offshore correlation diagram.
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i n S March I 0 comm. to R. Heck). Based on diatom zonation studies in the onshore Santa Maria Basin (Dumont, 1989), the upper limit of the early Pliocene is 3.4 million years old. The youngest age estimate for the early/late Pliocene boundary on the stratigraphic correlation charts of Bishop and Davis (1984) and Barron and others (1985) is 2.S to 3.0 million years old. Therefore, based on data from both the offshore and onshore Santa Maria Basins, the age of this unconformity is estimated to be between 4.0 and 2.8 million years old.
Stratigraphic and geomorphic relationships within the San Luis/Pismo structural block also can be used to constrain a minimum age for the mid-Pliocene unconformity east of the Hosgri fault zone.
The Squire Member of the Pismo Formation, which contains a pecten fauna restricted to the late Pliocene (Hall, 1973), is mapped in several areas of the San Luis/Pismo block. We consider the angular unconformity at the base of the Squire Member to be correlative with the mid-Pliocene unconformity in the offshore basin. The Squire Member is uplifted in the San Luis/Pismo block and beveled by a flight of marine terraces, the oldest of which are estimated to be over 1 million years old. The unconformity at the base of the Squire Member must be older than the late Pliocene age of the overlying pecten-bearing deposits of the Squire Member. The Bishop and Davis (1984) correlation chart for the onshore Santa Maria Basin indicates an age range of 1.7 to 2.8 million years for the contact between upper Pliocene and lower Pleistocene. Given this estimate and the onshore geologic relationships described above, we conservatively estimate a minimum age of 2.S million years for the mid-Pliocene unconformity east of the Hosgri fault zone.
RESULTS Table GSG Q3-1 provides the measured values of vertical separation of each of the three unconformities at several locations along the Hosgri fault zone. The greater number of values given for the central three reaches of the fault zone (the San Luis/Pismo, San Luis Obispo Bay, and Point Sal reaches) is due largely to the more complete Tertiary record that is preserved to the east of the fault zone along these reaches, and thus reflects our ability to make direct measurements of vertical separation. Based on the measured vertical separation and estimated ages of the top of Miocene and mid-Pliocene unconformities, we also provide time-averaged estimates of vertical slip rates (Table GSG Q3-I). The spatial distribution of these rates of vertical separation is shown on Figures GSG Q3-3 and GSG Q3-4 for the post-top of Miocene and post-mid-Pliocene periods, respectively. Late Pleistocene coastal uplift rates shown in the table are derived from the distribution of elevated marine terraces in the San Luis/Pismo structural block (see Response to Question Q43i, Attachment Q43i-2, January, 1989; Hanson and others, in review) and the Casmalia block (Clark and Slemmons, 1990).
The accuracy of the estimated vertical rates presented in Table GSG Q3-1 is a function of several variables including (1) the correct identification and dating of the unconformities, (2) the velocity model used to make the depth corrections, and (3) measurement errors. The identification of the unconformities is based on existing well data in the offshore Santa Maria Basin. The unconformities are correlated to other areas using conventional loop-tie geophysical techniques. Uncertainties in ages of the unconformities, particularly the potentially time-transgressive mid-Pliocene unconformity, must be considered in calculating rates of vertical deformation. To capture these uncertainties, we have calculated post-mid-Pliocene unconformity slip rates using a conservative minimum age of 2.S million years for the unconformity. The vertical rates based on this age are thus maximum rates because (I) this is a reasonable minimum age estimate for this unconformity given the geologic relationships described above in the onshore and offshore Santa Maria Basin, and (2) assuming that this is a time-transgressive unconformity, the time equivalent of the unconformity east of the Hosgri fault zone would lie within the sedimentary package above the unconformity west of the fault, and therefore, vertical separations based on the elevation of the unconformity are maximum values.
Potential errors resulting from inaccuracies in the velocity model used to convert time sections to depth sections are likely to be most significant in the deeper parts of the profiles adjacent to and within the Hosgri fault zone, and in the more deformed rocks east of the fault zone where the Oiabto Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
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Table GSG Q3-I n
VERTICALSLIP RATES BASED ON VERTICALSEPARATION OF UNCONFORMITIES ACROSS THE HOSGRI FAULT ZONE m
A TOTAL VERTICAL SEPARATION (m) VERTICALSLIP RATE (mm/yr)
A n
C<
2 SEISMIC LINE Top-B
(<<17 Ma) Ma)
(%)'aximum'ost
(<<17 to 5.3 Top-M (5.3 Ma)
Mid-P
(<<2.8 Ma)
Coastal Uplift Rates (mm/yr)
Post Top-M (5.3 Ma)
(mm/yr)
Top-M (mm/yr)
Post Mid-P (2.8 Ma)
(mm/yr)
Maximum'ost (mm/yr)
Mid-P orthe Reac CM-51 428 119 - 309 <<0 0.08 0.08 0.04 - 0.11 0.11 W-76A <<1922 (<<1335) 642 404 <<0 0.12 0.12 0.14 0.14
(<<69%)
J-113 <<2113 s1541 s285 <<0 0.12 0.12 0.10 0.10 (s72%)
San Luis/P'smo eac PGE-I 857 261 s0.23 0.16 s0.31 0.09 0.32 W-14 710 330 s0.23 0.14 s0.37 0.12 0.35 GSI-85 476 - 619 285 - 404 0.20 - 0.22 0.09 - 0.12 0.34 0.10 - 0.14 0.36 GSI-86 690 380 0.20 0.13 0.33 0.14 0.34 GSI-87 880 571 0.20 0.19 0.39 0.20 0.40 (0.26~)
~preferred value based on O O elevation of exposed basal Squire Member onshore 0
o B a
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Table GSG Q3-1 (contintted)
RS SSS TOTAL VERTICALSEPARATION (m) VERTICALSLIP RATE (mm/yr)
CO SSS SEISMIC LINE
(<<17
!R m
Top-B
(<<17 Ma) Ma)
(%)'op-M to 5.3 (5.3 Ma)
Mid-P
(<<2.8 Coastal Uplift Rate~
Post Top-M (5.3 Ma)
Maximums Post Top-M Post Mid-P (2.8 Ma)
Maximum Post Mid-CSS O Ma) (mm/yr) (mm/yr) (mm/yr) (mm/yr) (mm/yr)
A nCI San Luis Obis Reac 8 CM-119 881 595 <<0 0.17 0.17 0.21 0.21 SaS I-126 1095 571 <<0 0.20 0.20 0.20 0.20 GSI 598 <<0 0.20 0.20 0.21 0.21 GSI-100 Hosgri 1119 522 <<0 0.21 0.21 0.19 0.19
[Hosgri + Purisima] I [1238] [712] <<0 [0.23] [0.23] [0.25] [0.25]
~GE-3 Hosgri 1976 953 (48%) 1023 500 <<Q 0.19 0.19 0.18 0.18
[Hosgri + Purisima] [1834] [1238] [691] <<Q [0.23] [0.23] [0.25] [0 25]
~oinl Sal Raaa
~GSl- Ol Hosgri 1715 <<811 (47%) 904 357'.14 - 0.17 0.17 0.34 0 0 13'0 13'0.19']
[Hosgri + Purisima] [1477] [I] [1167] [547] [0.14 - 0.17] [0.22] [0.39] 19']
~204 Hosgri 857 571 0.14 - 0.17 0.16 0.33 0.20 0.37 g CI O G~Sl- 06 O O 3 O Hosgri 428-2095 -215s-1452 643 476 0.14 - 0.17 0.12 0.29 0.17 0.34 CO O
Y ((0 - 69%)
I0 ~ O
[Hosgri + Purisima] [2000] [I]
[1143] [571] [0.14 - 0.17] [0.22] [0.39] [0.20] [0.37]
'IJ CO O
Saa Sat 3
Table GSG Q3-1 (continued)
CD TOTAL VERTICALSEPARATION (m) VERTICALSLIP RATE (mm/yr)
Cl SEISMIC LINE DS DS Top-B (- 17 to 5.3 Top-M Mid-P Coastal Uplitt Post Top-M Post Mid-P
(%)'5.3 Ma) Maximum'ost Maximum'ost 833 CS
(-17 Ma) Ma) (-2.8 Rate (5.3 Ma) Top-M (2.8 Ma) Mid-P 888 Ma) (mmlyr) (mmlyr) (mmlyr) (mmlyr) (mmlyr) a 883 CD Southern Reach CD GSI-112C 8
833 Hosgri I I -0.14
-732'Hosgri
+ Purisima] [756] [1122] [536] [-0.14] [0.21] [0.35] [0.19] [0.33]
~GS1- 3 Hosgri 524s I I I -0.14
[Hosgri + Purisimal [1501] [I] [1024] [524] [-0.14] [0.19] [0.33] [0.19] [0.33]
G~S- 18 Hosgri I I 0.14
-357'Hosgri
+ Purisima] [976] [1000] [570] [-0.14] [0.19] [0.33] [0.20] [0.34]
~GS -123 Hosgri 1142 [I] -0.14 0.15 0.29
[Hosgri + Purisima] [1261] I [905] [476] [-0.14] [0.17] [0.31] [0.17] [0.31]
I - Indeterminate C7
'Percentage of vertical deformation of the top of basement unconformity that occurred prior to development of the top of Miocene unconformity.
CCS CD CD zLate Pleistocene uplift rate based on the elevation of the 120,000-year-old marine terrace.
D3 Q
CCS CD 8
Maximum value calculated by adding a maximum coastal uplift rate east of the fault zone to the vertical slip rate based on the evaluation of I
~
CD the unconformity west of the fault zone.
CD CD 'The presence of the unconformity on both sides of the fault zone allows for a direct measurement of the total vertical separation.
DS DS sDown on the east
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ueti n March 1 0 Pa e 10 X Vx EXPLANATION
~ Dhblo Canyon Power Plant Boundary between reaches of the Hosgri Fault
~0~ t ~ Point Estero zone PS Purlsima structure 0.20 Maximum post-top h%ocene (Top-M) vertical slip rate (mmtyr) across Hosgri fault zone Bay
[0.35) Maximum post-top Miocene (Top-M) verdcal sf o slip rate (mmiyr) across Hosgri fault zone and Purisima structure Verdcal uplift Indeterminate o
qgac qG~ cQ.
~ Axial trend of antidine or syndine Point Buchon A
~0 SA e Point San Luis 6
q'i5
~,5l ga>+ +eeac>
ps'~
est+
~
A%9 t g9~
o<
G gQ g91 io
~yX ~ o.
G Gg>
o
~ 'o>'~
g + Point Sai cd. ~tIS go@~
~ <Sa~
qeac GS~ Purisima Point q,eac 10 mi 6pe~
15 km Figure GSG Q3-3 Vertical slip rate based on the top of Miocene unconformity.
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u tion S M rch l Pa e ll EXPLANATION XVi =
k Diablo Canyon Power Plant Boundary between reaches of the Hosgrl Fault Zone
~ 0 < Point Estero PS Purislma structure 0.20 Maximum post-middle Pliocene vertical slip rate (mmfyr) across Hosgrl fault zone qA
@ a%
+ EStero 0.26 Preferred value based on direct measurement of vertical separation using onshore exposure of basal Squire member qG
[0.35] Maximum post-middle Pliocene vertical slip rate (mmfyr) across Hosgrl fault zone and Purisima structure 0
~eac QP l Vertical uplift indeterminate
"" Fault; dashed where inferred tyP N Point Buohon ~ Axial trend of antfdine or syndine G+
s,.s
~ ~'oint gA e'~
Qt~
San Luis
~,al
~a<<+ eao~ gSt+1 G %%9 s+ G G
~g6 g1
~ k'.
Sa".
q,eac>
sst<<ooss ~ o. a Q point sat e+
c'tie~
.,Sa'05
~eaG pe Purisima Point 1
9ea 10 mi sot n 15 km Figure GSG Q3-4 Vertical slip rate based on the mid-Pliocene unconformity.
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I. uesti n March 1 0 Pa e 12 seismic images are less distinct and the boundaries between rock units of differing velocities are less certain. Calculations of the depth to and vertical separation of the top of basement unconformity are most sensitive to the velocity model, and are estimated to be accurate to within 10 to 20 percent.
The velocities used to calculate the depths to the top of Miocene and mid-Pliocene unconformities, however, are better constrained. The critical points used to measure vertical separation across the entire fault zone are generally outside the most structurally complex areas. Comparison of depths calculated for selected seismic profiles using two techniques (a layer-cake model and a continuously varying velocity model) indicate that variation in depths due to the velocity model used is generally less than 10 percent.
The precision (+ 25 meters) with which depths can be measured from the depth sections given the scale of the sections most commonly used (1:48,000) and the thickness of the lines used to denote the unconformities, also leads to a possible error in calculating vertical separation. The possible error is between 10 and 15 percent for the mid-Pliocene and top of Miocene unconformities.
The combined effects of the errors due to the velocity model and measurement precision in calculating vertical slip rates based on estimated vertical separation is approximately 15 percent.
For the highest rates shown on Table GSG Q3-1, which are about 0.35 millimeter per year, this yields an error of about a 0.05 millimeter per year. However, as discussed previously, other factors, including our probable overestimation of vertical separation by not subtracting separation due to compaction and sediment draping, the use of a minimum age for the unconformity, and the addition of coastal uplift rates on the east side of the fault zone, indicate that the post-mid-Pliocene rates shown on Table GSG Q3-1 are maximum values.
The estimated vertical slip rates presented in Table GSG Q3-1 do not take into account the likelihood that lateral slip has occurred along the Hosgri fault zone during these time intervals. The vertical separation of units used to estimate vertical slip rates may be due, in part, to the juxtaposition of irregular or sloping unconformities by lateral slip. However, a comparison of seismic sections on opposite sides of the fault zone that would have been juxtaposed given a right-slip rate of 1 to 3 millimeters per year and the ages assigned to each of the unconformities indicates that lateral displacement would generally not affect the range of post-top of Miocene and mid-Pliocene vertical
'lip rates given in Table GSG Q3-l. East of the Hosgri fault zone, these unconformities probably attain maximum elevations within the elevated San Luis/Pismo and Casmalia blocks. West of the fault, these unconformities tilt to the south, subparallel to the Hosgri fault zone adjacent to the Northern and San Luis/Pismo and northern San Luis Obispo Bay reaches of the fault. Therefore, right-lateral slip along the fault would not increase the apparent vertical separation of these unconformities across the northern half of the Hosgri fault zone. Consequently, the values of separation that we have measured are maximum. Correction for lateral slip along the southern San Luis Obispo Bay and Point Sal reaches, however, may result in a higher post-mid-Pliocene vertical slip rate than is indicated on Table GSG Q3-1. The mid-Pliocene unconformity west of the Hosgri fault, as imaged on GSI-100, is at an elevation of about -760 meters. Assuming that this unconformity has been displaced laterally from 3 to 9 kilometers, a maximum vertical slip rate of 0.44 millimeter per year is calculated by adding a maximum late Pleistocene uplift rate for the Casmalia block (0.17 millimeter per year) to the vertical slip rate based on the elevation of the unconformity west of the fault zone in the vicinity of GSI-100 (760 meters+ 2.8 million years 0.27 millimeter per year).
The maximum vertical separation of basement across the fault may exceed the values given in Table GSG Q3-1. Based on the elevation of the top of basement unconformity west of the Hosgri fault zone, which ranges from about -1,800 to -2,700 meters, and the elevation of elevated basement (200 meters or greater) in the San Luis/Pismo and Casmalia blocks east of the fault zone, the maximum possible vertical separation of basement juxtaposed laterally across the fault zone may be on the order of 2,800 to 3,000 meters. As described below, because most of this vertical separation occurred during the Miocene and early Pliocene period of transtensional deformation along the Hosgri fault zone, we have not used the top of basement unconformity to evaluate vertical slip rates in the present tectonic setting.
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I
ue in M rch 1 0 P 1 DISCUSSION A northeast-side-up sense of vertical basement separation is apparent across all but the southern reach of the Hosgri fault zone. Along the southern reach of the Hosgri fault zone, basement in the region north of Purisima Point is elevated on the southwest side of the fault. In this region, active folding along the Purisima structure, which is 1 to 6 kilometers west of the Hosgri fault, has produced northeast-side-up vertical separation of basement. Recently proposed models of strain partitioning (see Response to Question GSG 2) suggest that oblique slip at depths may decompose up-section into separate dip-slip and strike-slip faults at the surface. For local strain partitioning, as defined in Response to Question GSG 2, characterization of the fault zone at depth should combine the vertical and horizontal slip along both surface structures. Because of the uncertainties of the relationship of the Hosgri fault zone to the Purisima structure at depth, we have presented in Table GSG Q3-1 alternative vertical slip rates that incorporate vertical slip across both structures.
Along the central reaches of the fault zone (San Luis/Pismo, San Luis Obispo, and Point Sal) much of the northeast-side-up separation of basement appears to be the result of deformation along the fault zone that occurred prior to formation of the top of Miocene unconformity. In areas where the unconformity can be traced across the fault, from 47 to 72 percent of the apparent vertical separation of the top of basement unconformity observed in the geophysical records occurred in the period between the formation of this unconformity (about 17 million years ago) and the top of Miocene unconformity (5.3 million years ago) (Table GSG Q3-1). During this 12-million-year interval, the Hosgri fault zone may have formed the eastern margin of a series of rapidly subsiding basins within a transtensional system (McCulloch, 1987; McCulloch and Lewis, 1988).
The Miocene episode of transtensional displacement along the Hosgri fault zone changed to transpression in the late Miocene to early Pliocene (McCulloch, 1987; PG&E, 1988; McCulloch and Lewis, 1988). The onset of regional northeast-southwest shortening across much of the North American-Pacific plate boundary is probably related to a change in plate motion from oblique divergence to oblique convergence (McCulloch and Lewis, 1988). Estimates of the age of this change in plate motion include about 5 million years ago (Cox and Engebretson, 1985), 3.2 to 5 million years ago (Pollitz, 1986), and 3.4 to 3.9 million years ago (Harbert and Cox, 1986; Harbert and Cox, 1989) ~
Inception and development of significant folding along the Hosgri fault zone prior to formation of the top of Miocene unconformity (5.3 million years old) is evidenced by more strongly deformed beds overlain by less deformed beds within the Miocene section (for example, see Attachment GSG
'l-A, San Luis/Pismo reach montage, line GSI-87A, shot point 400) and, locally, by the angular unconformity between Miocene sediments and lower Pliocene sediments (for example, see Attachment GSG Ql-A, San Luis Obispo Bay reach montage, lines J-126 and GSI-97, and San Luis/Pismo reach montage, line GSI-85).
Locally, there is evidence that significant deformation occurred during a relatively brief interval following the development of the top of Miocene unconformity and before the deposition of lower Pliocene sediments. For example, seismic line GSI-87 (Attachment GSG Ql-A) shows lower Pliocene sediments that onlap and appear to be buttressed against a preexisting fold. Bedding in the upper Miocene sediments subparallels the top of Miocene unconformity, indicating that the deformation that caused the fold occurred during or after formation of the unconformity. A similar relatively brief episode of shortening between 5 and 3 million years is recognized in the development of the Queenie structure, which lies in the offshore region between the Hosgri and Santa Lucia Bank fault zones (Clark and others, in press).
The above observations indicate there have been significant spatial and temporal variations in the location and rates of compressional deformation along the Hosgri fault zone since the late Miocene.
Therefore, long-term rates of vertical deformation based on the vertical separation of the top of Miocene unconformity only reflect an average of rates, which may have varied significantly over this period of time.
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I ue tion March 1 0 Pa e 14 Post-mid-Pliocene upward-diverging reverse faults that branch off the principal Hosgri fault traces (positive flower structures) and compressional structures (folds and underlying thrust and reverse faults) that subparallel the Hosgri fault zone in the South Basin compressional domain indicate there is probably a compressional component of slip along the Hosgri fault zone in this area. Vertical slip rates based on the apparent folding and displacement of the mid-Pliocene unconformity range from 0.1 millimeter per year to 0.4 millimeter per year (Table GSG Q3-1) and may be as high as 0.44 millimeter per year if a component of lateral slip is factored into the analysis.
CONCLUSIONS Analysis of the timing and rates of vertical separation of three distinct unconformities (top of basement, about 18 million years old, top of the Miocene section, 5.3 million years old, and between lower and upper Pliocene sediments, 2.8 million years old) across the Hosgri fault zone allow us to differentiate and quantify rates for different periods of deformation in the history of the Hosgri fault zone. Based on these analyses, we conclude that:
From about 47 to 72 percent of the northeast-side-up vertical separation of the top of basement. unconformity across the fault zone is the result of deformation that occurred between the formation of the unconformity (about 17 million years old) and the top of Miocene unconformity (5.3 million years old). Although much of this deformation can be attributed to basin-margin normal faulting during a period of Miocene transtensional deformation, some of the vertical separation is the result of a component of compression or transpression in the late Miocene.
- 2. Locally along the Hosgri fault zone, there is evidence that a significant amount of southwest-vergent folding occurred in a brief interval of time between formation of the top of Miocene unconformity (5.3 million years old) and deposition of the overlying early Pliocene sediments. A similar pulse of deformation is observed along the Queenie structure to the west (Clark and others, in press).
Based on the deformation of the mid-Pliocene unconformity, a compressional component of slip is occurring along the Hosgri fault zone in the present tectonic setting. This is consistent with relative plate motions that suggest the Hosgri fault zone is presently in a transpressional environment. Post-mid-Pliocene vertical slip rates across the Hosgri fault zone range from 0.1 to 0.4 millimeter per year (Table GSG Q3-1), but may be as if high as 0.44 millimeter per year the rate of right-lateral slip along the fault is greater than 1 millimeter per year. Comparison of these rates of vertical slip to lateral-slip-rate estimates (see Response to Question GSG 4) indicates that the Hosgri fault zone in the present tectonic setting is behaving as a strike-slip fault (see Response to Question SSC 2): Under certain extreme scenarios, the Hosgri fault zone may be an oblique-slip fault along the Point Sal and Southern reaches (that is, using minimum estimates of lateral slip and maximum estimates of vertical slip).
REFERENCES Barron, J. A., 1986, Updated diatom biostratigraphy for the Monterey Formation of California; in Casey, R. E., and Barron, J. A., eds., Siliceous Microfossils and Microplankton Studies of the Monterey Formation and Modern Analogs: Pacific Section, Society of Economic Paleontologists and Mineralogists, Book 45.
Barron, J. A., Keller, G., and Dunn, D. A., 1985, A multiple microfossil biochronology for the Miocene; jn Kennet, J. P., ed., The Miocene Ocean; Paleoceanography and Biogeography: Geological Society of America Memoir 163, p. 21-36.
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e in 3 Marchl 0 Pa e 1 Berggren, W. A., Kent, D. V., and van Couvering, J. A., 1985, The Neogene, Part 2, Neogene geochronology and chronostratigraphy; in Snelling, N. T., ed., The Chronology of the Geologic Record: Geologic Society of London Memoir 10, p. 211-260.
Bishop, C. L., and Davis, J. F., (Coords.), 1984, Correlation of stratigraphic units in North America, Southern California Province, Santa Maria Basin: American Association of Petroleum Geologists Memoir.
Clark, D. G., and Slemmons, D. B., 1990, Late Pleistocene deformation in the Casmalia Hills region, coastal central California (abs.): submitted to Geological Society of America.
Clark, D. H., Hall, N. T., Hamilton, D. H., and Heck, R. G., in press, Structural analysis of late Neogene deformation in the central offshore Santa Maria basin, California: Journal of Geophysical Research.
Cox, A., and Engebretson, D., 1985, Change in motion of Pacific Plate at 5 Myr BP.: Nature,
- v. 313, p. 472-272.
Crain, W. E., Mero, W. E., and Patterson, D., 1987, Geology of the Point Arguello field; in Ingersoll, R.V., and Ernst, W.E., eds., Cenozoic Basin Development of Coastal California: Rubey Vol. VI, Prentice Hall, Englewood Cliffs, New Jersey, p. 405-426.
Dumont, M. P., 1984, Miocene/Pliocene epoch boundary from diatom biostratigraphy in the Lompoc, California region: Geological Society of America Abstracts with Program, v. 16, no. 6, p. 496.
Dumont, M. P., 1989, The Monterey Formation and biostratigraphy: an overview; in Mackinnon, T. C., ed., Oil in the California Monterey Formation: American Geophysical Union Field Trip Guidebook T311, 28th International Geological Congress, p. 2S-32.
Dumont, M. P., Baldauf, J. G., and Barron, J. A., 1986, Thalassiosira praeoestrupii - a new diatom species for recognizing the Miocene/Pliocene epoch boundary in coastal California:
Micropaleontology, v. 32, no. 4, p. 372-377.
Dumont, M. P., and Madrid, V. M., 19S7, Magnetobiostratigraphy of the late Neogene Purisima Formation and the Miocene/Pliocene boundary in coastal California; in Barron, J. A., and Blueford, J. R., eds., Pacific Neogene Event Stratigraphy and Paleoceanographic History: Fourth International Conference on Pacific Neogene Stratigraphy, p. 29-31 ~
Fischer, T., 1987, Discovery of the Point Arguello oil field from a geophysical perspective: The Leading Edge, v. 6, no; 10, p. 16-21.
Hall, C. A., Jr., 1973, Geology of the Arroyo Grande quadrangle, California: California Division of Mines and Geology, Map Sheet 24, scale 1:48,000.
Hall, C. A., Jr., 1981, San Luis Obispo transform fault and middle Miocene rotation of the western Transverse Ranges, California: Journal of Geophysical Research, v. 86, no. B2, p. 1015-1013.
Hanson, K. L., Wesling, J. R., Lettis, W. R. Kelson, K. I., and Mezger, L., in review, Correlation and ages of Quaternary marine terraces, south-central California; in Alterman, I. B., ed.,
Seismotectonics of Central and Coastal California: Geological Society of America Special Paper.
Harbert, W., and Cox, A., 1986, Late Neogene motion of the Pacific Plate (abs.): EOS Transactions, America Geophysical Union, v. 67, no. 44, p. 1225.
Harbert, W., and Cox, A., 1989, Late Neogene motion of the Pacific Plate: Journal of Geophysical Research, v. 94, no. B3, p. 3052-3064.
Diablo Canyon Power Plant Paclflc Gas and Efectrfc Company Long Term Seismic Program
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u i n Mrhl P i McCulloch, D. S., 1987, Regional geology and hydrocarbon potential of offshore California; jn Scholl, D. W., Grantz, A., and Vedder, J., eds., Geology and Resource Potential of the Continental Margin of Western North America and Adjacent Ocean Basins, Beaufort Sea to Baja California:
American Association of Petroleum Geologists Circum Pacific Earth Science, v. 6, p. 353-401.
McCulloch, D. S., and Lewis, S. D., 1988, Offshore resource geology of central Californix'. S.
Geological Survey Research on Energy Resource, Circular 1025, 34 p.
Pacific Gas and Electric Company, 1988, Final report of the Diablo Canyon Long Term Seismic Program: U. S. Nuclear Regulatory Commission Docket Nos. 50-275 and 50-323.
Pollitz, F. F., 1986, Pliocene change in Pacific Plate motion (abs.): EOS Transactions, American Geophysical Union, v. 66, no. 44, 1062.
Turner, D. L., 1970, Potassium-argon dating of Pacific coast Miocene foraminiferal stages, Geological Society of America Special Paper 124, p.91-129.
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u ti n 4 March I Pa e I QUESTION GSG 4 Provide the analysis of the uplift rates across the Hosgri versus the lateral rate of displacement, and the variations along the length of the Hosgri as discussed at the meeting. Include the basis for their measurement, the uncertainties, and a discussion of the geometry of the fault and its effects upon the evaluation of the vertical and horizontal displacements. Also, summarize the evidence for strike-'slip and dip-slip along the 16 kilometer reach of the Hosgri fault that extends from the westernmost scarp at 59-meter ridge northward across the north Estero Bay slope break and explain how the geometry of the 30-meter high, scarp-like, part of the north Estero Bay slope break can be derived by right-lateral strike-slip.
The ratio of horizontal to vertical components of slip is a primary criterion used to differentiate strike-slip, oblique-slip and dip-slip faults. In Response to Question SSC 2, we provide published definitions of these three fault classes and the ratios of horizontal to vertical deformation expected for each. The ratios cited are calculated from the rake angle of the fault or the direction of displacement in the plane of the fault. For a strike-slip fault dipping 60 degrees, the ratio of horizontal to vertical slip is greater than 2:1. This ratio decreases with increasing fault dip (for example, a vertical strike-slip fault will have a horizontal to vertical slip ratio of greater than 1.7:1),
and increases with decreasing fault dip.
Because of the importance of these ratios in assessing style of faulting along the Hosgri fault zone, we quantified the horizontal and vertical components of slip along the fault using a variety of geophysical and geological data. In the Response to Question GSG 3, we present a detailed discussion of the timing and rates of vertical slip that have occurred during different periods in the history of the Hosgri fault zone. These rates of vertical slip incorporate all the data and lines of reasoning cited in Question GSG 3 in support of vertical slip along the Hosgri fault zone. In Response to Question GSG 12, we present a detailed discussion of lateral slip rates transferred between the southern San Simeon fault zone and the northern Hosgri fault zone via the San Simeon/Hosgri pull-apart basin.
In this response we (1) summarize the results of these analyses, in particular, the maximum rates of vertical separation across the Hosgri fault zone since the middle Pliocene, which can be used to evaluate the present rate of vertical separation across the Hosgri fault zone; (2) discuss estimates of the present rate of lateral displacement that is occurring along the Hosgri fault zone and relate these estimates to regional tectonism; (3) calculate ratios of horizontal to vertical slip rates; and (4) discuss possible spatial and temporal variations of these ratios along the length of the fault zone.
The question also requests information on the geometry of the fault zone and its effects upon the evaluation of vertical and horizontal displacements, and for information on fault activity along the north Estero Bay slope break and 59-meter ridge. The Response to Question SSC 2 provides a discussion of fault geometry in defining style of faulting, the methods that we use to evaluate true displacement (orientation and relative magnitude of slip) from apparent horizontal and vertical slip components, and the use of ratios of horizontal to vertical slip for assessing the present behavior of the Hosgri fault zone.
A discussion of the origin and tectonic significance of 59-meter ridge and the north Estero Bay slope break is provided in Attachment GSG Q4-A to this response. Based on a detailed examination of bathymetric, geophysical, and paleo-sea-level data, which we provide in Attachment GSG Q4-A, we conclude that the north Estero Bay slope break formed by a variety of processes, potentially at different elevations, and thus cannot be used as a strain gauge to assess deformation along the Hosgri fault zone. The Hosgri fault zone does not, at any location, cross the north Estero Bay slope break, as stated in the question, and only for a distance of 3 kilometers do the fault and slope break coincide. The 16-kilometer reach of the fault extending north of 59-meter ridge forms the western margin of the San Simeon/Hosgri pull-apart basin.
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I u ti n 4 Mrchl Pa e2 COMPONENTS OF SLIP ALONG THE HOSGRI FAULT ZONE The Hosgri fault zone lies entirely offshore, where direct observations of fault geometry and conventional paleoseismological investigations of fault behavior cannot be used to assess style and rates of deformation. However, constraints on the rates of both vertical and horizontal slip along this fault zone are provided by analysis of offshore geophysical data, in combination with both onshore and offshore geological observations, and by assessment of the regional kinematic and tectonic setting of the fault zone.
Vertical Slip Rates Vertical rates of late Pliocene and Quaternary slip along the Hosgri fault zone are based on an assessment of the vertical separation of the early/late Pliocene (mid-Pliocene) unconformity (2.8 million years old) acros's the fault zone along its entire length. A detailed discussion of the method and uncertainties involved in this assessment is presented in the Response to Question GSG 3.
Conservative estimates of the rate of vertical separation of this unconformity across both low-angle and high-angle components of the fault zone along its entire length range from 0.1 to 0.4 millimeter per year (Figure GSG Q4-1). Along the Point Sal and Southern reaches of the Hosgri fault zone, we conservatively consider the Purisima structure to be the result of local strain partitioning along the Hosgri fault zone and include the vertical structural relief of the structure in estimating vertical separation across the Hosgri fault zone. A slightly greater (0.44 millimeter per year) maximum rate of vertical separation of the mid-Pliocene unconformity is possible if stratigraphic horizons at different structural levels are juxtaposed by right-lateral slip along the fault zone.
Horizontal Slip Rates There are no recognized marker horizons or features that can be correlated across the Hosgri fault zone as piercing points to measure directly rates of horizontal slip during the late Pliocene and Quaternary. This is not surprising, because it has been long recognized that quantification of lateral slip along faults is extremely hard to find. Crowell (1959, p. 2654), for example, cautions that "since many faults have significant components of strike-slip, no longer can we assume logically that a fault has only dip-slip until proved otherwise". As cited by Stone (1986), "Haites (1960, p. 37) has gone even further by stating that 'field evidence suggesting the presence of a normal or reverse fault is not conclusive unless absence of strike-slip movements is proved'". Thus, while horizontal slip is a necessary feature of strike-slip faulting, it often cannot be definitively recognized and measured.
Regional tectonic and structural relationships indicate that horizontal slip is transferred between the San Simeon fault zone to the north and the northern Hosgri fault zone via the San Simeon/ Hosgri pull-apart basin (see Response to Question GSG 12). Several studies quantify the rate of horizontal slip across the major strands of the San Simeon fault zone. Based on an analysis of deflected stream drainages and detailed marine terrace mapping, supplemented by drilling and soil-profile development studies in the San Simeon Point region directly north of San Simeon Bay, PG&E (1988),-
Hanson and others (1987), and Hanson and Lettis (in review) estimate right-lateral slip rates of 0.5 to 6 millimeters per year, with a preferred estimate of 1 to 3 millimeters per year during the past 214,000 years. Similarly, PG&E (1988) and Hall and Hunt (in review) estimate a minimum Holocene right-lateral slip rate of 1 to 2 millimeters per year, on the basis of offset fluvial deposits exposed in several trench excavations.
These rates are similar to the estimated rate of horizontal slip on the Hosgri and San Simeon fault zones that is required to develop the San Simeon/Hosgri pull-apart basin in the right en echelon stepover region between these faults (see Response to GSG Question 12). Based on empirical and theoretical studies of pull-apart basins worldwide that relate dimensions and age of basins to rates of slip along the master bounding faults, we estimate that 1 to 4 millimeters per year of right-lateral slip is transferred between the San Simeon fault zone and the Hosgri fault zone via the stepover.
Based on direct observations in the San Simeon region, the best estimate of the horizontal slip rate along the Hosgri fault zone is I to 3 millimeters per year. This range is consistent with the modeled estimate based on the dimensions of the pull-apart basin in the stepover region.
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uesti n 4 March l 0 Pa e EXPLANATION XVi 0
=
Otabio Canyon Power Plant Boundary between reaches of the Hosgri Fault Zone PS Purislma structure 0.2O.Maximum postmlddle Pliocene vertical slip rate (mnuyr) across Hosgrl fault zone 0.qa
+ Estero 026 Preferred value based on direct measwement of verdcal separation using onshore exposure of basal Squire member
[0.35] Maximum post~lddie Pliocene vertical slip rate (mmlyr) across Mosgri fault zone and Pudslma structure 0H 0
q,pa< I Vortical upliftIndeterminate 0P N Point Fault; dashed where btforred
~
~ 'oint
~qA
+o W Axhl trond of antidino or syndine ga GS San Luis 0.+'E0 Sao< ac> pe>~
qiS 0 O i ops
~ ~'.
Sa~.
O+~59
~",a=
,.<00 qG o.
<06 GSi SgK go~~
~ ~eac,
~gG cs~- Punsima Point gS 10 mi 15 km Figure GSG Q4-I Rates of post-middle-Pliocene vertical separation along the Hosgri fault zone.
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uesti n S 4 M rch I 0 Pa 4 RATIOS OF HORIZONTAL TO VERTICAL SLIP ALONG THE HOSGRI FAULT ZONE The ratio of horizontal to vertical slip rate is an important criterion for evaluating style of faulting along the Hosgri fault zone. The range in the ratio along the Hosgri fault zone given a lateral slip rate of 1 to 3 millimeters per year and a vertical slip rate of 0.1 to 0.44 millimeter per year is 2.3:1 to 30:1. These ratios indicate that although there is a component of dip slip, the Hosgri fault zone is behaving predominantly as a strike-slip fault (see Response to Question SSC 2), and not as either an oblique-slip or reverse/thrust fault.
The ratio of horizontal to vertical slip rates is not constant along the fault zone, nor does it vary monotonically from north to south along the fault zone. Figure GSG Q4-2 graphically shows the distribution of horizontal and vertical components of slip along the Hosgri fault zone and the ratio of slip components from reach to reach of the fault zone. The lateral slip of 1 to 3 millimeters per year, which is transferred from the San Simeon fault zone to the northern part of the Hosgri fault zone, probably progressively decreases southward as slip is consumed by crustal shortening along bordering, more westerly trending, reverse faults and folds within the Los Osos/Santa Maria domain (PG&E, 1988; Lettis and others, 1989). Because faults and folds in the offshore Santa Maria Basin to the west of the Hosgri fault zone either do not exist or, in the southern part of the zone, are subparallel to the Hosgri fault zone, a similar decay of slip offshore does not occur. A graphic representation of this model of southward decay of lateral slip in the Los Osos/Santa Maria domain along the Hosgri fault zone is presented in Figure GSG Q4-2. For the Los Osos fault zone and faults along the southwestern boundary of the San Luis/Pismo block, including the Olson, San Luis Bay, and Wilmar Avenue faults, we use rates of slip estimated for these faults based on displaced marine terraces. The resolved component of slip tangential to the Hosgri fault zone is a maximum of about 0.2 millimeter per year for each. Similarly, we assume that these rates are applicable for other faults that intersect the Hosgri fault zone, such as the Casmalia and Lion's Head fault zones. Recent studies by Clark and Slemmons (1990) indicate the uplift rate for the Casmalia Hills between the Casmalia and Lion's Head faults is lower than the uplift rate of the San Luis/Pismo block. Based on the estimated vertical separation across these fault zones, the resolved component of slip tangential to the Hosgri fault zone is less than 0.2 and 0.05 millimeter per year for the Casmalia and Lion's Head fault zones, respectively. As depicted on Figure GSG Q4-2, if one assumes a horizontal slip rate of 1 millimeter per year at the northern end of the Hosgri fault zone, the lower value of the preferred range, the southward decay of horizontal slip by crustal shortening in the Los Osos/Santa Maria domain brings the ratio of horizontal to vertical slip to a value of less than 2:1 along the Point Sal and Southern reaches of the Hosgri fault zone. In these southern parts of the fault zone, the range in ratios of horizontal to vertical slip indicates that the fault zone may be either an oblique-slip or a strike-slip fault.
Luyendyk and others (1980, 1985), PG&E (1988) and Lettis and others (1989) relate the southward decreasing rate of lateral slip along the Hosgri fault zone to the clockwise rotation of the Western Transverse Ranges. They suggest that rotation of the Western Transverse Ranges (underlain by relatively thick (about 33 kilometers) crust), compresses the Los Osos/Santa Maria domain (underlain largely by Franciscan basement about 12 to 20 kilometers thick), against the relatively stationary Santa Lucia and San Rafael ranges (underlain in part by more-rigid Salinian granite). This results in north-northeast crustal shortening. The structural grain in the domain is oriented roughly N50'W to N80'W, and becomes progressively more westerly trending southward in the domain. This structural trend is oblique to the N20'W to N40'W trend of the Hosgri fault zone and the subparallel folds and reverse faults in the offshore Santa Maria Basin. The change in orientation of structural grain between the Los Osos/Santa Maria domain and offshore Santa Maria Basin occurs abruptly in a narrow, elongate zone along the Hosgri fault zone. The change in orientation of crustal shortening reflected by the change in structural trends requires lateral slip along the Hosgri fault zone.
Using geodetic data in south-central coastal California, Fiegl and others (1989) estimate that the integrated rate of deformation across the Los Osos/Santa Maria domain is 7 + 1 millimeters per year, oriented N03'E + 13'. Based on a model that assumes uniform strain and no rotation, these authors decompose the rate of deformation into 6 a 2 millimeters per year of crustal shortening on the general structural trend of N30 E, and 3 + 1 millimeters per year of right-lateral shear across the Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
I ueti n 4 M,rchl Pa e5 Northern San Luis/Pismo'each San Luis Obispo Point Sal Southern Reach Bay Reach Reach Reach San Simeon- Southwestern Hosgri Los Osos Boundary Casmalia Lion's Head Southern stepover fault zone faults fault zone fault zone Termination I I
'P' Pp
."p 2 r.
Maximum VO~ g P~~
Preferred~ Hosgn Hosgn and Punslma 0
H'V H'V H:V ~ H:V ~
H:V 10:1 to30:1 l 2:1 to11:1 2.4:1 to 14:1 l >1:1 to 18:1l 21:1 to >7:1 Post-middte Pliocene (2.8 Ma) horizontal slip rate Maximum post-middle Pliocene (2.8 Ma) vertical slip rate across the Hosgri Fault Zone Maximum post-middle Pliocene (2.8 Ma) combined vertical slip rate across the Hosgri fault zone and Purisima structure Figure GSG Q4-2 Distribution of lateral and vertical slip rate along the Hosgri fault zone, assuming that lateral slip decays southward due to crustal shortening within the Los Osos/Santa Maria domain.
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ueti n 4 March 1 0 Pa e axis. The orientation of crustal shortening is based on an average N60'W trend of reverse faults and folds in the Los Osos/Santa Maria domain. The rate of right-lateral shear (3 + 1 millimeters per year) determined by these authors is consistent with our estimate of 1 to 3 millimeters per year of right-lateral shear along the Hosgri fault zone. No other right-lateral fault has been identified within the Los Osos/Santa Maria domain that can accommodate the right-lateral shear determined by Feigl and others (1989).
CONCLUSIONS We have evaluated and quantified rates of horizontal and vertical slip along the Hosgri fault zone.
The rate of horizontal slip is judged to be 1 to 3 millimeters per year. This rate is based on paleoseismic and Quaternary mapping studies conducted along the San Simeon fault zone, evaluation of the rate of slip necessary to produce the San Simeon/Hosgri pull-apart basin, and consideration of regional tectonics and geodetic rates of crustal shortening in south-central coastal California. The pattern and style of deformation within the Los Osos/Santa Maria domain and the rate and orientation of geodetically determined rates of crustal shortening virtually requires that horizontal slip is occurring along the Hosgri fault zone. The rate of vertical slip ranges from 0.1 to 0.44 millimeter per year. These rates have been assessed from vertical separation of the mid-Pliocene unconformity across the Hosgri fault zone, combined with uplift of the coast based on elevated Pleistocene marine terraces.
The ratio of horizontal to vertical slip ranges from 2.3:1 to 30:1. These ratios indicate rake angles of less than 30 degrees for the Hosgri fault zone. These rake angles, or direction of slip on the fault plane, meet our definition and published definitions (see Response to Question SSC 2) for a strike-slip fault. In the extreme case where horizontal slip along the northern Hosgri fault zone is assumed to be the minimum preferred value of 1 millimeter per year and this slip rate decreases southward by crustal shortening within the Los Osos/Santa Maria domain, the ratio of horizontal to vertical slip may drop to below 2:1 along the Point Sal and Southern reaches of the Hosgri fault zone. These ratios yield rake angles of greater than 30 degrees for the Hosgri fault zone in these areas, suggesting oblique-slip. In this extreme scenario, the oblique-slip character of the Hosgri fault zone is confined to the Point Sal and Southern reaches of the fault; all other reaches of the fault, including the San Luis/Pismo reach opposite Point Buchon and the Diablo Canyon Power Plant, meet the definition of a strike-slip fault. All other scenarios or combinations of scenarios (for example, horizontal slip rates greater than 1 millimeter per year, absence of or less extreme southward decay of lateral slip) provide horizontal to vertical slip rate ratios indicative of strike-slip faulting along the entire length of the Hosgri fault zone.
REFERENCES Clark, D. G., and Slemmons, D. B., 1990, Late Pleistocene deformation in the Casmalia Hills region, coastal central California (abs.): submitted to Geological Society of America.
Crowell, J. C., 1959, Problems of fault nomenclature: American Association of Petroleum Geologists Bulletin, v. 43, p. 2653-2674.
Feigl, K. L., King, R. W., and Jordan, T. H., 1989, The Santa Maria fold and thrust belt as a transition zone in southern California tectonics: EOS, v. 70, no. 43, p. 1353-1354.
Hall, N. T., and Hunt, T. D., in review, Holocene behavior of the San Simeon fault zone, south-central coastal California; in Alterman, I. B., (ed.), Seismotectonics of Central and Coastal California:
Geological Society of America Special Paper.
Hanson, K. L., and Lettis, W. R., in review, Estimated Pleistocene slip rate for the San Simeon fault zone, south-central coastal California; in Alterman, I. B., (ed.), Seismotectonics of Central and Coastal California: Geological Society of America Special Paper.
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uesi n 4 Mrchl Pa e7 Hanson, K. L., Lettis, W. R., and Mezger, L., 1987, Late Pleistocene deformation along the San Simeon fault zone near San Simeon, California (abs.): Geological Society of America Abstracts with Program, Cordilleran Section, v. 19, no. 6, p. 386.
Lettis, W. R., Hall, N. T., and Hamilton, D. H., 1989, Quaternary tectonics of south-central coastal
~ California (abs.): 28th International Geological Congress, v. 2, p. 2-285.
Luyendyk, B. P., Kamerling, M. J., and Terres, R. R., 1980, Geometric model for Neogene crustal rotations in southern California: Geological Society of America Bulletin, Part I, v. 91, p. 211-217.
Luyendyk, B. P., Kamerling, M. J., Terres, R. R., and Hornafius, J. S., 1985, Simple shear of southern California during the Neogene: Journal of Geophysical Research, v. 90, p. 12,454-12,466.
Pacific Gas and Electric Company, 1988, Final report of the Diablo Canyon Power Plant Long Term Seismic Program, U. S. Nuclear Regulatory Commission Docket Nos. 50-275 and 50-323.
Stone, D., 1986, Characteristic features of wrench faults; in Foster, N. H., and Beaumont, E. A.,
Structural concepts and techniques: American Association of Petroleum Geologists Reprint Series.
Weber, G. E., 1983, Geologic investigation of the marine terraces of the San Simeon region and Pleistocene activity of the San Simeon fault zone, San Luis Obispo County, California: U.S.
Geological Survey Technical Report 66 p.
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tt chment 4- March 1 0 Pa e 1 ATTACHMENTGSG Q4-A ORIGIN AND TECTONIC SIGNIFICANCE OF THE NORTH ESTERO SLOPE BREAK INTRODUCTION The responses to Questions GSG 3 and GSG 4 address the vertical and lateral rates of deformation on the Hosgri fault zone. With regard to this deformation, questions were raised concerning the origin and tectonic significance of a prominent slope break in Estero Bay, the location and elevation of the 18,000-year-old lowstand strandline, its coincidence with the slope break, and its usefulness in evaluating vertical rates of deformation across the Hosgri fault zone. This attachment describes the analyses performed to assess tectonic deformation and associated morphologic features in the north Estero Bay region, and includes:
- 1. A detailed bathymetric map (2-meter contour interval, 1:48,000 scale) used to evaluate the location and origin of the slope break in Estero Bay.
- 2. Common-depth-point(CDP) and high-resolution seismic data, which provide evidence of no faulting across the northern part of the north Estero Bay slope break.
- 3. A discussion of the elevations of globally recognized Pleistocene sea-level lowstands, including the most recent lowstand, 18,000 years ago.
- 4. An assessment of the usefulness of the strandline of the 18,000-year-old lowstand as a geomorphic strain gauge to measure rates of vertical and lateral deformation across the Hosgri fault zone.
To assess the origin and tectonic significance of seafloor geomorphic features along the Hosgri fault zone, we conducted detailed bathymetric, morphologic, and geophysical investigations. Preliminary results of these investigations were presented by Niemi and others (1987) for the offshore region between Estero Bay and San Luis Obispo Bay, and submitted to the NRC in response to Question 431, January 1989. Niemi and others identified a seafloor geomorphic trend in Estero Bay that was referred to as the north Estero Bay slope break. This geomorphic trend, as mapped on the 1987
'athymetric map (Response to Question 431, January 1989) extends for a distance of 11.2 kilometers subparallel to and southwest of a trace of the Hosgri fault zone (Figure GSG Q4-A.1). At the time of publication in 1987, detailed analysis of the high resolution seismic data was not completed in this region, so preliminary bathymetric maps were used and the location of the Hosgri fault zone was not well constrained. Because of the proximity to and subparallel trend of this slope break with the Hosgri fault zone, and the presence of disrupted subsurface reflectors and surface depressions at the southern end of this feature (Figure Q431-3), Niemi and others (1987) concluded that this feature was, in part, fault-controlled.
Subsequent to the study conducted by Niemi and others (1987), we completed the analysis of the high resolution seismic data and prepared detailed bathymetric maps of the seafloor at a contour interval of 2 meters. These analyses indicate that although the southern part of the north Estero Bay slope break as mapped by Niemi and others (1987) coincides with a strand of the Hosgri fault zone, the slope break generally deviates from and lies west and north of the active traces of the Hosgri fault zone in northern Estero Bay (PG&E, 1988, Plate 4). The slope break is crossed by a number of both CDP and high-resolution seismic lines (PG&E, 1988, Plates 1 and 2). These lines provide good evidence of no faulting being associated with the northern part of the slope break in either the shallow or deep part of the underlying Tertiary section. In addition, the sense of apparent vertical separation across the Hosgri fault zone in this region is down on the east, opposite to the west-facing scarp. For these reasons, the north Estero Bay slope break is not interpreted to be a tectonic geomorphic feature, and the height of this slope break was not used in assessing rates of vertical separation across the Hosgri fault zone.
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At chment GSG 4-A March l Pa e2 Lw i San Simeon" Point
'L "L
% ~
0 5 mt I 0 10 km Point Estero PA C IF I C OCEAN Morro Bay EXPLANATION A Fault; dashed where inferred; sawteeth on upper plate of a thrust fault
- ,."..,y
- :~~~@ Regional break ln slope between middle and outer zones of continental shelf 'oint 8uchon Diablo Canyon Northern Estero Bay slope break (Nlemi and Power Plant others, 1987) i::::.. Pan't
';:.';.l, San Luis o
Figure GSG Q4-A.1 Map showing location of the north Estero Bay slope break and regional shelf break between San Simeon Point and Point San Luis.
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I t chm n 4-A arch 1 0 BATHYMETRICDATA Bathymetric maps were prepared at a contour interval of 2 meters for the entire coastal region from Estero Bay to San Luis Obispo Bay to a water depth of at least 220 meters (Plates Q431-1 and Q431-2, Response to Question 431, January 1989). Preliminary bathymetric maps and analyses prepared by Niemi and others (1987) are also provided in the response to Question 431.
The bathymetric maps were prepared from copies of original boat sheet survey data collected by the U. S. Coast and Geodetic Survey (1934 and 1935). These data sources and areas of coverage are listed in the Final Report (PG&E, 1988, Plate 19) and are discussed in the response to Question 431.
These data were supplemented and integrated with bathymetric data from echo sounder and high-resolution geophysical records collected during the offshore geophysical surveys.
Subsequent to the submittal of the Response to Question 431, January 1989, we modified slightly the bathymetric map that covers Estero Bay (Plate Q431-1) and extended comparable coverage throughout the study region from Cape San Martin south to Point Arguello. In this response, we provide the updated bathymetric maps for the coastal region between San Simeon Point and Point San Luis (Plate GSG Q4-A.l).
LOCATION AND CHARACTERISTICS OF THE SHELF BREAK On the basis of both submarine geology and physiographic expression, Niemi and others (1987; Response to Question 431, January 1989) subdivided the shelf area in the offshore region near the Diablo Canyon Power Plant into inner, middle, and outer shelf zones (Figure Q431-1). These shelf zones are similar to the subdivisions established by Cacchione and others (1983) for the California continental shelf north of San Francisco. The inner shelf is the rocky nearshore zone corresponding to the general seaward limit of the seabottom outcrop. The gradational boundary between the inner and middle shelf zones occurs at approximately -70 to -80 meters and is marked by a decrease in gradient and seafloor roughness, even though isolated rock outcrops are present locally within the middle shelf. The outer shelf exceeds depths of -115 to -135 meters, and typically has a slope greater than 2 degrees. The boundary between the middle and outer shelf zones is a steep seaward-facing increase in slope that is very pronounced on the Niemi and others (1987) bathymetric map (Figure Q431-1). This corresponds to similar shelf breaks found worldwide; a discussion of these shelf breaks and their elevations is provided later in this attachment.
The detailed 2-meter contour interval bathymetric maps prepared subsequent to the Niemi and others (1987) study (Plate GSG Q4-A.I) provide excellent elevation control for assessing the origin of the north Estero Bay slope break and its relationship to the more regional shelf break. The top of the slope face that defines the prominent shelf break between the middle and outer zones in the region between San Simeon Point and Point San Luis is at an elevation between -120 and -130 meters.
The basal elevation of the slope face generally is at an elevation of -160 to -180 meters. Locally, the shelf break is a composite slope consisting of a steeper (up to 9 degrees) upper slope and a less steep (4 to 5 degrees) lower slope. The steeper part of the slope break is clearly imaged on seismic profiles. The north Estero Bay slope break as defined by Niemi and others (1987) coincides with the upper steeper part of the shelf break. Other, less prominent seafloor inflections and slope breaks are also superimposed on the general slope face that defines the regional shelf break. Although discontinuous, many of these minor slope breaks are at similar elevations. This observation, as well as the fact that these features parallel the isobath contours, suggests they may be submerged shorelines. One of the more common of these seafloor inflections occurs at an elevation of approximately -160 meters.
Niemi and others (1987) noted that the prominent shelf break in Estero Bay can be traced northward to the Point Sur region. The shelf break is less distinct in the area west and southwest of 59-meter ridge in the southern part of Estero Bay. South of this area,'n elongate slope failure marked by a prominent slump headwall is present locally along the lower part of the shelf break for a distance of more than 30 kilometers (Niemi and others, 1987).
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tahmn 4-A March I p 4 The 11.2-kilometer-long north Estero Bay slope break coincides along its entire length with the prominent shelf break between the middle and outer shelf zones. An active trace of the Hosgri fault zone coincides with the southern 3 kilometers of the north Estero Bay slope break in the region northwest of 59-meter ridge. It is in this area that Niemi and others (1987) noted disrupted subsurface reflectors in the vicinity of the slope break, as imaged on Fairfield line 248 (Figure Q431-3).
GEOPHYSICAL IMAGING OF THE SHELF BREAK Representative high resolution profiles across the shelf break between the latitude of Morro Bay and San Simeon Point are presented on Plate GSG Q4-A.1. These profiles, as well as other high resolution reflection data presented on the Piedras Blancas, San Simeon/Hosgri stepover, and Northern reach montages (see Attachment GSG Ql-A) clearly show undeformed late Pliocene strata beneath the shelf break throughout most of this region.
The north Estero Bay slope break lies close to but is coincident with an active trace of the Hosgri fault zone only along its southernmost 3 kilometers. Along most of its length, seismic profiles indicate that the slope break is a geomorphic feature that does not reflect'folding or faulting of the underlying late Pliocene sediments (see CM-49 and CM-45, Plate GSG Q4-A.1 and CM-43, San Simeon/Hosgri stepover montage, Attachment GSG Ql-A). A comparable slope break is imaged in seismic profile CM-55 (Plate GSG Q4-A.1) beyond the northern mapped trace of the Hosgri fault zone.
At its southernmost end, the prominent north Estero Bay slope break, which is approximately 20 to 25 meters high along most of its length, is marked by a lower (less than 10 meters high) slope break at the base of a more gradual shelf break that spans the two active traces of the Hosgri fault zone (see CM-39, San Simeon/Hosgri stepover montage, Attachment GSG Ql-A).
South of this area, the shelf break changes in character where it impinges on the elevated Miocene bedrock east of the Hosgri fault zone in the vicinity of 59-meter ridge. A distinct slope break or, locally, an escarpment coincides with the active trace of the Hosgri fault zone at the margin of the elevated Miocene section. The basal elevation of this slope break or scarp, which is lower and is locally less pronounced than the north Estero Bay slope break (see CM-35, Plate GSG Q4-A.1; and CM-33, Northern reach montage, Attachment GSG Ql-A) ranges from about -130 to -120 meters.
. South and west of this elevated region, a series of subtle seafloor inflections (at elevations of about
-160 to -140 meters) occur within a broad region that narrows and becomes a more well defined shelf break to the south (Plate GSG Q4-A.1).
PLEISTOCENE LOW SEA LEVELS To better understand the relationship of the regional shelf break and, in particular, the north Estero Bay slope break to paleo-sea-level stands, we conducted a review of existing data and literature describing and correlating low sea-level stands throughout the world. Estimates of ice volume based on the oxygen isotope record in marine deep-sea cores indicate there have been several periods of glacio-eustatically lowered sea level (Shackleton and Opdyke, 1973; Chappell and Shackleton, 1986).
Although relative isotopic differences have been used to compare ice volume and hence sea level during the glacial extremes (Shackleton, 1987), these comparisons do not provide precise definitions of sea levels during these periods.
Elevations of relative sea level lower than present are difficult to constrain and are generally approximated from estimates of glacial ice volume, the marine oxygen-isotope record and, most commonly, from stratigraphic or morphologic evidence on submerged continental shelves. Chappell (1987), however, cites numerous problems in defining the elevations of low sea-level stands on the basis of submarine evidence on the outer continental shelf. Because the evidence is at depths greater than scuba divers can reach, it is often sampled by dredges or drill cores from a ship. This results Diabio Canyon Power Plant Pacfffc Gas and Electric Company Lang Term Seismic Program
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Attachment 4-A March 1 0 Pa e in sparse, scattered data having considerable age/depth uncertainty. Thus, even the elevation of the most recent lowstand, which culminated about 18,000 years ago, is not known with certainty.
Although the elevation of this lowstand is commonly cited as about -120 meters (Curray, 1965; Nardin and others, 1981; Shackfeton, 19S7; Ota and Machida, 1987), radiocarbon dates on fossil wood, peat, and shell from sedimentary deposits on continental shelves and in shallow coastal embayments throughout the world indicate that sea level was about 100 to 150 meters below its present position during the last glacial maximum, between 15,000 and 20,000 years ago (Bloom, 1977). In addition, both higher and lower elevations have been reported for this lowstand. Data from the Black Sea basin (for example, Badyukov and Kaplan, 1979), suggest that the post-glacial sea-level rise started from -80 meters between 17,000 to 18,000 years ago (Pirazzoli, 1987). Values of -90 to -100 meters have been cited for the West Atlantic (Milliman and Emery, 1970) and for the Texas Gulf (Curray, 1960). More recent studies, however, indicate a lower elevation for the late Wisconsinan lowstand. On the outer margins of the north Australian shelves, samples dated between 14,000 and 18,000 years from lagoonal facies at -135 meters, intertidal beach rock at -150 meters, and coral below a terrace at -165 meters indicate a lower sea level of approximately -135 to -165 meters (Chappell, 1987). Along the coastline of eastern China, sea level is estimated to have been at an elevation of -150 to -160 meters approximately 15,000 years ago (Ota, 1987).
Variations in the post-1 S,000-year-old sea-level rise, as well as the present elevation of the 18,000-year-old strandline around the globe, are expected as a result of the deformation of the earth' surface and its geoid by changing ice and water loads (Walcott, 1972; Cathles, 1980; Clarke and others, 1978). According to the model developed by Clark and others (1978), the melting of a realistic ice load in 1,000-year steps since 16,000 years ago gives a non-uniform sea-level change relative to the 16,000-year-old shoreline. As shown by Clark and others (1978), maximum submergence in the offshore south-central California coastal region may be slightly in excess of the eustatic rise in sea level; thus, the elevation of the submerged lowstand strandline would be below the global average.
Glacial periods more extreme than the most recent one, culminating about 18,000 years ago, which correlates to marine oxygen-isotope Stage 2, are evident in deep-sea cores (Shackleton, 1987). Based on isotopic differences within benthonic data sets, Shackleton (1987) concludes that marine oxygen-isotope Stages 12 and 16 were clearly more extreme than Stage 2, and that ice volumes during these two stages were about 15 percent greater than at the last glacial maximum. Sea-level stands lower than or equivalent to the 18,000-year-old lowstand may have produced submerged shorelines on the continental shelf below -120 meters. Multiple shorelines, where preserved, result in a complex
- series of submerged slope breaks that converge and diverge laterally. Where two or more shorelines coincide, they can produce a locally enhanced slope break or escarpment on the shelf.
ORIGIN OF THE NORTH ESTERO BAY SLOPE BREAK Submerged escarpments in near-shore coastal environments such as Estero Bay may be caused by a variety of tectonic and geomorphic processes. Tectonic processes include faulting and folding of the sea floor. Geomorphic processes include fault-line erosion, erosion along contacts between contrasting bedrock lithologies, development of submerged shorelines during sea-level lowstands or stillstands, progradation of deltaic deposits, or sediment draping over preexisting erosional or tectonic scarps.
There is strong evidence from geophysical data that the north Estero Bay slope break is not underlain by or associated with a fault or fold along most of its length. The lack of late Pliocene and Quaternary deformation indicates that the slope break between the middle and outer shelf zones in Estero Bay is not a tectonic fault scarp or an erosional fault-line scarp.
Undeformed bedding surfaces are clearly imaged on seismic sections across the slope break (Plate GSG Q4-A.1). The shelf break may be, at least in part, a progradational feature. Curray (1965) notes that shelf breaks are observed worldwide at an average elevation of -130 meters, and that the close correspondence of the depth of the shelf break worldwide is probably the effect of deposition Diablo Canyon Power Plant Pacific Gas and Electric Company long Term Seismic Program
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t achment 4-A M rch 1 Pa e and erosion during lowered sea level. Seaward-facing scarps at the edge of the continental shelf in the Monterey Bay region are interpreted to be erosional features at the landward edge of young foreset beds on a prograding continental slope (Greene, 1977). E. A. Silver (pers. comm. to Greene, 1977) attributes similar features along the northern California coast to wave erosion and accompanying deposition during lowered sea level.
The shelf break between San Simeon Point and Point San Luis is at or slightly below the estimated range in elevation (-120 to -160 meters) of the late Wisconsinan (18,000-year-old) lowstand. Wave erosion during this lowstand or earlier lowstands, which may have been slightly lower in elevation than the late Wisconsinan lowstand, probably caused or significantly contributed to the origin of the north Estero Bay slope break. In addition, draping of sediments over the slope break and wave erosion during and after these lowstand(s) has certainly modified the slope break.
The north Estero Bay slope break appears to be a composite of two or more individual submerged shorelines. As shown on Plate GSG Q4-A1, the slope break consists of at least two distinct smaller slope breaks that converge northward near 59-meter ridge and, thus, locally enhance the escarpment in northern Estero Bay.
The width and definition of the shelf break vary along its length and it cannot be traced as a continuous feature between San Simeon Point and Point San Luis. In particular, the shelf break is difficult to map in the area near and to the south of 59-meter ridge in the southern part of Estero Bay (Plate GSG Q4-A.1). This may be due, in part, to the presence of 59-meter ridge and other elevated bedrock east of the Hosgri fault zone, which locally blocks sediment transport across the shelf and effectively inhibits development of a progradational wedge of sediment on the shelf break.
Also, as reported by Niemi and others (1987) and shown on Plate GSG Q4-A.1, the headwall of a large submarine slump locally disrupts and modifies the shelf break in southern Estero Bay.
We conclude, therefore, that the offshore slope break observed in Estero Bay probably resulted from wave erosion and associated deposition during periods of lowered sea level, and has been modified locally by sediment progradation, slope failure, and possible erosional scour at the base of the slope break. In addition, where the slope break coincides with active fault traces of the Hosgri fault zone near 59-meter ridge, it may be modified by tectonic deformation.
USEFULNESS OF THE 18,000>> YEAR-OLD SHORELINE TO ASSESS TECTONIC DEFORMATION The submerged strandline of the 18,000-year-old lowstand is not a clearly defined geomorphic feature in the offshore region between San Simeon Point and Point San Luis. Niemi and others (1987) assumed a value of -120 meters for this lowstand, based on reported elevations elsewhere along western North America (Curray, 1960; Nardin and others, 1981). Other than the escarpment bordering 59-meter ridge, there is little or no bathymetric expression of a prominent terrace feature at this elevation (Plate GSG Q4-A.1) to support this assumption. Also, along the Santa Monica shelf in southern California, where a Holocene sediment-filled channel suggests that sea level was -117 meters during the most recent lowstand, there is no wave-cut terrace this level (Nardin and others, 1981). In the offshore region between San Simeon Point and Point Buchon, the base of the most prominent slope break is best expressed at an elevation of about -160 a 10 meters. Given the range
(-120 to -160 meters) in estimates for the 18,000-year-old lowstand worldwide, it is possible that this feature developed during the most recent lowstand. Alternatively, it may be a composite of erosionally and depositionally modified features formed during one or more of the earlier more extreme glacial periods described above.
Nowhere along its entire length does the Hosgri fault zone clearly displace submerged shoreline features that can be correlated across the fault zone. For this reason, in addition to the uncertainties in the paleo-sea-level at which the 18,000-year-old shoreline formed, we did not use this shoreline to evaluate rates of deformation across the Hosgri fault zone.
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At chmen 4-A March I Pa e7 CONCLUSIONS The utility of the north Estero Bay slope break as an 18,000-year-old strain gauge to assess deformation is severely limited for several reasons:
~ The slope break in no instance crosses the Hosgri fault zone and, thus, cannot be used as a direct measure of lateral or vertical displacement.
~ The origin of the north Estero Bay slope break is very likely complex. The slope break appears to be a composite escarpment formed during two or more periods of low sea level, subsequently modified by a variety of erosional and depositional processes. The elevation and location of the actual 18,000-year-old lowstand is uncertain.
~ The north Estero Bay slope break is part of a regional break in slope between the middle and outer continental shelf zones. In Estero Bay, this slope break coincides with an active strand of the Hosgri fault zone for a distance of approximately 3 kilometers.
However, the 20- to 30-meter-high west-facing slope break persists beyond the zone of active faulting and, in these areas, clearly is not the result of tectonic deformation.
~ Slope breaks and seafloor inflections subparallel to isobaths at elevations ranging from
-114 to -180 meters are present elsewhere along the south-central California coast.
These slope breaks may be submerged shorelines that formed during one or more Pleistocene lowstands or during stillstands in the post-glacial rise to the present level.
An accurate, consistent, global or western North America elevation for the 18,000-year-old stillstand is not available and, thus, cannot be used to calibrate the initial elevation of the lowstand in Estero Bay from which to assess subsequent deformation (either faulting or folding).
For these reasons, we have not used the north Estero Bay slope break to assess or quantify rates of deformation along the Hosgri fault zone.
REFERENCES Badyukov, D. D., and Kaplan, P. A., 1979, Sea-level changes on the coasts of the USSR during the last 15,000 years: Proceedings of the 1978 International Symposium on Coastal Evolution in the Quaternary, Sao Paulo, Brazil, p, 135-169.
Bloom, A. L., 1977, Atlas of sea-level curves: IGCP Project 61, Sea Level Project, UNESCO, Paris.
Cacchione, D. A., Drake, D. E., Grant, W. D., Williams, A. J., III., and Tate, G. B., 1983, Variability of seafloor roughness within the coastal oceanic dynamics experiment'(ODE) region: Woods Hole Oceanography Institute of Technology Report WHO1-83-25, 50 p.
Cathles, L. M., 1980, Interpretation of postglacial isostatic adjustment phenomena in terms of mantle rheology; in Morner, N. A., ed., Earth Rheology, Isotasy and Eustasy: Wiley and Sons, New York,
- p. 11-44.
Chappell, J., 1987, Late Quaternary sea-level changes in the Australian region; in Tooley, M. J.,
and Sherman, I., eds., Sea-level Changes: Institute of British Geographers Special Publication Series,
- v. 20, p. 296-331.
Chappell, J., and Shackleton, N. J., 1986, Oxygen isotopes and sea level: Nature, v. 324, p. 137-140.
Clarke, J. A., Farrell, W. E., and Peltier, W. R., 1978, Global changes in post-glacial sea level; a numerical calculation: Quaternary Research, v. 9, p. 265-287.
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A chm n S 4-A March 1 Curray, J., 1960, Sediments and history of Holocene transgression, continental shelf, northwest Gulf of Mexico; in Sheppard, F. P., ed., Recent Sediments, Northwest Gulf of Mexico: American Association of Petroleum Geologists, p. 2211-2266.
Curray, J., 1965, Late Quaternary history, continental shelves of the United States; in Wright, H.
E., Jr., and Frey, D. G., eds., The Quaternary of the United States: Princeton University Press, Princeton, New Jersey, p. 723-736.
Greene, H. G., 1977, Geology of the Monterey Bay region: U.S. Geological Survey Open-File Report 77-718, 347 p.
Milliman, J. D., and Emery, K. O., 1970, Sea levels during the past 35,000 years: Science, v. 162,
- p. 1121-1122.
Nardin, T. R., Osbourne, R. H., and Bottjer, D. J., 1981, Holocene sea-level curves for Santa Monica shelf, California, California continental borderland: Science, v. 213, p. 331-333.
Niemi, T. M., Hall, N. T., and Shiller, G. T., 1987, Seafloor scarps along the central reach of the Hosgri fault zone, southern Coast Ranges, California (abs.): Geological Society of America Abstracts with Program, v. 19, no. 7, p. 789.
Ota, Y., 1987, Sea-level changes during the Holocene, the Northwest Pacific; in Devoy, R. J. N.,
Sea Surface Studies: Croom Helm, London, p. 348-374.
Ota, Y., and Machida, H., 1987, Quaternary sea-level changes in Japan; in Tooley, M. J., and Sherman, I., eds., Sea-level Changes: Institute of British Geographers Special Publication Series, v.
20, p. 182-224.
Pacific Gas and Electric Company, 1988, Final report of the Diablo Canyon Power Plant Long Term Seismic Program: U. S. Nuclear Regulatory Commission Docket Nos. 50-275 and 50-323.
Pirazolli, P. A., 1987, Sea-level changes in the Mediterranean; in Tooley, M. J., and Sherman, I.,
eds., Sea-level Changes: Institute of British Geographers Special Publication Series, v. 20, p. 152-181.
Shackleton, N. J., 1987, Oxygen isotopes, ice volume, and sea level: Quaternary Science Reviews,
- v. 6, p. 183-190.
Shackleton, N. J., and Opdyke, N. D., 1973, Oxygen-isotope and paleomagnetic stratigraphy of equatorial Pacific core V28-238: oxygen isotopic temperatures and ice volumes on a 10 -year and 10 -year scale: Quaternary Research, v. 3. p. 39-55.
Walcott, R. I., 1972, Past sea levels, eustasy and deformation of the earth: Quaternary Research,
- v. 2, p. 1-14.
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H ue tion S 5 March 1 0 P el QUESTION GSG 5 Provide the tsunami analysis used to determine the location, moment, and magnitude of the 4 November 1927 "Lompoc" earthquake including the uncertainties, and the letter from Dr. Abe pertaining to his tsunami magnitude determi>iation and the Hilo tide gauge recording. Discuss potential timing errors that arise from clock error, marking error, or other causes inherent in the San Diego or San Francisco marigrams. Describe hotv'he tsunami analysis technique has been calibrated against data.
METHOD The method used in the tsunami analysis is described by Satake (1989). Tsunami propagation is calculated by finite-difference representations of the linear long-wave equation and the equation of continuity. For coastal regions in California and Hawaii, a 1-minute bathymetric grid was compiled from the NOS 15-second and NOAA 5-minute grids, and interfaced with a 5-minute grid derived from the NOAA grid for the oceanic path between Hawaii and California. The Californian and Hawaiian tide gauge stations used in the analysis are shown in Figures GSG Q5-1 and Q5-2, respectively.
CALIBRATION The source region of the tsunami of the 1975 Kalapana, Hawaii earthquake has a location and tsunami volume that is known from Hawaiian tsunami recordings (Cox, 1980; Hatori, 1976; Figure GSG Q5-2). Estimates of the location and tsunami volume of the Kalapana earthquake derived from tsunami modeling of tide gauge recordings in California provide an opportunity to evaluate the accuracy of estimates of location and seismic moment of the 1927 earthquake derived from tsunami analysis. Also, the oceanic portion of the path between the Kalapana earthquake and the tide gauge station at Port San Luis is similar to the oceanic portion of the path between the 1927 Lompoc earthquake and the Hilo tide gauge station, as seen in Figures GSG QS-1 and Q5-2, providing a calibration of that path.
Tsunamis were calculated at La Jolla, Long Beach, and Port San Luis using a uniform uplift of 3
. meters over the source region, as estimated by Hatori (1976). The arrival time and peak amplitudes of the calculated tsunamis are compared with the recorded ones in Table GSG Q5-1.
Table GSG Q5-I COMPARISON OF OBSERVED AND CALCULATED ARRIVALTIMES AND AMPLITUDES OF THE
, TSUNAMI OF THE 1975 KALAPANAEARTHQUAKE Arrival Time (h:m) Amplitude (cm) obs. cal. obs. cal.
La Jolla 5.37+ 2 5:35 20 16 Long Beach 5.46 + 2 5:47 Port San Luis 5.17 + 2 5:14 26 26 Diabio Canyon Power Plant Pacific Gas and Electric Company Long Term SeismIc Program
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uestion GSG 5 March 1 0 Pa e2 124 '&123 122 '21 '20 119 118 117 '16 '9oN Fort Point 38'00 Cati forni a
+o
~o ~o
.0 0
~o 37'ong 000 ~ 0 Port San Luis (o
Beach La Jolla o
32'1'XPLANATION 1000 Bathymetry contours in meters Station location Area shown in Figures GSG Q5-4 and Q5-5 Figure GSG 5-1 Locations of tide gauge stations in California used in the tsunami analysis.
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u 157'W i n oo March Maui 1
156 '55 '54 P e
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O ~Q Haw ass 20'ilo Kalapana 000 19'000 18'XPLANATION 1000 Bathymetry contours in meters Station location 1975 Kalapana earthquake Figure GSG Q5-2 Location of the Hilo, Hawaii tide gauge station, and source region of the 1975 Kalapana earthquake.
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ue tion 5 March 1 0 Pa e4 The calculated arrival times are within the uncertainty range of the recorded ones, indicating that the location of the Kalapana tsunami source region derived from the California tide gauge stations by tsunami modeling is consistent with that derived by Cox (1980). The calculated amplitudes of the first cycle of motion are also in good agreement with the recorded ones, indicating that the tsunami volume of the Kalapana earthquake derived from the California tide gauge stations by tsunami modeling is compatible with that derived by Hatori (1976). We conclude that the accuracy of tsunami computations between Hawaii and California is about a few minutes in travel time and a factor of 1.5 in amplitude.
LOCATION The tide gauge recordings of the 1927 Lompoc earthquake at Fort Point (San Francisco), La Jolla, and Hilo are shown in Figure GSG QS-3. The location estimate is based principally on the times of the tsunamis at La Jolla and Hilo, as these two recordings have documented clock corrections (of-3 and +2 minutes, respectively). The procedure is to compute tsunami propagation from the tide gauge station to the epicentral area. For each of the two stations, the location locus shown by the solid line in Figure GSG Q5-4 corresponds to the time difference between the recorded arrival time at the station and the origin time of the earthquake. We estimate the tsunami source location to be at the intersection of the Hilo and La Jolla loci, at latitude 34.2'N, longitude 120.75 W.
We estimate the uncertainty of the clock corrections to be 1 minute. These corrections were noted directly on the marigrams, evidently in real time while the marigram was still on the drum, with arrows associating a corrected time directly to a point on the tide record. We estimate the marking error to be 1 minute. The overall uncertainty in timing is estimated to be two minutes. This is consistent with the estimate of two minutes derived from the Kalapana earthquake. The uncertainty in location corresponding to this estimate of uncertainty in timing is shown in Figure GSG Q5-4 by the dotted lines on either side of the solid line. The recording at Fort Point (San Francisco) does not have a documented clock correction, and its arrival time thus places uncertain constraint on the location. A clock correction of only a few minutes at Fort Point, similar to those at La Jolla and Hilo, is consistent with the location derived from La Jolla and Hilo. The Honda location is at the southern end of the Hosgri fault, just opposite the town of Honda, and provides a test of whether any location on the Hosgri fault is compatible with the recorded tsunamis.
The source model of the Lompoc earthquake used in generating synthetic tsunami records is shown in Table GSG Q5-2. The focal mechanism is from the analysis of teleseismic and regional body waves described in the Final Report. The focal depth of 10 kilometers, obtained from modeling teleseismic body waves, is assumed to represent the centroid depth of a rupture surface that spans Table GSG 5-2 PARAMETERS OF THE TSUNAMI SOURCE MODEL OF THE 1927 LOMPOC EARTHQUAKE Latitude 34.35'N Longitude 120.9'W Strike of fault N20'W Dip of nodal plane 66'NE Rake angle 95'8 Fault length kilometers Fault width 14 kilometers Average dislocation 2.5 meters Focal depth extent 3 to 16 kilometers Seismic moment 3 x 10 dyne-centimeters Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
I uestion March 1 0 Pa e5 Fort Point La Jolla Hilo, Hawaii EXP LANATlON Arrival time of tsunami Figure GSG Q5-3 Tide gauge recordings on November 4, 1927, at Fort Point (San Francisco), La Jolla, and Hilo.
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ue i n 5 March I 0 121 '20 119
'6'N 35
+ Gawthrop (1978) location
+ Honda location Long Term Seismic Program revised location
~~200 ~oo
+o 0 o
33 EXPLANATION 1000 Bathymetry contours in meters Best estimate ot tooos
2-minute uncertainty in travel time
+ 1927 Lompoc earthquake Figure GSG Q5-4 Tsunami source location of the 1927 earthquake using arrival times of tsunamis in Hilo, Hawaii, and La Jolla, California.
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uestion S 5 March I Pa e7 the depth range of 3 to 16 kilometers, corresponding to a fault width of 14 kilometers, on the nodal plane dipping northeast at 66 degrees. From the source duration of 6 seconds derived from teleseismic body wave modeling, we estimate a source area of 200 square kilometers from the relation of Cohn and others (1982), corresponding to a fault length of 14 kilometers and a fault width of 14 kilometers. However, the seismic moment of 3 x 102o dyne-centimeters estimated from the tsunami records, as described below, is three times that estimated from the body waves, suggesting that a larger source region ruptured at very long periods. Accordingly, we have assumed a fault length of 28 kilometers and a fault width of 14 kilometers for the tsunami source model, corresponding to an average dislocation of 2.5 meters and a static stress drop of approximately 100 bars.
Using this source model, we computed tsunamis for the Fort Point, La Jolla, and Hilo stations for the three source locations shown in Figure GSG Q5-5. These computed tsunamis are compared with the recorded tsunamis (after removal of the tide) at the same amplitude scale and at absolute time in Figures GSG Q5-6, Q5-7, and Q5-8, respectively.
Although the timing of the waveform of the Fort Point record places weak constraint on the location due to the absence of a documented clock correction as noted above, its high frequency waveform strongly indicates an offshore location rather than a near-shore location such as that of Gawthrop (1978). The waveforms of the La Jolla and Hilo recordings (Figures GSG Q5-7 and Q5-8) are also matched better by the offshore location, especially in the first few cycles of motion, which are less contaminated by the response of the coastal region near the tide gauge. Based on the waveforms of the recorded tsunamis, we estimate that the water depth in the source region of the Lompoc earthquake was not less than 200 meters.
The large delays in arrival time for the Gawthrop and Honda locations relative to the data are also evident in Figures GSG Q5-7 and Q5-8, and are due to the long time required for tsunamis to cross the shallow coastal waters along the paths shown in Figure GSG QS-5. These results indicate that no near-coastal location for the Lompoc earthquake is compatible with the tsunami records. The timing, amplitude, and waveform of the synthetic tsunamis calculated for the Long Term Seismic Program location is insensitive to whether the faulting occurred on the steeply northeast dipping fault plane, as assumed in the Long Term Seismic Program model, or on the shallowly southwest dipping fault plane, as assumed in the conjugate model shown in Figures GSG Q5-6, Q5-7, and Q5-
- 8. The timing, amplitude, and waveform of the synthetic tsunamis are also relatively insensitive to the dimensions of the assumed rupture surface, and to whether faulting extended upward to displace the sea floor.
In Figure GSG Q5-9, we summarize evidence bearing on the location of the 1927 Lompoc earthquake derived from the Final Report (PG&E, 1988), the response to Question 46, February 1989, the response to Question GSG 8 of this submittal, and the tsunami analysis presented above. The best estimate of the location is that derived from the analysis of teleseismic and regional body waves in the response to Question 46 and confirmed by the further analysis of regional body waves in the response to Question GSG 8. The uncertainty of this location is shown by the irregular boundary drawn around the location in Figure GSG Q5-9. In our response to Question 46, we estimated an uncertainty of 25 kilometers in that location; this is represented by the circular segments of the boundary. The northwest-trending segment of the boundary to the northeast of the epicenter is the 200-meter-depth bathymetry contour, and represents the constraint on water depth in the tsunami source region imposed by the tsunami waveforms. The rhomboidal southeast segment of the boundary includes the location estimates based on SSS-S travel times, as well as the uncertainty of the tsunami location.
The geometry of the fault model used to model the tsunami records is shown in the epicentral region.
The surface projection of the 28-kilometer-long by 14-kilometer-wide northeast-dipping fault plane is shown by the dashed lines, and its surface outcrop projected updip is shown by the solid line on the western side. This model assumes a bilateral rupture centered on the epicenter. The tsunami data suggest that rupture may have propagated farther in the south-southeast direction than in the north-northwest direction.
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ueti n M rch I Pa 122 W 121 120 36'N To Gawthrop (1978) location Fort Point Honda location Long Term Seismic Program revised location To Hilo happ 200 zoo To La Jolla~
O 0O O
~
EXPLANATION 1000 8athymetry contours in meters Tsunami path
+ 1927 Lompoc earthquake Figure GSG 5-5 Tsunami paths from three locations of the 1927 earthquake to three tide gauge stations.
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uestion GS 5 March I 0 Pa e Fort Point OBSERVATION 0
5 LTSP (Bkm) 0 El, surface o
-5 200 m depth 0
Honda 0
0 20 40 60 80 100 120 min from 6:00 Figure GSG QS-6 Comparison of recorded tsunami waveforms at Fort Point, San Francisco, from the 1927 earthquake (top trace) with synthetic waveforms for the three locations shown in Figure GSG Q5-2 (middle three traces), computed using the fault parameters in Table GSG Q5-2. The bottom trace shows the synthetic waveform for the revised Long Term Seismic Program location, assuming the shallowly southwest dipping nodal plane.
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l ue ion March I Pa e 10 La Jolla OBSERVATION 0
5 LTSP (Bkm) 0 El surface o
-5 200 m depth 0
Honda 0
0 80 40 60 80 100 180 min from 6:00 Figure GSG QS-7 Comparison of recorded tsunami waveforms at La Jolla from the 1927 earthquake (top trace) with synthetic waveforms for the three locations shown in Figure GSG Q5-2 (middle three traces),
computed using the fault parameters in Table GSG Q5-2. The bottom trace shows the synthetic waveform for the revised Long Term Seismic Program location, assuming the shallowly southwest dipping nodal plane.
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uesi n M rch I Pa e 11 Hilo observation 10 0
-10 LTSP 0
-10 g 10 Honda 0
~-10 Gawthrop 0
-10 conjugate 0
-10 0 20 40 60 80 100 120 min from 8:00 Figure GSG Q5-8 Comparison of recorded tsunami waveforms at Hilo, Hawaii, from the 1927 earthquake (top trace) with synthetic waveforms for the three locations shown in Figure GSG QS-2 (middle three traces),
computed using the fault parameters in Table GSG QS-2. The bottom trace shows the synthetic waveform for the revised Long Term Seismic Program location, assuming the shallowly southwest dipping nodal plane.
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ue ti nG March 1 0 Pa e 12 1983 Coalinga earthquake Azimuth to De Bilt
~ tatlall rra t
r (k " <</Dia IoBanyo wo Plant~..
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--Aolnpoc L
~Santa Bar L
S p ~~
~Back azr rnllrh rn
~a>adona ...,. " -'""'
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~
0 20 km a ~ ~
S$ SaS,.Sanli Lucia Bank...- ----"".......
Santa Barbara ata 1210 1204 1927 Lompoc earthquake epicenters Area encloses SSS-S Qe Byerly (1930) location estimates and uncertainty in tsunami S Hanks (1979) location Oo Gawthrop (1978)
Q PG&E (revised) ~~~Surface projection of Arcs drawn from De Bilt and Santa Barbara seismograph stations l rupture zone Surface projection of fault plane Figure GSG Q5-9 Location of the 1927 earthquake and estimates of uncertainty in location derived from tsunamis and teleseismic and regional body waves.
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uesti n M rch I Pa e I SEISMIC MOMENT The synthetic tsunamis in Figures GSG Q5-6, Q5-7, and Q5-8 were calculated using a seismic moment of 3 x 10 dyne-centimeters, and are shown on a common scale with the recorded tsunamis.
The recorded and synthetic amplitudes of the first few cycles of motion are within a factor of two or less at each station. We estimate the uncertainty in the seismic moment to be a factor of 1.5, or one-tenth of a unit of moment magnitude. This is consistent with the estimate of a factor of 1.5 uncertainty in moment estimated from the Kalapana earthquake.
A notable feature of both the recorded and synthetic amplitudes is the fact that the amplitude at Hilo is approximately twice that at San Francisco and La Jolla, despite its much greater distance. The large amplitude of the Hilo recording is due to the resonance of Hilo Bay (Wiegel, 1970). This resonance is dependent on the azimuth of the tsunami source, as indicated by the large variability in the correction terms for Hilo derived by Abe (1979) and noted by him. His correction terms for Hilo range from 0.0 to -0.6 for different azimuths. Because of the sparsity of data, he did not compute a correction term for tsunamis originating in California and, instead, used the average correction term of -0.3 for Hilo in estimating the tsunami magnitude of 7.6 for the 1927 Lompoc earthquake from the Hilo record. The use of an uncalibrated correction term appears to be the cause of the large value of tsunami magnitude that he obtained from the Hilo record. In his letter of April 28, 1989 (Table GSG Q5-3), Abe noted that he had obtained an estimate of 6.9 from the California records.
Our estimate of the seismic moment, derived from waveform modeling of the tsunami, directly includes the effect of the bathymetry between the source and the tide gauge stations and, thus, requires no correction. Both the Hilo station and the California stations are consistent with a seismic moment of 3 x 10 dyne-centimeters, which corresponds to a moment magnitude of 7.0. This value is consistent with the surface wave magnitude of 7.0, but larger than the seismic moment of 1 x 10 dyne-centimeters (corresponding to a moment magnitude of 6.6) derived from the modeling of long-period body waves described in the Final Report (PG&E, 1988).
REFERENCES Abe, K., 1979, Size of great earthquakes of 1837-1974 inferred from tsunami data: Journal of Geophysical Research, v. 84, p. 1561-1568.
Cohn, S. N., Hong, T. L., and Helmberger, D. V., 1982, The Oroville earthquakes: a study of source characteristics and site effects: Journal of Geophysical Research, v. 87, p. 4585-4594.
Cox, D. C., 1980, Source of the tsunami associated with the Kalapana, Hawaii, earthquake of November 1975: Report of the Hawaii Institute of Geophysics HIG-80-8, University of Hawaii, Honolulu, Hawaii, 46 p.
Gawthrop, W. H., 1978, Seismicity and tectonics of the central California coastal region; in Silver, E. A., and Normark, W. R., eds., San Gregorio-Hosgri fault zone, California: California Division of Mines and Geology Special Report 137, p. 45-46.
Hanks, T. C., 1979, The 1927 Lompoc, California earthquake (November 4, 1927, M = 7.3) and its aftershocks: Bulletin of Seismological Society of America, v. 69, p. 451-462.
Hatori, T., 1976, Wave source of the Hawaiian tsunami in 1975 and the tsunami behavior in Japan:
Zisin, v. 29, p. 355-363.
Pacific Gas and Electric Company, 1988, Final report of the Diablo Canyon Long Term Seismic Program: U. S. Nuclear Regulatory Commission Docket Nos. 50-275 and 50-323.
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uestion March I 0 Pa e l4 Table GSG Q5-3 LETTER FROM K. ABE TO K. SATAKE REGARDING THE 1927 LOMPOC EARTHQUAKE EARTHQUAKE RESEARCH INSTlTUTE THE UNIVERSITY OF 'K)KYO ADDRESS'VNKYO.KV, TOKYO, JAPAN OIS)
CABLC ADDRCSS t ZISINKCN TOKYO TELEX NVNBCRt 222 2IAS(ERI TOIO April 28, 1989
Dear Dr. Ken)i Satake:
Some years ago, I received the same questionnaires about the Lompoc tsunami of 1927, one from Tom Hanks and the other from Robert Page. There was a concern, at that time, with respect to the nuclear reactor. They recommended me to study further, but I have not, though I am still concerned over it. Do you remember that I previously summarized this topic at the seminor in Hokkaido University 2 I rigorously obtained MtN7.6 +/- from the records at Hilo and Honolulu in Hawaii. In the late study, I found that records in California give the smaller estimate, i.e. 6.9, as far as my method for use of local tide gage data is applied to the records. This difference between the near-source and the far-field Mt is suggestive of a strong directivity, but I am not confident of discussing any more without detailed study.
It is quite interesting that only the Lompoc tsunami was recorded at the Hawaiian Islands across the Pacific.
Sincerely,
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ueti n 5 March 1 0 P I Satake, K., 1989, Inversion of tsunami waveforms for the estimation of heterogeneous fault motion of large submarine earthquakes; the 1968 Tokachi-oki and 1983 Japan Sea earthquakes: Journal of Geophysical Research, v. 94, p. 5627-5636.
)Viegel, R. L., 1970, Tsunamis; in Wiegel, R. L., ed., Earthquake Engineering: Chapter 11, Prentice-Hall, New Jersey, 518 p.
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I ueti n S 7 March I Pa el QUESTION GSG 7 In U. S. Geological Survey Professional Paper 1223, "Seismic Intensities of Earthquakes of Conterminous United Slates--Their Prediction and Interpretation," Everenden, Kohler, and Clow used the observed intensity data from the 4 November 1927 earthquake and their predictive model to evaluate several estimates of the epicenter, fault length, and if fault orientation of this event. The authors conclude that, the general applicability of the predictive model is accepted, the intensity data from the Lompoc earthquake require a location on or very near the Hosgri fault. Provide a discussion of the conclusion from this study in light of PGd'cE's analysis of the earthquake.
Evernden and others (1981) describe an expanded procedure, based on their previous work, to calculate expected earthquake intensities for areas in the United States. Their model uses earthquake source parameters of fault length, fault strike, and focal depth, combined with regionally specified intensity attenuation with distance to predict the intensity expected at an arbitrary location. The site intensity can also be modified to reflect local geologic conditions assessed on a 1/2-minute-by-1/2-minute grid. Evernden and others (1981) developed a set of statistical analyses to compare observed and calculated intensity data. The statistics can then be used to evaluate various combinations of source parameters and intensity attenuation rates.
For their study of the November 4, 1927, earthquake, Evernden and others (1981) used the Rossi-Forel intensity data compiled by Byerly (1930), and assumed intensity attenuation and focal depth parameters typical for other earthquakes they previously studied in western California. The unknowns in their 1927 earthquake analysis were the event location, fault orientation, and fault rupture length. After analyzing 12 different models for fault location and orientation, and analyzing these models using various assumptions about site conditions, Evernden and others concluded that the intensity data for the 1927 earthquake "require a location" on or no more than a few kilometers west of the Hosgri fault, with an epicentral location just offshore of Point Sal and a rupture length of about 60 kilometers.
Recent studies of instrumental recordings of the 1927 earthquake have resulted in strong constraints on the earthquake source that were not available to Evernden and others (1981). Body-wave modeling (PG&E, 1988) of teleseismic and regional data indicates that the source has a nearly pure reverse-thrust mechanism, with planes striking north-northwest, and body-wave moment of 1 x 10 o dyne-centimeters. A variety of regional and teleseismic phase data have been analyzed to revise the epicenter to a location about 35 kilometers southwest of Point Arguello, with an uncertainty of 25 kilometers (PG&E, 1988; Response to Question 46, February 1989; Response to Question GSG 8).
Timing and modeling of tsunami records also constrain the location to be well offshore of Point Arguello and to have a long-period moment of 3 x 10 dyne-centimeters (Response to Question GSG 5).
The explanation for the inconsistency between the instrumental and intensity locations is based on the inadequacies of the available intensity data to adequately constrain the location of the event. The analytical procedure described by Evernden and others (1981) is predicated on the assumption that the intensity data of Byerly (1930) were adequately distributed around the epicenter to provide an accurately convergent result. The inadequacies in this data base are noted in the following drscussron.
Evernden and others (1981, page 6) note that if the intensity data for an earthquake are too limited at short ranges, the statistical analysis "can fail" even though data from all four quadrants may be available. Figure GSG Q7-1 shows the revised PG&E location of the 1927 earthquake southwest of Point Arguello plotted on the isoseismal map from Byerly (1930). The tsunami analysis discussed in Response to Question GSG 5 indicates that the waveforms as well as arrival times of the tsunami are inconsistent with the earthquake having occurred beneath water of depth shallower than about 200 meters (about 15 kilometers offshore). The closest land to the earthquake is Point Arguello Diablo Canyon Power Plant Pacific Gas and Electric Company long Term Seismic Program
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PG&E revised 85'100 Versura Monrovia Ox San Gabriel location Ouane Los Angeles Ponona e ~
21'204 1194 Redondo PL Vincent Pedro 11
~ Salsa Ana Figure GSG Q7-1 Isoseismal map for the November 4, 1927, earthquake from Byerly (1930), with the PG&E revised location, Hosgri fault zone, and azimuthal ranges of intensity coverage also indicated.
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uesi n 7 March I 0 Pa e itself, which sits on the southwest corner of the south-central California coast. As can been seen in Figure GSG Q7-1, because this corner points toward the epicenter, the land area nearest to the epicenter is very small in comparison with the ocean area. It is quite evident, therefore, that the short-range intensity data are severely limited at short ranges. Thus, for the purposes of the Evernden and others'nalysis, the short-range intensity data set appears inadequate to evaluate the PG&E location of the 1927 earthquake.
Evernden and others (1981, page 6) also note that their statistical analysis should work well for intensity observations that are well-distributed in distance, even when the range of azimuths from the event to the intensity reports is as small as 120 degrees. The implication of their statement is that for smaller ranges of azimuthal coverage, the statistical convergence will degrade. For the PG&E location of the 1927 earthquake shown in Figure GSG Q7-1, the azimuthal range for intensities VIII(R/F) and larger is about 85 degrees, and the range for intensity VI-VII(R/F) and larger is 110 degrees.
At best these ranges would appear to be marginal according to Evernden and others.
All the other earthquakes analyzed by Evernden and others appear to have at least 180 degrees of coverage. The range for higher intensities is particularly important, for these provide most of the possible location constraint. Since this range is less than one fourth of the desired coverage for the 1927 event, the azimuthal range of intensity data does not appear adequate to statistically test the PG&E location.
In the course of testing various fault location models for the 1927 earthquake, Evernden and others (1981, page 28) make a "major point" to place the modeled fault "near Point Arguello in order to explain the observed [Rossi/Forel intensity] IX values in this area." The intensity IX ratings were assigned by Byerly using the field data he collected shortly after the earthquake; this information is presented in Byerly (1930, Table 1, page 53) and is as follows:
~ Honda--Several hundred thousand cubic feet of sand were shaken down from the cliff to the beach below.
~ Roberds Ranch [east of Surf]--Man thrown from feet; house shifted on foundations; chimney thrown down, earthquake fountains; earth lurched; cracks in ground.
~ Surf--People thrown from beds; sand-blows and cracks in sand; concrete highway cracked; small dirt-falls; railroad bridge thrown out of line.
~ White Hills--Poorly built block walls collapsed.
The first three of these reports are primarily geologic features indicative of liquefaction, lateral spreading, and landsliding. As noted by Toppozada and Parke (1982, pages 5 and 6) in their compilation of earthquake damage due to earthquakes in California, earthquake intensities derived from ground failures "are often much higher than those indicated by structural damage in the same area." The structural damage cited above that is independent of geologic failures appears more consistent with Rossi/Forel intensity VIII-,"considerable [damage] in poorly built or badly designed structures; some chimneys broken" (Evernden and others, 1981, page 55). Thus the use of intensity IX as a strong constraint on the intensity modeling seems unwarranted.
Evernden and others (1981) note another problem with intensity IX. In several preferred cases for the location of the 1927 earthquake along the Hosgri fault zone near Point Sal, Evernden and others comment that "they predict too large an area of intensity IX." The cause of the intensity IX predictions is the placement of the assumed source very close to shore and the use of saturated ground conditions to increase the predicted intensities. Evernden and others argue that the unobserved intensity IX predictions may be due to a locally incorrect adjustment for the intensity effects of alluvial site conditions and to a lack of intensity observations near shore, rather than the use of an incorrect location in the source model.
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u tin 7 Mrchl p 4 In summary, the predictive intensity model that Evernden and others (1981) applied to a number of California earthquakes appears to be generally applicable when the source and attenuation parameters are accurately defined and when site conditions are accurately known. However, when the model is used to invert intensity observations for an unknown source location, rupture length, and orientation, as Evernden and others attempted for the 1927 earthquake, inadequacies in the observational data can provide inadequate constraints for the inversion procedure to correctly locate the earthquake. Because Evernden and others did not have confident independent knowledge of the location and other source characteristics of the 1927 earthquake, they were not able to recognize that the intensity data set itself was inadequate in both short-range coverage and azimuthal coverage for proper convergence. And finally, geologic effects need to be separated from structural damage in evaluating high intensities.
The consistency of the intensity data with the PG&E location of the 1927 earthquake is further addressed in the Response to Question GSG 6.
REFERENCES Byerly, P., 1930, The California earthquake of November 4, 1927: Seismological Society of America Bulletin, v. 20, p. 53-66.
Evernden, J. F., Kohler, W. M., and Clow, G. D., 1981, Seismic intensities of earthquakes of conterminous United States--their prediction and interpretation: Geological Survey Professional Paper 1223, 56 p.
Toppozada, T. R., and Parke, D. L., 1982, Areas damaged by California earthquakes, 1900-1949:
Open-File Report 82-17 SAC, California Division of Mines and Geology, 64 p.
Diablo Canyon Power Plant Pacific Gas and Electric Company long Term Seismic Program
I u i n March I 0 Pa e2 1983 Coalinga earthquake Azimuth to De Bilt station aa,
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~ ~ ~oatr,o paean t 'na 24 ktn ntn ta a ~t ~ BSss,.Santa,t,nofa Bantt 4 ~ ASS P>tiga 344 PC 1204 121'XPLANATION 1927 Lompoc earthquake epicenters 0 Earthquakes Oe Byerly (1930) a Santa Lucia Bank (10/22/1969)
I IB) Hanks (1979) 0 Gawthrop (1978)
PGRE (revised) b Santa Lucia Bank (11/5/1969) c Point Sal (5/27/1 980) d Point Conception (08/27/1949)
Arcs drawn from De Bilt and Santa Barbara seismograph stations Figure GSG Q8-1 Location of the 1927 earthquake and estimates of uncertainty in location derived from teleseismic and regional body waves.
Diablo Canyon Power Plant s Pacific Gas and Electric Company Long Term Seismic Program
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uesti n 8 March 1 0 P e North-South Lompoc Mainshock 11-04-27 W-A .8sec Lornpoc A ftershock 11-05-27 W-A 6.sec Point Conception 8-27-49 W-A 6.scc Santa Lucia Banks 11-05-69 W-A .8sec 18 888SEC Figure GSG Q8-2 IUaveforms of earthquakes off Point Conception recorded at Pasadena (PAS); amplitudes are normalized.
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uestion 8 March 1 p 4 example, the Point Sal and both Santa Lucia Bank events.) This result is not surprising, because the first half-cycle of the Pwave for this data set is about 2 seconds long.
The minimum in the TAN power versus BAZ curve is robust, and is illustrated in Figure GSG Q8-3, along with the algorithm described above. The curve shown is for the Point Sal event having a minimum in the power at BAZ 295.3 for a north-south shift of 0.0 second. The actual BAZ at Point Sal is 293.3.
The back azimuth vector results are summarized in Figure GSG Q8-4. The solid stars are from locations cited above, and the open symbols are BAZ estimates from this study. The results from the different instruments are shown as different open symbols; the error bars signify the uncertainty due to digitizer error in time origin placement on the record. The bar spans the results from the different north-south shifts, and the symbols are plotted at the average of these points. The estimates from the long-period data (Benioff 1-90 and W-A 6-second torsion) agree well with those from the assumed locations. The Point Conception BAZ estimates from long periods are almost 5 degrees larger than our assumed location BAZ, but this event occurred in 1949 and its location is less precise. However, both Santa Lucia Bank events and the Point Sal event have BAZ estimates from long periods that are very close to those of our assumed locations.
For the Lompoc mainshock and aftershock, the assumed location is the revised Long Term Seismic Program (PG&E, 1988) location described in the Response to Question 46, February 1989, and shown in Figure GSG Q8-1. For the aftershock, the long-period BAZ calculation is very close to our mainshock location BAZ; the uncertainty bar encompasses this location. A significant finding is that the aftershock BAZ calculation is very different from that of the Point Sal event, which is close to Gawthrop's (1978) location for the Lompoc mainshock.
The short-period BAZ estimates are consistently smaller than those from the long-period estimates and from assumed locations. This is probably due to receiver effects at PAS. Unfortunately, the only existing PAS records for the Lompoc mainshock are the short-period Wood-Anderson records.
The BAZ estimate for the mainshock of 269 degrees is about 11 degrees smaller than that for the revised PG&E location, consistent with the difference in BAZ estimates from assumed locations using short periods of the other events (Figure GSG Q8-4). If we increase the BAZ estimate for the Lompoc mainshock by 11 degrees, which is the average difference between assumed values and those estimated from short-period Wood-Anderson seismograms for the Point Conception and Santa Lucia Bank (November 5, 1969) events, we obtain an azimuth of 280 degrees. This azimuth intersects the Santa Barbara S-P arc of Hanks (1979) at a latitude of 34.35'N at a point that is identical to the revised PG&E (Response to Question 46, February 1989) location of the Lompoc mainshock.
We conclude that the particle motions of P waves recorded at Pasadena are consistent with the latitude of the PG&E (1988) location of the Lompoc mainshock, and inconsistent with the latitude of Gawthrop's (1978) location near Point Sal.
LONGITUDE OF THE 1927 LOMPOC EARTHQUAKE BASED ON S-P TIMES AT SANTA BARBARA We have re-read the S-P times of the aftershocks of the 1927 Lompoc earthquake from the original Santa Barbara seismograms. The S-P times were measured from both the north-south and east-west components for aftershocks having an impulsive P-wave and a clearly visible S-wave onset. In this distance range (about 100 kilometers), the first P and S arrivals are direct waves that propagate entirely with the crust (P and S ). Of the approximately 390 aftershock S-P times cited in the unpublished California Institute of Technology (CIT) tables used by Hanks (1979), only 27 aftershocks met the above criteria. However, the mean of our S-P times is 12.8 seconds, which agrees with the mean of the CIT times, 12.8 seconds, cited by Hanks (1979). In Table GSG Q8-1, our times are listed together with the times originally read at CIT. The correlation between our times and the CIT times is shown in Figure GSG Q8-5. We do not see a systematic difference between the two sets of times.
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e i n March 1 Pa e FOR NS umc shift = -0.2, -0.1, 0.0, 0.1, 0.2 Point Sal 1-90 NS time shift = 0.0 FOR angle = BAZ-40 to BAZ+40 Compute TAN from NS and EW Compute gN TAN,-
i=i nexr angle Save minimum gN TAN; & its BAZ i=i 250 270 290 310 330 rrexr NS time shift Back Azimuth Figure GSG QS-3 Illustration of the procedure used to estimate back azimuth from the Pasadena records.
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uesti n M rch l 0 Pa e6 300 290 Explanation 280 Benioff 1-90 W-A 6sec Torsion 270 Oi 0 W-A .8sec Torsion 260
- From other studies 250 O
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uesti n 8 March 1 Pa e7 Table GSG QS-1 Ts TIMES OF THE NOVEMBER 1927 LOMPOC AFTEREHOCKS RECORDED AT SANTA BARBARA (SBC)
Ch I,rEP Date TIIIle (sec) (sec) 11/04/27 10:52 12 12.2 16:17 16(') 13.9 19:28 11.5+ 12.0 20:06 12.0 12.4 20:43 12.0 12.6 21:30 12.5 14.5 11/05/27 19:00 12.5 11.7 20:26 12.8x 12.2 20:51 13 1 1.8 20:53 12.0a 12.4 20:58 12 11.5 23:09 12.1 12.4 11/06/27 01:05 13.4 13.4 02:29 12.5 12.6 02:40 13.5 14.0 02:49 12.5 12.5 04:49 14.5 14.7 09:11 14 12.4 09:39 12.0+ 11.9 12:02 12 1 1.8 17:27 13 13.0 23:02 13 13.3 11/07/27 01:30 12.5+ 12.6 03:43 13 14.4 04:55 14 13.8 06:20 13.5 13.2 07:17 12 15.1 NOTES:
Time is given as hours:minutes and is approximate time of aftershock.
CIT is the Ts > time as it appears in the Caltech unpublished tables.
LTSP is the new estimate of Ts p, yielding an average of 12.9 seconds.
Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
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Figure GSG Q8-5 Comparison of CIT and Long Term Seismic Program S-P times at Santa Barbara for aftershocks of the 1927 Lompoc earthquake.
Diabio Canyon Power Plant Pacific Gas and Electric Company Lang Term SeismIc Program
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ueti n M rch 1 Pa Hanks (1979) used this S-P time of 12.8 seconds to draw an arc from Santa Barbara to locate the 1927 earthquake, using the travel-time curve of Richter (1958) for the southern California region. To evaluate this travel-time curve, we read the S-P times at Santa Barbara of more recent earthquakes located in the vicinity of the 1927 earthquake, specifically those larger than magnitude 3 between latitude 34'5'N and 34'35'N and longitude 120'35W and 121'10'W between 1980 and 1989 (Table GSG Q8-2). The S-P times are plotted against epicentral distance in Figure GSG Q8-6, together with Richter's curve and the S-P time of Hanks (1979). These results indicate that west of Santa Barbara, the Richter curve overestimates the epicentral distance for a given S-P time by less than 10 kilometers on average, which is within the uncertainty of 10 kilometers assigned to the Santa Barbara arc, as shown in Figure GSG Q8-1.
We conclude that the CIT S-P times, and the arc drawn by Hanks (1979) from Santa Barbara based on these times are accurate. This arc was used in constructing the revised PG&E location shown in Figure GSG QS-1. This in turn indicates that the longitude of the PG&E location, at 120.9'W, is also accurate.
LOCATION OF THE 1927 LOMPOC EARTHQUAKE BASED ON S-P TIMES AT SANTA BARBARA AND AZIMUTHOF P WAVES AT PASADENA Combining the longitude of 120.9 W estimated from our reading of S-P times at Santa Barbara with the azimuth from Pasadena from the short-period Wood-Anderson (280 degrees; corrected by 11 degrees as described above), we obtain a location for the 1927 Lompoc earthquake that is identical to the revised PG&E location derived from S-P times at Santa Barbara and S-P and SSS-S times at De Bilt. This location, described in the response to Question 46, February 19S9, and substantiated further here, is at a latitude of 34.35'W and a longitude of 120.9'W, as shown in Figure GSG QS-1.
REFERENCES Eaton, J. P., 1984, Focal mechanisms of near-shore earthquakes between Santa Barbara and Monterey, California: U. S. Geological Survey Open-File Report 84-477.
Gawthrop, W. H., 1978, Seismicity and tectonics of. the central California coastal region; in Silver, E. A., and Normark, W. R., eds., San Gregorio-Hosgri fault zone, California: California Division of Mines and Geology, Special Report 137, p. 45-46.
Hanks, T. C., 1979, The 1927 Lompoc, California earthquake (November 4, 1927, M 7.3) and its aftershocks: Bulletin of the Seismological Society of America, v. 69, p. 451-462.
Hileman, J. A., Allen, C. R., and Nordquist, J. M., 1973, Seismicity of the southern California region, 1 January 1931 to 31 December 1972: Seismological Laboratory, California Institute of Technology, 487 p.
Richter, C. F., 1958, Elementary Seismology: W. H. Freeman, San Francisco, 343 p.
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I u i n Mr h1 a el0 Table GSG QS-2 S-P TIMES OF RECENT EARTHQUAKES OFF POINT CONCEPTION Time Latitude Longitude Depth Mag- Ts p No. (Figure Date hrmin sec ~N ~W ~km n~iud GSR2~6 07/18/80 05:14 50.99 34 34.00 120 41.95 4.75 3.00 12.7 05/10/85 15:48 00.34 34 25.12 120 45.60 7.64 3.69 11.8 11/23/86 02:08 59.77 34 17.69 120 39.30 13.27 3.07 02/27/87 22:43 19.32 34 31.00 120 46.51 0.00 3.61 08/06/88 05:35 10.86 34 30.46 120 48.59 12.21 3.00 13.1 09/24/88 07:33 44.60 34 24.45 120 47.77 13.19 3.20 +
01/09/89 23:01 17.16 34 29.61 120 43.73 9.11 4.00 12.3 01/10/89 00:34 37.65 34 25.25 120 45.15 5.64 3.00 11.6 01/10/89 12:45 42.32 34 28.97 120 42.38 8.67 3.10 11.2 01/10/89 17:21 21.51 34 29.64 120 41.52 4.38 3.10 11.1 04/26/89 14:47 10.40 34 26.79 120 46.29 5.95 3.80 12.3 10
~NT~E:
Illegible seismogram No time marks Time period: 1980-1989 Magnitudes: 3 or greater Latitudes: 34'5'o 34'35'20'35'o Longitudes:
121'10'acific Diablo Canyon Power Plant Gas and Electric Cantpany Long Term Seismic Program
I uetion S 8 M rch I Pa e ll 15 14 13 0
0 1
0 0 12 6 10 0
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10 95 105 110 115 Distance (km)
Figure GSG QS-6 Comparison of S-P times at Santa Barbara for recent earthquakes off Point Conception with times predicted by the Richter (1958) curve. The 1927 Lompoc earthquake is indicated by an L.
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ueti n S Pa el QUESTION GSG 9 The seismogenic structure on which the 4 November 1927 earthquake occurred should be identi fied and evaluated for maximum earthquake potential and its closest approach to the Diablo Canyon site. Account for this in both the probabilistic and the deterministic analyses.
STRUCTURAL ASSOCIATION OF THE 1927 LOMPOC EARTHQUAKE Considerable recent work has been directed toward locating the 1927 earthquake and estimating its magnitude and mechanism. To reduce the uncertainty in epicentral location and place better constraints on the mechanism and magnitude of the 1927 Lompoc earthquake, regional and teleseismic waveform data were analyzed (PG&E, 1988, p. 2-54 to 2-63; Response to Question 46, February 1989). A discussion of the tsunami data and modeling results is provided in Response to Question GSG 5, and the analyses of intensity data and seismograms updated subsequent to our Final Report in 1988 is provided in the Responses to Questions GSG 6, GSG 7, and GSG 8.
Analyses of these data constrain the location of the event to be about 35 kilometers southwest of Point Arguello at approximately 34.35'N, 120.9 W (Figure GSG Q9-1). The total uncertainty in the location is estimated to be 25 kilometers (Response to Question 46, p. 5). The location constraints clearly do not permit an interpretation of an epicenter near Point Sal (for example, Gawthorp, 1978),
nor an association with either the Hosgri fault zone or faults associated with the Casmalia block (for example, the Casmalia, Orcutt, Pezonni, or Lion's Head faults). In addition, the epicentral location is well south of the southern end of the Lompoc fold, thus precluding any postulated association with this structure (for example, Yerkes and Lee, 1979). The earthquake mechanism is nearly pure reverse slip on a plane striking N20'W and dipping either 66'NE or 23'SW (PG&E, 1988, Figure 2-20); the focal depth is 10 kilometers. The seismic moment, estimated from long-period body waves, is 1 x 102 dyne-centimeters, corresponding to a moment magnitude of 6.6. The seismic moment estimated from tsunami records is 3 x 10 dyne-centimeters, corresponding to a moment magnitude of 7.0. The surface-wave magnitude is 7.0 (PG&E, 1988, p. 2-62 to 2-64).
Because of its far offshore location and epicentral uncertainty, it is difficult to associate the 1927 Lompoc earthquake with a specific geologic structure. The epicentral area is about 35 kilometers southwest of Point Arguello, west of areas of recent petroleum interest. Consequently, available "geophysical coverage in the region is minimal. In the 1986 deep crustal study, Rice University acquired a common-depth-point (CDP) seismic line (RU-10) that trends nearly north-south a few kilometers east of the epicenter (Figure GSG Q9-1). Interpretation of these data indicate that several northeast-dipping reverse faults are present in the vicinity of the epicenter (Figure GSG Q9-2)
(Nicholson and others, 1989; C. Nicholson pers. comm., 1990). The more northerly fault near shot point 920 lies beneath an anticline that deforms late Tertiary and potentially younger Quaternary strata, suggesting that the fold and underlying reverse fault are active. The underlying basement, however, is displaced down on the east, suggesting the contemporary reverse displacement is a reactivation of former basin-margin normal or transtensional strike-slip faults. These faults lie within a transitional zone between thick Tertiary deposits in the southeastern offshore Santa Maria Basin to the northeast, and thin Tertiary deposits over shallow basement in what appears to be the southeastern extension of the Santa Lucia Bank High (Figure GSG Q9-2). The sense of basement separation indicates that the faults separate the southern part of the Santa Lucia Bank on the west from the southern part of the offshore Santa Maria Basin on the east.
As shown on Figure GSG Q9-2, the structures observed on RU-10 are in a region containing numerous north-northwest to northwest-trending reverse faults and folds. The northwest-trending faults may be the southeastern extension of the Santa Lucia Bank fault zone, which separates the Santa Lucia Bank High from the offshore Santa Maria Basin to the northeast (Figure GSG Q9-1).
The Santa Lucia Bank fault zone consists of several near-vertical to steeply northeast dipping fault strands that extend N25'W to N30'W for more than 150 kilometers along the western margin of the offshore Santa Maria Basin (McCulloch, 1987). The fault separates unlike basement referred to by Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
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>> /r 0 20 km 1224 1214 1204 EXPLANATION Marine reflection line Airgun refraction line Sonobouy recording High explosive shot Refraction recorders Seismograph station Note: Base map exphnatlon provided In PG&E (I Qee, Phte 3).
Figure GSG Q9-1 Location of the 1927 Lompoc earthquake and seismic reflection line RU-IO.
Diablo Canyon Power Plant Pacific Gas and Electric Company long Term Seismic Program
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0 15 kilometers Uncertainly ondoses SSS-S Sourcos of fault data: tocabon estimatos and uncertainty
~ Mccultoch, 1987 in tsunami ication (seo Response
~ Willingham and Hamilton, 1982 to Question GSG 5)
~ pGaE, 1988 Figure GSG Q9-2 Map of geologic structures in the epicentral region of the 1927 Lompoc earthquake.
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ue in Pa e4 Howell and others (1986) and McCulloch (1987) as the Patton and San Simeon terranes to the west and east of the fault zone, respectively (Figure GSG Q9-3). The Santa Lucia Bank basement high lies within the Patton terrane to the west and the southern offshore Santa Maria Basin lies within the San Simeon terrane to the east (Figure GSG Q9-1).
The Santa Lucia Bank fault, like the fault observed near shot point 920 on RU-10, has experienced multiple periods of activity. McCulloch (1987) reports a significant period of normal or transtensional faulting in the Miocene that produced down-on-the-east displacement and foundering of the western margin of the offshore Santa Maria Basin. Subsequent compressional tectonism in the Pliocene and Quaternary reactivated parts of the fault zone with reverse up-on-the-east displacement. As observed on seismic reflection data (for example, PGE 1, 3), there are small anticlines that deform late Tertiary strata above the fault along the eastern margin of the Santa Lucia Bank high. These anticlines are evidence of the probable late Cenozoic reactivation of the Santa Lucia Bank fault as a high-angle, northeast-dipping reverse fault or transpressional strike-slip fault.
The estimates of the size of the 1927 earthquake (Ms 7.0, seismic moment 1 - 3 x 10 dyne-centimeters) imply a length of rupture of about 30 kilometers or more, based on empirical relations between magnitude and rupture length and rupture area (for example, Wells and others, in preparation; Wyss, 1979). Evidence from worldwide surface ruptures has shown that in nearly all cases, faults do not rupture their entire length during individual earthquakes (Wentworth and others, 1969; Slemmons, 1982; Schwartz and Coppersmith, 1986; Schwartz, 1988; Knuepfer, 1989) and typically they rupture less than half their length during individual events (Wentworth and others, 1969; Slemmons, 1982). This implies that the source structure for the 1927 event is probably a significant fault zone whose total length is about 100 kilometers or more. The only known prominent structure in the source region having these dimensions is the Santa Lucia Bank fault zone.
The uncertainty in epicentral location of the 1927 event encompasses a large part of the southern Santa Lucia Bank High, a small part of the southern offshore Santa Maria Basin, and the transition zone between these two areas (Figure GSG Q9-2). The 1927 event may have occurred on a fault within one of these areas. Although no prominent faults have been identified within these areas, seismic data are sparse, especially in the southern Santa Lucia Bank region, and unidentified folds
~
and faults may be present. Farther north within the central Santa Lucia Bank High, numerous northwest-trending, laterally continuous faults have been identified by McCulloch (1987). The occurrence of earthquakes in 1969 having reverse mechanisms and broadly distributed microseismicity indicates that at least some structures within the bank are seismically active. Data are not available to evaluate the southern extension of these structures into the southern Santa Lucia Bank High. McCulloch (1987) also identifies several folds and faults in the southern offshore Santa Maria Basin (Figure GSG Q9-2). These structures, as mapped, are relatively short and laterally discontinuous. They do not appear to be likely candidate structures for an event as large as the 1927 earthquake, although microseismicity occurs in the region and the presence of a larger fault cannot be precluded using existing data from the region.
To summarize, we conclude that the 1927 Lompoc earthquake probably occurred on the southern Santa Lucia Bank fault zone. Alternative candidate structures for the earthquake include a currently unrecognized fault within the southern Santa Lucia Bank High, or a fault within the southernmost part of the offshore Santa Maria Basin. We favor the Santa Lucia Bank fault zone as the most likely candidate structure for several reasons:
Spatial association. The epicenter of the earthquake is within the zone of surface faults identified by McCulloch (1987) as the southern Santa Lucia Bank fault. The epicenter is close to a prominent fault-and-fold couple observed on the RU-10 common-depth-point seismic reflection line (Nicholson and others, 1989) that lies along trend of the Santa Lucia Bank fault and appears to have a history of activity and contemporary style of deformation similar to that of the Santa Lucia Bank fault.
Style of deformation. The contemporary style of deformation on the Santa Lucia Bank fault is consistent with the compressional mechanism of the earthquake. Although located Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
I i n Pa e 125e 124e 123e 121'20e MFZ e e e King Range San Simeon e
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~ EXPLANATION Pateosuhtuctton zone Strike slip Point Arena e
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~r e t 125e 124'23e 122'21'2oe Modified from Mcculloch (1 987)
Figure GSG Q9-3 Map showing distribution of basement terranes along the western margin of California. See Howell and others (1986) for a description of basement lithologies.
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uestion S Pa e in the western Santa Lucia Bank area, the two 1969 earthquakes also had compressional focal mechanisms, similar to the 1927 Lompoc earthquake.
Fault geometry. The strike and dip of the Santa Lucia Bank fault zone are consistent with the N20 W strike and 66'NE dip of the focal plane of the earthquake.
Fault dimensions. The rupture dimensions necessary to produce the 6.6 - 7.0 Mw, 7.0 Ms 1927 Lompoc earthquake are about 30 to 60 kilometers. Evidence from rupture segmentation suggests it is likely that the 1927 rupture occurred on a segment of a relatively large, laterally continuous fault probably more than 100 kilometers long. The Santa Lucia Bank fault is a long-recognized, prominent fault zone in offshore south-central California. There are no other large, prominent faults or folds identified in the source region.
CHARACTERIZATIONOF SANTA LUCIA BANK FAULT AS A SEISMIC SOURCE The location of the 1927 earthquake and the uncertainties in its location place the earthquake in a region where it may be associated with one of the following structures: the Santa Lucia Bank fault zone along the eastern side of the bank, faults within the Santa Lucia Bank, or other faults/folds in the region of intersection between the southern offshore Santa Maria Basin and the western Transverse Ranges structures. As discussed previously, the 1927 earthquake was probably associated with a prominent, laterally continuous fault zone. Hence, the Santa Lucia Bank fault zone is the most likely candidate structure of the possibilities given above. Further, the Santa Lucia Bank fault zone in its northern reach is the closest structure to the site and its lateral continuity implies a larger maximum earthquake than the other candidate structures. Therefore, from the standpoint of seismic source and ground motions, the Santa Lucia Bank fault zone will represent a more conservative scenario for evaluation than the other two structures. We specifically consider the earthquake potential of the Santa Lucia Bank fault zone below.
To characterize the Santa Lucia Bank fault for probabilistic and deterministic ground motion analyses, we must evaluate the source location relative to the site, the sense of slip, the maximum earthquake magnitude, and earthquake recurrence. Each of these characteristics is discussed below, followed by the implications to the ground motions analyses.
The 1927 Lompoc earthquake occurred about 80 kilometers from the site, based on our best estimate of its location. As discussed previously, several lines of evidence suggest that the most likely candidate structure for this event is the southern Santa Lucia Bank fault zone, although the offshore location of the event precludes an unequivocal association with any structure. We assume for this analysis that the northern and southern parts of the Santa Lucia Bank fault zone act as a single structure in terms of accommodating regional strain. Assuming that the northern Santa Lucia Bank fault zone is the source of interest, we consider the easternmost fault in this analysis because it is the most laterally continuous and is closest to the site. As discussed previously, the Santa Lucia Bank fault along the eastern side of the Santa Lucia Bank high is interpreted to be a steeply northeast dipping reverse fault or transpressional strike-slip fault in the present tectonic regime. The focal mechanism for the 1927 earthquake indicates reverse slip. Given the northern Santa Lucia Bank fault is a high-angle reverse or oblique strike-slip fault related to the southern Santa Lucia Bank fault, we consider for seismic source characterization purposes a 150-kilometer-long fault zone whose closest distance is about 45 kilometers from the site.
The assessment of the maximum earthquake magnitude along the Santa Lucia Bank fault is hindered by the paucity of high-quality seismic reflection data to define the map pattern and, thereby, potential segmentation of the fault zone. Clearly, our assumption that the 1927 event is associated with the fault places a minimum limit on the maximum magnitude of 6.6 -7.0 Mw, 7.0 M>. The map pattern of the Santa Lucia Bank fault (McCulloch, 1987; PG&E, 1988, Plate 3) is based on a limited number of seismic lines and, as a result, is probably represented as a more continuous, less complex fault zone than it actually is. However, accepting the map pattern as it is, we can identify changes Diablo Canyon Power Plant Pacific Gas and Electric Company long Term Seismic Program
I ueti n P e7 in the fault zone that may be points of segmentation. A possible segmentation point lies just north of 35'N latitude, which is marked by an apparent discontinuity in the fault trace, a right-stepping pattern, and a bend of about 15 degrees to the north. This point is also at the latitude marking the approximate northern limit of observed seismicity beneath the Santa Lucia Bank. Another possible segmentation point is identified to the south at about 34'30'N, where the Santa Lucia Bank fault is inferred by McCulloch (1987) to bend over 30 degrees to a more easterly orientation (Figure GSG Q9-1). McCulloch (1987) shows the fault zone as branching into a number of fault traces, although this map pattern is based on limited geophysical data. Willingham and Hamilton (1982) and preliminary results from RU-10 suggest that the prominent basement displacement mapped by McCulloch (1987) as the Santa Lucia fault to the north continues on trend to the south without a 30-degree bend. Based on the limited data and published interpretations available, however, the Santa Lucia Bank fault does appear to branch into a wider, more diffuse zone southward at about 34'30'.
These two segmentation points, at 35'N and 34'30'N, separate the fault into segments having lengths of 65, 60, and 30 kilometers, north to south.
The northern two segments are equidistant from the site and only the longer northern segment is treated here as a potential seismic source. The magnitudes that would be associated with a 65-kilometer-long segment assumed to have a down-dip width of about 12 kilometers (the average maximum depth of seismicity in the region) are given below as a function of the sense of slip:
LeenLh~km ~ra ~kmz 65 Reverse. 7.35 Strike-slip 7.15 780 Reverse or Strike-slip 6.93 The calculated magnitudes are from the regressions by Wells and others (in preparation; Attachment GSG Q14-A). Both reverse and strike-slip regressions are used due to uncertainty in the sense of slip on the Santa Lucia Bank fault, particularly in its more northern reaches closest to the site. From the calculations, we estimate the maximum magnitude on the northern segment of the Santa Lucia Bank fault to be 7.1 Mw. Ground motions at the site (84th-percentile peak ground acceleration) from this event at a distance of 45 kilometers would be 0.16 g, well below ground motions associated with the Hosgri fault zone.
To evaluate the contribution that the Santa Lucia Bank fault might make to the probabilistic seismic hazard analysis, earthquake recurrence rate must be estimated. The seismicity associated with the fault zone is insufficient to estimate the recurrence rate because of the low levels of seismicity during the historical record, as well as probable catalog incompleteness of moderate magnitudes due to the distant offshore location. For other faults in the site region, we have used late Quaternary geologic fault slip rates to estimate recurrence rates. However, we do not have similar data for the Santa Lucia Bank fault. We do know that recent interpretations of relative plate velocities (for example, DeMets and others, 1987), when compared to fault slip rates in the onshore and near-offshore region, do not require significant amounts of additional strain in the offshore region to account for interplate motion. This conclusion precludes the Santa Lucia Bank from having very high slip rates. An alternative approach to the recurrence problem is to begin with the probabilistic seismic hazard calculated for the Hosgri fault zone (which contributes over 95 percent to the site hazard) and to calculate the recurrence and slip rate that would be required for the Santa Lucia Bank fault to contribute equivalently to the hazard (see PG&E, 1988, Chapter 6 for discussion of probabilistic hazard results). This calculation shows that to equal the Hosgri's contribution to the site hazard, the Santa Lucia Bank fault would be required to have a totally unrealistic slip rate of 1000 millimeters per year. For potentially realistic slip rates on the Santa Lucia Bank fault, say in the 1.0-to 10-millimeter-per-year range, the contribution to the site hazard is three to four orders of magnitude below the Hosgri. We therefore conclude that given any realistic assessment of the Santa Lucia Bank fault, it has a negligible contribution to the probabilistic seismic hazard at the site (Figure Q9-4).
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ue in 10 Hosgri 10 Santa Lucia Bank 10 10-'.
10 7
10 5 .5 .75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 Spectral Acceleratiort, 8 to 8.5 Hz (g)
Figure GSG Q9-4 Frobabilistic seismic hazard curves comparing the site hazard due to the Hosgri fault and the Santa Lucia Bank fault. Note that the range of spectral acceleration important to the plant is 1.25 g to 2.0 g.
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ue ion Pa e REFERENCES DeMets, C., Gordon, R. G., Stein, S., and Argus, D. F., 1987, A revised estimate of Pacific-North America motion and implications for western North America plate boundary zone tectonics:
Geophysical Research Letters, v. 14, no. 9, p. 911-914.
Gawthrop, W. H., 1978, Seismicity and tectonics of the central California coastal region; in San Gregorio-Hosgri fault zone, California: California Division of Mines and Geology Special Report 137, p. 45-56.
Howell, D. G., Champion, D. E., Vedder, J. E., 1986, Terrane accretion, crustal kinematics and basin evolution, southern California; in Ingersoll, R., and Ernst, G., eds., Evolution of Sedimentary Basins, Southern California: Rubey Vol. VI, Prentice-Hall, Inc., Englewood Cliffs, New Jersey.
Knuepfer, P. L. K., 1989, Implications of the characteristics of end-points of historial surface fault ruptures for the nature of fault segmentation; in Fault Segmentation and Controls of Rupture Initiation and Termination: U. S. Geological Survey Open-File Report 89-315, p. 193-228.
McCulloch, D. S., 1987, Regional geology and hydrocarbon potential of offshore central California; in Scholl, D. W., Grantz, A., and Vedder, J., eds., Geology and Resource Potential of the Continental Margin of Western North America and Adjacent Ocean Basins, Beaufort Sea to Baja California:
American Association of Petroleum Geologists Circum Pacific Earth Science, v. 6, p. 353-401.
McCulloch, D. S., Heston, U. S., and Rubin, D. M., 1980, A summary report of the geology and geologic hazards in proposed lease sale 53, central California outer continental shelf: U.S. Geological Survey Open-File Report 80-1095, 12 maps, scale 1:25,000, 76 p.
McCulloch, D. S., Utter, P. A., and Menack, J. S., 1985, Maps showing locations of selected pre-Quarternary rock samples from 34'30'N lat to 42'N lat, California continental margin: U. S.
Geological Survey Miscellaneous Field Studies, MF 1719, 4 maps, scale 1:250,000, 38 p.
Nicholson, C., Sorlien, C., and Luyendyk, B. P., 1989, Reprocessing of Line RU-10, Offshore southern Santa Maria Basin, California (abs.): EOS Transactions, American Geophysical Union, v.
70, no. 43, p. 1214-1215.
Pacific Gas and Electric Company, 1988, Final report of the Diablo Canyon Long Term Seismic Program: U. S. Nuclear Regulatorty Commission Docket Nos. 50-275 and 50-328.
Schwartz, D. P., 1988, Geology and seismic hazards; moving into the 1990's: Earthquake Engineering and Soil Dynamics II - Recent Advances in Ground Motion Evaluation, ASCE Geotechnical Special Publication 20, p. 1-42.
Schwartz, D. P., and Coppersmith, K. J., 1986, Seismic hazards: new trends in analysis using geologic data; in Active Tectonics: National Academy Press, Washington, D.C., p. 215-230.
Slemmons, D. B., 1982, Determination of design earthquake magnitudes for microzonation:
Proceedings of the Third International Earthquake Microzonation Conference, v. 1, p. 119-130.
Wells, D. L., Coppersmith, K. J., Slemmons, D. B., and Zhang, X., in preparation, Earthquake source parameters; updated empirical relationships among magnitude, rupture length, rupture area, and surface displacement.
Wentworth. C. M., Bonilla, M. G., and Buchanan, J. M., 1969, Seismic environment of the sodium pump test facility at Burma Flats, Ventura County, California: U.S. Geological Survey Open-File Report, 42 p.
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i n P el Willingham, C. R., and Hamilton, D. H., 1982, Neogene structure and stratigraphy of the offshore region on trend with the western Transverse Ranges: Geological Society of American, Abstracts with Program, v. 14, no. 4, p. 245.
Wyss, M., 1979, Estimating maximum expectable magnitude of earthquakes from fault dimensions:
Geology, v. 7, pp. 336-340.
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ueti n 10 March 1 0 Pa el QUESTION GSG 10 Provide a critique of the presentation made by Jay Namson. Discuss and evaluate any consistencies or inconsistencies of his model with the PGd'cE geologic and geophysical field data.
Namson and Davis (1990) propose a tectonic model for the crustal structure of south-central coastal California derived from the construction of two subparallel retrodeformable cross sections. Applying the theories and techniques of fault-bend folding and fault-propagation folding (Suppe, 1983; Suppe and Medwedeff, 1984), they interpret the regional crustal structure as a system of active southwest-verging blind thrust faults above a basal detachment within basement at a depth of 11 to 14 kilometers. This system of thrust faults and related upper crustal folds is interpreted by them to accommodate crustal shortening in the region approximately normal to the San Andreas fault. From this model, they estimate the amount of crustal shortening and rates of convergence for the region and comment on the seismic hazard potential posed by the structures that accommodate the shortening.
The modeling technique applied by Namson and Davis (1990) can be a powerful quantitative tool for modeling crustal deformation in regions of crustal shortening and may provide useful insights into the neotectonic development of south-central coastal California. However, the structural model developed by Namson and Davis using these techniques is inaccurate for several reasons and, in our opinion, does not portray the contemporary style of deformation occurring in south-central coastal California. Thus, their interpretation of crustal structure and rates of deformation is not appropriate for assessing seismic hazard in the contemporary tectonic setting. In Response to Question GSG 11, we provide an alternative retrodeformable cross section through the Los Osos/Santa Maria domain that, although it continues to violate many of the assumptions inherent in the modeling technique described in this response, is better constrained by and consistent with the known geology in the region.
Reasons why we disagree with the structural modeling by Namson and Davis (1990) are described fully in Attachment GSG Q10-A to this=response and generally fall into two principal categories:
(1) assumptions inherent in the modeling approach are not valid for the Los Osos/Santa Maria domain, and (2) geologic data and interpretations made by Namson and Davis in the construction of their model are not consistent with many known, observed geologic and kinematic relationships in the region. The critical issues that relate to the inaccuracy of Namson and Davis'odel are summarized in Table GSG Q10-1. These issues are divided into "Generic Issues," related to the modeling technique and assumptions required by the technique regardless of the regional geology, and "Geologic Issues," related to specific geologic conditions in the Los Osos/Santa Maria domain.
The critique summarized in Table GSG Q10-1 (and described fully in Attachment GSG Q10-A) indicates that the fault-propagation and fault-bend fold modeling technique has limited use for describing the contemporary style and rates of deformation in the Los Osos/Santa Maria domain.
Namson and Davis (1990) use selected geologic data and observations to constrain their model.
They do not use available Quaternary data or observations of surface deformation to constrain the locations and geometries of deeper crustal structures. We disagree with both the application of the modeling technique to this region (because of the inherent assumptions of rock behavior and style of deformation that are clearly violated, not because of problems with the technique itself) and with the geologic data and interpretations of surface deformation used by Namson and Davis (1990) to model the underlying crustal structure. In our opinion, the interpretations of crustal structure, deformation style, rates of shortening, geometry and dimension (length, width) of fault surfaces, and estimates of maximum earthquake magnitude made by Namson and Davis (1990) are not supported by known geologic relationships.
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ue ti n 10 March 1 Pa e2 Table GSG Q10-I CRITICAL ISSUES REGARDING THE STRUCTURAL MODEL OF SOUTH-CENTRAL COASTAL CALIFORNIA PREPARED BY NAMSON AND DAVIS (1990)
Critical issues are divided into "primary" issues and "other" issues. "Primary" issues are those that invalidate the application of the modeling technique in this area or require significant alteration of the model. "Other" issues are those that require modification of the model interpretation but do not necessarily invalidate the model.
ENERI ISSUES Re uired Fa lt-Pr a ion Fault-Bend F Id The r Cross section construction method Primary Issues None, the cross section construction method outlined by fault-propagation and fault-bend fold theory is technically sound and not violated by Namson and Davis (1990).
Other Issues None Assumptions inherent to modeling technique Primary Issues Namson and Davis (1990) assume that deformation within their cross sections proceeds by plane strain (that is, mass is conserved in the plane of the section). However, the presence of recently active regional strike-slip faults and probable post-Miocene block rotations strongly indicate the occurrence of non-plane strain. Both the amount and style of this deformation are not accounted for in their model.
~ Prominent strike-slip faults include the Hosgri and Rinconada fault zones and possibly the San Luis Obispo Transform fault as postulated by Hall (1981); less prominent strike-slip faults include the San Miguelito and ancestral Los Osos fault zones, and locally the West Huasna fault zone.
~ Block rotations are indicated by paleomagnetic data from the Morro Rock-Islay Hill dacite complex (Greenhaus and Cox, 1979) and the Monterey Formation at Shell Beach (Khan and others, 1988).
Namson and Davis (1990) assume that basement deforms in a manner analogous to brittle, bedded strata; that bed length and thickness are preserved and bedding-parallel slip is the most important deformation mechanism. However, basement in south-central coastal California consists of heterogenous lithologies having extreme variation in rheology, ranging from Franciscan Complex melange to Salinian granite, and deformation as a brittle, layered medium cannot be assumed.
Franciscan Complex melange is a chaotic assemblage of lithologies and structures; there are no throughgoing discrete horizons to accommodate layer-parallel slip.
Melange is a pervasively sheared anisotropic medium that accommodates shear strain in a distributed form rather than by frictional sliding along discrete faults.
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uestion 1 March 1 0 Pa e
~ Layer-parallel competency contrasts, required for the formation of ramp-flat thrust geometries, do not exist in the Franciscan Complex melange.
Other Issues Namson and Davis (1990) assume that the style and rate of deformation are constant.
However, geologic, geophysical, and seismologic data indicate that neither is constant.
The Pliocene Pismo syncline is inactive, but is currently uplifting. Therefore, a change in style of deformation has occurred.
~ Changes in rates and patterns of fold deformation are recorded in the offshore Santa Maria Basin (Clark and others, in press), where relationships between structure stratigraphy are well-preserved and imaged on geophysical data.
~ Fold and thrust belts typically propagate into their foreland or hinterland; they are not everywhere continuously and simultaneously active.
Namson and Davis (1990) assume that a mid-Cenozoic unconformity was horizontal prior to the onset of late Cenozoic compression. However, geologic data clearly indicate pre-compression non-horizontality of the unconformity.
Regional extension (or transtension) during the Miocene, prior to compression but following development of the unconformity (Oligocene in age), produced subsiding Tertiary basins and deformed the basal mid-Cenozoic unconformity (Hall, 1981).
Inherited deformation (tilting) of the unconformity produces significant errors in the model, both in the construction of crustal structures and in the restoration and assessment of cumulative shortening.
E L I I E Inconsis encie with Known e i Primary Issues Namson and Davis (1990) omit several known geologic structures from their structural model.
~ Faults along the southwestern boundary of San Luis/Pismo structural block (Wilmar Avenue, Olson, San Luis Bay and Pecho faults).
~ Los Osos fault zone.
~ Faults and folds within the San Luis/Pismo structural block (Edna, San Miguelito and Indian Knob faults; parasitic folds on the limbs of the Pismo syncline).
~ Oceanic/West Huasna fault.
Namson and Davis (1990) interpret the presence of specific geologic structures that are either not present or incorrectly defined based on geologic data and observations.
~ Point San Luis anticline.
~ Santa Lucia Range anticlinorium.
~ La Panza Range anticlinorium.
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uestion 10 March 1 0 P e4 The dimensions and orientations of these postulated structures subsequently are used by Namson and Davis (1990) to constrain the nature, dimensions, and rates of mid- to upper-crustal structural deformation.
The structural model proposed by Namson and Davis (1990) predicts rates and patterns of uplift that differ significantly from known rates and patterns of uplift. This disparity strongly indicates that the interpretation of mid- to upper-crustal structure by Namson and Davis (1990) is inaccurate in terms of style of deformation, location and geometry of deformation, and/or rates of crustal shortening.
Other Issues Namson and Davis (1990) interpret the Hosgri fault zone to be an inactive, rotated former basin-margin normal fault.
There is overwhelming geological, geophysical, and seismological data indicating that the Hosgri fault zone is an active, fundamental structure in the tectonic setting of south-central coastal California.
Regardless of sense of slip, an active Hosgri fault zone cannot be accommodated by the model of Namson and Davis (1990).
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uesti n 1 March I Pa e REFERENCES Clark, D. H., Hall, N. T., Hamilton, D. H., and Heck, R. G., in review, Structural analysis of late Neogene deformation in the central offshore Santa Maria Basin, California.
Greenhaus, M. R. and Cox, A., 1979, Paleomagnetism of the Morro Rock-Islay Hill Complex as evidence for crustal block rotations in central coastal Californix Journal of Geophysical Research,
- v. 84, p. 2393-2400.
Hall, C. A., Jr., 1981, San Luis Obispo transform fault and middle Miocene rotation of the western Transverse Ranges, Californix Journal of Geophysical Research, v. 86, p. 1015-1031.
Kahn, S. M., Coe, R. S., Barron, J. A., 1988, Magnetic polarity stratigraphy of the Miocene Monterey Formation at Shell Beach, Pismo Basin, central Californix EOS Transactions, American Geophysical Union, v. 69, no. 44, p. 1160.
Namson, J. S., and Davis, T. L., 1990, Subsurface study of the late Cenozoic structural geology of the Santa Maria Basin, western Transverse Ranges and southern Coast Ranges, California, American Association of Petroleum Geologists, in press.
Suppe, J., 1983, Geometry and kinematics of fault-bent folding: American Journal of Science,
- v. 283, p. 684-721.
Suppe, J., and Medwedeff, D. A., 1984, Fault propagation folding (abs.): Abstracts with Program, Geological Society of America, v. 16, p. 670.
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i ATTACHMENTGSG Q10-A Critique of Structural Model Proposed by Namson and Davis (1990)
For Geologic Evolution of Santa Maria Basin and Surrounding Region Diablo Canyon Power Plant Paclflc Gas and Electric Company Long Term Seismic Program
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A achment S 10-A M rch 1 0 TABLE OF CONTENTS Page INTRODUCTION TECHNIQUES AND ASSUMPTIONS OF FAULT-BEND FOLD AND FAULT-PROPAGATION FOLD MODELS
SUMMARY
OF NAMSON AND DAVIS'ODEL CRITIQUE OF NAMSON AND DAVIS'ODEL 10 Invalid Assumptions 10 Assumption of Plane Strain Deformation 10 Assumption of Constant Style and Rate of Deformation 16 Assumption that Pre-Tertiary Basement (Franciscan Complex and Salinian Granite) Behaves as Brittle, Bedded Sedimentary Strata 18 Assumption of Pre-Compression Horizontality of Tertiary Strata 19 Inconsistencies with Known Geology 19 Southwestern Boundary of the San Luis/Pismo Structural Block 19 Los Osos Fault Zone 23 Oceanic/West Huasna Fault Zone 23 Interpretation of Anticlinal Deformation 23 Point San Luis Anticline 23 Santa Lucia Range Anticlinorium 26 La Panza Range Anticlinorium 27 Uplift Rates and Patterns (predicted versus observed) 27 IMPLICATIONS FOR SEISMIC HAZARD ASSESSMENT 28
SUMMARY
31 REFERENCES 32 Diablo Canyon Poorer Plant Pacific Gas and Electric Company Long Term'Seismic Program
t hmen S l0-A March 1 0 Pa e 1 INTRODUCTION This attachment to Question GSG 10 provides a critique of the structural model and analyses performed by Namson and Davis (1990) in the southern Coast Ranges. Their report, entitled "Surface study of the late Cenozoic structural geology of the Santa Maria basin, western Transverse Ranges and southern Coast Ranges, California" presents a tectonic model for the crustal structure in the region derived from two sub-parallel retrodeformable cross sections constructed using the theories of fault-bend folding and fault-propagation folding developed by Suppe'1983) and Suppe and Medwedeff (1984). Locations of the two cross sections are shown on Figure GSG Q10-A.1. Their cross sections, A-Aand B-B and accompanying stratigraphic correlation diagram are reproduced here on Figures GSG Q10-A.2, A.3, and A.4, respectively.
The model presented by Namson and Davis (1990) inaccurately portrays the crustal structure of the Los Osos/Santa Maria domain. Although the modeling techniques developed by Suppe (1983) and Suppe and Medwedeff (1984) are powerful tools for assessing crustal structure in areas of active fold and thrust belt deformation, these techniques are inappropriately applied in the Los Osos/Santa Maria domain. Firstly, many of the assumptions that are required for constructing retrodeformable cross sections probably are not valid for the region in question, in particular, the area defined as the San Luis/Pismo structural block (Figure GSG Q10-A.S) (PG&E, 1988). Secondly, geologic interpretations incorporated into their model disagree with observed Quaternary and bedrock geology and kinematic data recorded by Hall (1973a, 1973b), Dibblee (1976), Hart (1976), Hall and others (1979), Greenhaus and Cox (1979), Khan and others (1988) and PG&E (1988). As a result, the structural geometry of their model and the implications for seismic hazard in the region are not well constrained and their conclusions are not appropriate.
In this attachment, we describe in detail the general and specific problems associated with the model proposed by Namson and Davis (1990). We begin by outlining the techniques employed by them to construct their retrodeformable cross sections, and we provide a summary of their results. This is followed by a point-by-point critique of the assumptions that we believe are violated in their application of the modeling technique to this region, and a description of the inconsistencies of their geologic interpretations with known geologic data and relationships in the region. We conclude with a summary of the implications of Namson and Davis'tructural model for seismic hazard assessment.
TECHNIQUES AND ASSUMPTIONS OF FAULT-BEND FOLD AND FAULT-PROPAGATION FOLD MODELS The construction of retrodeformable, or "balanced," cross sections requires the assumption that cross sectional areas and line-lengths are maintained in both the deformed and undeformed sections. The construction techniques used to accomplish this were developed initially by geologists working in the fold and thrust belts of the Canadian Rockies and southern Appalachians (Bally and others, 1966; Dahlstrom, 1969; Boyer, 1976; Boyer and Elliot, 1982) where the relatively simple "layer-cake" stratigraphy and vertical plane strain deformation conditions of these regions allowed for the basic assumptions to be satisfied. Recently, however, cross-section balancing has been applied with varying success to regions where the applications are not as straightforward. These areas include extensional terranes, regions dominated by blind thrusting, regions where the original stratigraphy is not horizontal, and regions of oblique compressional tectonics. The application of retrodeformable cross section analysis to such geologically complex settings as these has been facilitated by the development of more powerful section construction techniques, such as the theory of fault-bend and fault-propagation folding developed by Suppe (1983) and Suppe and Medwedeff (1984). In several instances, the application of these techniques has been severely criticized (for example, Yeats, 1989; Weldon and Humphreys, 1989).
Namson and Davis (1990) incorporate the geometric techniques developed by Suppe (1983) and Suppe and Medwedeff (1984) in the construction of several retrodeformable cross sections across the Santa Maria Valley and southern Coast Ranges (Los Osos/Santa Maria Valley). These geometric techniques are based upon the fundamental theory that there is an intimate geometric and kinematic Diablo Canyon Power Plant Paclflc Gas and Electric Company Long Term Seismic Program
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Orcutt fault, RHF = Red Hills fault, RNF = Rinconada-Nacimiento fault, SF Suey fault, SLO =
San Luis Obispo, SLRA = Santa Lucia Range anticlinorium, SM = Santa Maria, SYF = Santa Ynez fault, WHF = West Huasna fault.
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to tua 1 I on o la o Awe I n\ H& ~ ltJle Idlt Figure GSG Q10-A.3 Namson and Davis'1990) regional cross section A-A and interpretation across the southern Coast Ranges from the offshore Santa Maria Basin to the San Andreas fault. The anticlinoria are interpreted to be caused by thrust fault ramps off a regional detachment. Only late Cenozoic convergence is restored in the restoration, so preconvergence structures still illustrate deformation and the trajectories of the late Cenozoic thrusts are shown. The restoration shows 26.8 km of late Cenozoic shortening on the regional detachment and 21.8 km on the mid-Cenozoic unconformity.
The shortening discrepancy yields a 4 percent error in the interpretation. Abbreviations are as in Figure GSG Q10-A.2. The dashed line within the KJf-Jop is an arbitrary structural reference horizon that illustrates late Cenozoic convergent deformation.
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The interpretation shows the Point San Luis anticline to be a fault-bend fold associated with the Point San Luis thrust. Slip on the Point San Luis thrust is shown to reverse to a northward direction on the Purisima-Soloman thrust to form the Lompoc-Purisima anticline and the Orcutt anticline.
The restoration suggests 9.2 km of convergence across the Santa Maria Basin, which includes 2.8 km of slip transferred southward to form the Santa Ynez Mountains. The dashed line within the KJf-Jop is an arbitrary structural reference horizon that illustrates late Cenozoic convergent deformation.
Abbreviations are as in Figure GSG Q10-A.2.
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Fold axes Fault, dotted where inrerred 0 SS k~ MSVdtyd kb dq Sk dbi kb JM.S b~ b 14 EQ Satinran Terrene 0 20 mi EG Stanley Mountain Terrene Sur Obispo Composite Ssn Simeon Terrene iMDCunoch, 19atl Patton Terrene 0 20 ttm ggg Orablo Canyon Power Plant Figure GSG Q10-A.5 Map of the Los Osos/Santa Maria domain illustrating the boundaries of the structural sub-blocks within the domain, and general distribution of major basement terranes (modified from PG&E, 1988).
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At achment G 1 -A March 1 0 Pa e7 relationship between folding and faulting in compressional terranes. This relationship is manifested as three classes of folds, all of which result from displacement along fault planes that underlie the folds. These are fault-bend folds, fault-propagation folds, and lift-offfolds. Fault-bend folds (Figure GSG Q10-A.6a) are caused by bending of the hanging wall block as it rides over changes in dip of the underlying fault (Suppe, 1983); fault-propagation folds (Figure GSG Q10-A.6b) are caused by compression of material in front of the leading tip of a fault splay (Suppe and Medwedeff, 1984; Suppe, 1985); and lift-off folds are caused by buckling above a bedding parallel decollement (lift-offfolds are not a component of Namson and Davis'1990) model and therefore are not addressed in this response). By assuming kink-style parallel folding and ramp-flat thrust geometries, Suppe (1983, 1985) derives equations relating the interlimb angles of these folds to the initial step-up or "cutoff'ngles of the fault ramp, and the angle between the forelimb of the resulting fold and the fault surface. Using these techniques, one is able to construct an admissable representation of the mid- to lower-crustal geometry of a fold and thrust belt.
Inherent in these techniques are several rigid assumptions concerning the mechanisms and style of deformation. These assumptions are necessary to satisfy the geometric constraints that allow for derivation of the fundamental equations and for the resulting section to be retrodeformable. They are derived from observations made in thin-skinned fold and thrust belts throughout the world and, although not universal, have been successfully applied to many regions (for example, Suppe, 1980, 1983; Namson and Davis, 1988a). These assumptions and their consequences on the geometric and kinematic properties of the deforming rocks are outlined below.
Deformation occurs by plane strain. Mass is conserved from the deformed to the undeformed state, requiring that there is no loss of material out of the plane of section.
Deformation occurs under low-temperature conditions. This implies that layer-parallel shortening and interbedding plane shear (flexural flow) are minimized, Bedrock deforms in a manner analogous to brittle sedimentary rocks and bedding-parallel slip and frictional sliding on faults are the most important deformation mechanisms. This implies that bedding thickness is preserved.
Layer-parallel slip is limited to that produced by folding. This requires that (1) there is no angular shear of vertical lines at the trailing edge of restored sections; and (2) displacement on faults produces unique fold shapes.
Thrust faults step up abruptly as ramps from decollements in the transport direction.
This implies that higher level structures are deformed by lower level structures, but not vise-versa.
The movement of hanging wall beds past bends in fault planes causes those beds to fold.
Footwall beds are unaffected except by movement on deeper faults. This implies that many (but not all) axial planes terminate at bends in faults.
SUMMARY
OF NAMSON AND DAVIS'ODEL Using the geometric techniques and assumptions of fault-propagation and fault-bend fold theory, Namson and Davis (1990) construct two retrodeformable cross sections through the Los Osos/Santa Maria domain (Figures GSG Q10-A.1, A.2, A.3, A.4). These cross sections trend roughly subparallel to one another, orthoganal to the regional structural grain of the Los Osos/Santa Maria domain, and approximately 60 degrees to the trend of the Hosgri fault zone.
Cross section A-A (Figure GSG Q10-A.3) extends from the eastern margin of the offshore Santa Maria Basin just west of the Hosgri fault zone, trends N20'E across the coastline roughly 7 kilometers north of Point San Luis, through the San Luis Range to the Los Osos Valley, where it bends to a N30'E trend across the Santa Lucia Range, the La Panza Range, and the Carrizo Plain Diablo Canyon Power Plant Pacific Gas and Electric Company tong Term Selsmlc Program
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Figure GSG QIO-A.6a Kinematic model of a fault-bend fold forming over a ramp in a horizontal detachment. The front limb (A-A) forms as the hanging wall cutoff (X-Y) is translated from the ramp (X-Y) onto the upper bedding-plane detachment. The back limb (B-B) forms as the hanging wall is translated up the ramp (from Suppe and Namson, 1979).
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Figure GSG QIO-A.6b Kinematic model of a fault-propagation fold. The bedding-plane detachment extends up-section, with fault slip progressively decreasing as shortening is transferred to folding. Fault slip is fully accommodated by folding in units stratigraphically above the fault tip (from Suppe and Namson, 1979).
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A achment 10-A March 1 0 Pa to the San Andreas fault. There is a 20-kilometer offset in the section directly north of the La Panza fault to include additional well data and a more complete Tertiary section. Section B-B (Figure GSG Q10-AA) begins at the southern margin of the onshore Santa Maria Basin and trends north across the basin to the San Rafael Range. Ne restrict our comments to cross section A-A and the Point San Luis anticline in cross section B-B.
Cross section A-A (Figure GSG Q10-A.3) shows several prominent northwest-trending structures interpreted by Namson and Davis (1990) to exist on the basis of geologic and geophysical data and, in some cases, because they are required by the modeling techniques. The Hosgri fault zone is depicted as an inactive, basin-bounding normal fault along the eastern margin of the offshore Santa Maria Basin. The fault is interpreted by Namson and Davis (1990) to have been rotated to its present steeply northeast-dipping attitude by the formation of the Point San Luis anticline. The Point San Luis anticline is modeled as a fault-bend fold produced by southwestward translation of a large thrust sheet of Franciscan Complex basement over a postulated blind thrust ramp (the Point San Luis thrust) stepping up from a basal detachment at a depth of 11 to 14 kilometers. This detachment is interpreted to be of regional extent and to separate the overlying fold and thrust belt from the lower crust. Northeast of the Point San Luis thrust, a southwest-verging imbricate thrust fan consisting of three fault splays branches upward from the postulated basal detachment. Two of these faults are blind: the Santa Lucia thrust fault, and the Black Mountain thrust fault. The northeasternmost fault splay, the La Panza fault, reaches the surface, where it has been mapped by Ballance and others (1983) on the basis of displaced Tertiary strata and basement rocks. Each fault underlies a major antiformal structure modeled by Namson and Davis (1990) as a fault-propagation fold. Another structure, which Namson and Davis (1990) refer to as the Rinconada-Nacimiento fault, lies between the Black Mountain thrust fault and Santa Lucia thrust fault and is carried as an inactive, passive structure in the hanging wall of the Santa Lucia thrust fault.
Namson and Davis (1990) calculate the amount of shortening represented on their deformed-state cross sections by restoring a mid-Cenozoic unconformity to horizontal. They relate this unconformity to the Ynezian Orogeny of Zemorrian age and assume that it represents an originally horizontal surface prior to the onset of compressional deformation. Based upon stratigraphic relationships across the Orcutt anticline (cross section B-B, onshore Santa Maria Valley), changes in the intensity of folding across key Tertiary stratigraphic units in the Pismo and Huasna basins (synclines), and a change in Pacific Plate motion 5.0 to 3.4-3.9 million years ago, as indicated by studies of the Hawaiian-Emperor Seamount chain (Cox and Engebretson, 1985; Harbert and Cox, 1988), Namson and Davis assume that the onset of compression began roughly 4 million years ago and continues to the present. From their restoration of the mid-Cenozoic unconformity, Namson and Davis (1990) calculate the total amount of shortening for each cross section and the convergence rate implied by assuming constant slip since the onset of shortening 4 million years ago.
The total calculated shortening for cross section A-A is 26.8 kilometers for the regional detachment and 21.8 kilometers for the mid-Cenozoic unconformity. The 5.0-kilometer discrepancy results from slip on structures not accounted for in the cross section or from layer-parallel shortening. These estimates of shortening yield an average convergence rate of 6.7 millimeters per year for restoration of the detachment and 5.5 millimeters per year for restoration of the mid-Cenozoic unconformity.
Namson and Davis (1990) also calculate total shortening and slip rates for individual structures.
Using these slip rates, they estimate average recurrence intervals for 1- and 2-meter slip events on each structure. These estimates are given in Table 1 of Namson and Davis (1990).
The total estimated shortening for cross section B-B is 9.2 kilometers, an amount calculated from line-length balancing of an arbitrary datum within the Franciscan Complex basement. Balancing errors result in a maximum deviation of 1.3 kilometers from the 9.2 kilometers in their restored section. Slip rates and estimates of recurrence intervals for I- and 2-meter slip events on each structure are also given in Table 1 of Namson and Davis (1990).
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ttachment 10-A M rch 1 0 Pa 1 CRITIQUE OF NAMSON AND DAVIS'ODEL The crustal model and interpretation of deformation and seismic hazard presented by Namson and Davis (1990) are incorrect for two primary reasons. First, many of the assumptions they must make about the style of deformation required by the modeling technique to construct their retrodeformable cross sections are not valid for this region. Second, their model is not consistent with observed geologic relationships and fault kinematics in the area. Because their model is based upon several invalid assumptions and violates known geologic relationships, the conclusions that Namson and Davis draw from their model cannot be substantiated.
Invalid Assumptions In the construction of cross sections A-A and B-B (Figures GSG Q10-A.2 and A.3), Namson and Davis (1990) make a series of assumptions regarding rock rheology and the geometry and kinematics of contemporary tectonic deformation in south-central California. Several of the more critical assumptions, however, are not valid for the Los Osos/Santa Maria domain. These assumptions are critiqued below and include:
~ Assumption of plane strain deformation
~ Assumption of constant style and rate of deformation
~ Assumption that pre-Tertiary basement (Franciscan Complex and Salinian granite and gneiss) behaves as brittle, bedded sedimentary strata
~ Assumption of pre-compression horizontality of Tertiary strata Assumption of Plane Strain Deformation. To model regional deformation in the Los Osos/Santa Maria domain as a simple fold and thrust belt, Namson and Davis (1990) assume that deformation conforms to the conditions of plane strain. This implies that the primary tectonic transport direction is perpendicular to the faults and fold axes in the region, and that the effects of post-Miocene strike-slip faulting and block rotation are minimal. To meet these conditions, they assume that the oblique convergence of the North American and Pacific plates is completely partitioned into normal and tangential components relative to the San Andreas fault zone. In this model (Figure GSG Q10-A.7), tangential strains are consumed entirely by strike-slip on a weak San Andreas fault and normal strains are manifested as a fold and thrust belt parallel to the San Andreas fault (Namson and Davis, 1988a). This model is supported by studies of the stress field in central California that show that the principal compressive stress is oriented normal to the San Andreas fault (Zoback and others, 1987; Mount and Suppe, 1987). Although Namson and Davis (1988b) and Namson (1987) believe that this assumption is valid for other regions of central California, there are several reasons why this assumption is not valid to assess deformation in the Los Osos/Santa Maria domain and surrounding region.
First, fold axes and faults within this region are not parallel to the San Andreas fault. Rather, they are oriented west-northwest, transitional between the east-west-trending structural grain of the western Transverse Ranges to the south, and the north-northwest-trending structural grain of the central San Andreas fault zone to the northeast (Figure GSG Q10-A.8; PG&E, 1988; Lettis and others, 1989). Therefore the contractile strains observed in the Los Osos/Santa Maria domain are oblique to the San Andreas fault, not normal to it as Namson and Davis (1990) suggest. This is supported by earthquake focal mechanisms for the region (Figures GSG Q10-A.9 and A.10) (PG&E, 1988), which show a fairly even distribution of strike-slip and reverse mechanisms, with preferred nodal planes that strike west-northwest, parallel to the local structural grain, and suggesting an average maximum horizontal P-axis oblique to the trend of the San Andreas fault zone.
Second, geologic evidence indicates that this region has experienced widespread (post-Miocene) strike-slip deformation. Because Namson and Davis (1990) must assume that post-Miocene Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
ttachment S 10-A March 1 90 Pa 11 PACIFIC PLATE NORTH AMERICAN PLATE San Andreas Fault S c A Brlttlo-Ductllo Transltlon QA Plato Boundary Uthosphero Aesthenosphere Coast Ranges 0 0 Qa
~QT Plato Boundary TectonIc Thlckenlng Inclplent Subduction Uthos phere Aesthenosphere Figure GSG QIO-A.7 Kinematic model showing the partitioning of the oblique convergence of the North American and Pacific plates into normal and tangential strains across the San Andreas fault. Normal manifested as a fold and thrust belt perpendicular to the strike of the San Andreas fault; strains tangential are strains are consumed by slip on the San Andreas fault (from Namson and Davis, 1988a).
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- Faultl dotted where approximatefy located, Inferred, or concealed Figure GSG QIO-A,8 Map showing distribution and orientation of major structural elements in south-central California.
Note the transition in structural grain from east-west at the Transverse Ranges to northwest-southeast at the San Andreas fault (from PG&E, 1988). Insert illustrates the trend of major fold axes for the region (modified from Page, 1981).
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s A achment S 1 -A March I 0 Pa e 14 30'0'0'5O
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0 km 121'0 50'0 30'0'XPLANATlON Lower hemisphere first-motion plots White Dilation Black Compression R Diablo Canyon Power Plant Figure GSG QIO-A.IO Compilation of focal mechanisms for the period 1980-1988 in the region of the San Luis/Pismo structural block. The magnitudes of the events range from 0.9 to 3.2. Note the dominant strike-slip character (modified from PG&E, 1988).
Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
ttachment GS 1 -A March 1 0 Pa e 15 deformation proceeds by plane strain, their model does not account for the effects of post-Miocene strike-slip faulting within their cross sections. This assumption conflicts with documented observations of strike-slip deformation on the Hosgri fault zone (Hall and others, 1979; PG&E 1988), the Rinconada fault zone (Dibblee, 1976; Hart, 1976) and the San Miguelito fault (Hall, 1973a; PG&E, 1988). Although post-Miocene lateral offsets may be small on some faults, (for example, on the San Miguelito) compared to the regional contractile strains proposed by Namson and Davis (1990), their slip behavior strongly suggests wrench-style tectonics are locally dominant, rather than simple fold and thrust tectonics. On other faults, the lateral cumulative offsets are large (for example, on the Hosgri and Rinconada faults), invalidating the assumption of plane strain.
Bedrock mapping by Hall (1973a), and Hall and others (1979) indicates that the San Miguelito fault is a zone of anastomosing west-northwest-trending traces approximately 9 kilometers in length juxtaposing Miocene and Pliocene volcanic and sedimentary rocks to the northeast with Mesozoic basement rocks to the southwest (Response to Question GSG 16, Plate GSG Q16-1). Detailed mapping and trenching investigations indicate the fault is composed of three segments (PG&E, 198S). The northern and southern segments are subvertical and trend northwest. Kinematic indicators on shear planes exposed in trenches across these segments show that the most recent and possibly long-term displacements on this fault are strike-slip. The central segment trends east-west and dips moderately to the north. A shear plane oriented N78 W, 37'NE, having mullions and slickensides indicating dip slip, was observed in a trench along this segment. The kinematics and geometry of these segments are consistent with a small left-stepping restraining bend in a right-slip fault. Disruption of the Squire Member of the Pismo Formation by this fault indicates it was active during the late Pliocene or Quaternary; thus, the fault must be considered in any interpretation of post-Miocene deformation in the region.
Detailed investigation by offshore geophysical surveys of the Hosgri fault zone, onshore field mapping and trenching of the related San Simeon fault zone, and analysis of seismicity data indicate that the San Simeon/Hosgri fault system is a major right-slip fault system. Quaternary lateral slip rates are estimated at between 1.0 and 3.0 millimeters per year, and vertical slip rates at between 0.2 to 0.4 millimeters per year (PG&E, 1988; Response to Questions GSG 3 and GSG 4). The Hosgri fault zone is depicted by Namson and Davis (1990) however, as a relatively minor, inactive normal fault bordering the eastern margin of the Santa Maria Basin that was subsequently rotated to its present sub-vertical attitude by growth of the Point San Luis anticline. Namson and Davis (1990) do not consider the Hosgri fault zone to be a significant structure in the contemporary geologic setting. This interpretation contradicts not only the data presented by PG&E (1988), but also data and interpretations presented by a number of researchers of the offshore Santa Maria Basin, who consider the Hosgri fault zone to be a fundamental tectonic structure that has played a significant role in the Neogene tectonic evolution of south-central coastal California(for example, Silver, 1978; Graham and Dickinson, 197S; Hamilton and IVillingham, 1978; Hall, 19S1; Crouch and others, 1984; Luyendyk and others, 1985; McCulloch, 19S7).
The Rinconada fault appears on Namson and Davis'1990) map and cross section A-A (Figure GSG Q10-A.3) as a northeastward-dipping, low-angle fault that they call the Rinconada-Nacimiento fault. Evidence cited by Dibblee (1976), Hart (1976), and Page (19S1), however, indicates that the Nacimiento and Rinconada faults are two distinct structures in the area of Namson and Davis'ross section A-A. Page (1981) considers the Nacimiento fault as part of a major, northwest-striking, high-angle, strike-slip fault, the Sur-Nacimiento fault zone, extending from Point Sur in the northwest to the Transverse Ranges in the southeast. This fault zone separates the Salinian and Franciscan terranes along its entire length. Dibblee (1976), on the other hand, suggests that the Rinconada fault formed at a later time within the Salinian block and extended south, truncating the Sur-Nacimiento fault zone and following its trace southeastward this area. According to his definition, the Nacimiento and Rinconada faults are coincident with the Transverse Ranges. Hart (1976) generally agrees with Dibblee, showing the Sur-Nacimiento fault zone truncated by the Rinconada fault. However, his Nacimiento fault is a complex, ill-defined zone of both vertical strike-slip and low-angle, southwest-dipping thrust fault segments that do not necessarily juxtapose Salinian and Franciscan basement.
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ttachment SG 10-A March 19 0 Pa e I Mapping by Dibblee (1976) also indicates that the Rinconada fault is subvertical in this region.
Lithologic relationships across the fault described by Dibblee (1976) indicate that right-lateral separation of 23 to 56 kilometers has occurred since post-early Miocene time. Detailed mapping of the Rinconada fault by Durham (1965) indicates that approximately 18 kilometers of right slip has occurred since the early Pliocene. This conclusion is supported by the investigations of Hart (1976) and Dibblee (1976), who show that the Pleistocene Paso Robles Formation is involved in the faulting.
Hart (1976) also indicates that alluvium of probable late Pleistocene age is deformed by the fault and notes that there is sufficient geomorphic expression in the form of sag ponds, aligned depressions, and offset drainages to suggest the possibility of Holocene displacement. The Rinconada fault in Namson and Davis'1990) model does not offset his arbitrary structural reference horizon within the basement, indicating there has been no post-Miocene slip on this fault. This clearly conflicts with the known geologic observations of significant lateral displacement since Miocene time described above.
Third, in addition to probable strike-slip, non-plane strain, Neogene crustal block rotations are also well documented both for the western Transverse Ranges (Luyendyk and Hornafius, 1983; Luyendyk and others, 1985; Hornafius, 1985; Hornafius and others, 1986), and the Santa Maria Basin (Khan and others, 1988; Greenhaus and Cox, 1979). Rotation of regional and sub-regional crustal blocks will result in non-plane strain deformation and loss of mass from the plane of section. Hornafius (1985) has modeled fault geometries in the Santa Maria Basin and southern Los Osos/Santa Maria domain to account for post-Miocene clockwise rotation of the western Transverse Ranges. His model suggests that rigid-block rotation and right-lateral translation along the Hosgri fault zone must be coeval with crustal shortening and block translation in the Santa Maria basin to permit continued northward rotation of the range (Figure GSG Q10-A.11). These crustal-scale fault kinematics are not accounted for in the partitioned strain fold and thrust model presented by Namson and Davis (1990).
Fourth, investigations by PG&E (1988) show that the San Luis/Pismo structural block is being uplifted along its bounding faults (Los Osos fault zone and a system of faults along the southwestern block boundary) as a rigid block. Uplift rates determined from marine terrace elevations decrease toward the southeast along the southwest margin of this block (Figure GSG Q10-A.7). If we assume that the rate of slip is similar on faults along the southwestern boundary, the decrease in uplift rate suggests that the faults controlling uplift of the southwestern boundary may have gentler dips to the southeast, causing clockwise rotation of the block, a consequence consistent with the data obtained by Khan and Barron (1988) and the regional kinematic model of Luyendyk and others (1980) and Hornafius (1985).
These observations indicate that mechanisms other than simple tectonic telescoping by fold and thrust deformation are responsible for the present state of strain in the region, particularly for the San Luis/Pismo structural block. Any tectonic model for this region must account for coeval strike-slip and compressional deformation, rigid block uplift, and the possibility of block rotations in the Los Osos/Santa Maria domain. Namson and Davis'1990) model does not account for regional non-plane strain and, therefore, cannot be considered valid.
Assumption of Constant Style and Rate of Deformation. Namson and Davis (1990) assume that the style and rate of deformation has remained constant since roughly 4 million years ago. Based on this assumption, they interpret that the style of deformation recorded by deformed late Tertiary strata reflects the style of deformation in the contemporary tectonic setting. This assumption is incorrect for several reasons.
First, Namson and Davis (1990) use the geometry and rate of development of the Pismo syncline to model their Point San Luis anticline and underlying Point San Luis thrust fault. Investigations of marine and fluvial terraces (PGAE, 1988; Killeen,-.1988; Lettis and others, 1989, and in review; Hanson and others, 1989, and in review), however, indicate that the syncline is not deforming internally in the contemporary tectonic setting. The syncline, which until late Pliocene or early Quaternary time was a subsiding marine depocenter, is now rising as a rigid structural block along Diablo Canyon Power Plant Long Term Selsmlc Program Pacific Gas and Electric Company
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Atta hm n 10-A March 1 Pa e 17 Onshore Santa Maria Basin Hosgri Fault Zone Western Transverse Ranges Late Miocene Present Figure GSG QIO-A.II Development of the present-day fault geometry within the Santa Maria Basin, as a consequence of post-Miocene clockwise rotation of the Santa Ynez Range according to Hornafius (1985) (modified from Hornafius, 1985).
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t achment S 1 -A March I 90 Pa e 1 bordering reverse faults. Namson and Davis'odel does not account for this evolution or transition of tectonic style of deformation.
Second, loci of active faulting have clearly changed from late Pliocene to Quaternary time. Field mapping and trenching investigations across the San Miguelito, Edna, and Indian Knob faults (Hall and others, 1979; Hall, 1973a, 1973b; Hall and Corbato, 1967; PG&E, 1988; Lettis and others, in review) show that these structures displace the late Pliocene Squire Member of the Pismo Formation and locally displace the Pliocene and early Quaternary Paso Robles Formation, but do not displace mid- to late Quaternary marine and fluvial terrace deposits. Thus, these faults played an active role in the late Tertiary tectonic development of the San Luis/Pismo block, but are not active in the contemporary tectonic setting. Namson and Davis'1990) model does not account for the presence of or change in loci of activity on these faults.
Third, geophysical studies of the Hosgri fault zone and offshore Santa Maria Basin indicate either a pulse of late Pliocene folding and reverse faulting or a progressive dying out of contractional deformation in many areas. Both the Queenie structure (PG&E, 1988; Clark and others, in press) and the Purisima structure (Point Sal montage, seismic line PG&E-3, Response to Question GSG 1),
for example, show evidence of Pliocene activity and either abrupt or progressive decay of rate of deformation into the post-mid-Pliocene sediments. In addition, many upward-diverging splay faults along the Hosgri fault zone in the San Luis/Pismo reach deform pre-mid-Pliocene sediments but do not deform the mid-Pliocene unconformity or younger deposits (San Luis/Pismo reach montage, seismic lines GSI-86 and IV-14, Response to Question GSG 1).
These several observations indicate that both the style and rates of deformation have not remained constant in south-central coastal California from the early Pliocene to the present. There are several known examples of mid- to late Pliocene deformation, such as the Pismo syncline and Queenie structure, that are not representative of either the style, geometry, or locus of contemporary tectonic deformation. The assumption made by Namson and Davis (1990) that the rates and style of deformation have remained constant over the past 4 million years, and thus that Pliocene deformation is representative of late Quaternary deformation, is not valid.
Assumption that Pre-Tertiary Basement (Franciscan Complex and Sallnlan Granite) Behaves as Brittle, Bedded Sedimentary Strata. Namson and Davis (1990) assume that the Franciscan Complex basement deforms as a bedded brittle medium and that shear strain is restricted to bedding planes during the folding process (flexural slip) or to localized faulting on discrete ramps and a horizontal detachment. However, in a highly sheared and foliated tectonite such as the Franciscan Complex melange of south-central California (Hsu, 1969; Cowan, 1985), Tertiary deformation may not be localized along discrete throughgoing faults; rather, it may be distributed pervasively throughout the argillaceous matrix by reactivation of pre-existing shear fractures. The few extensive, relatively narrow fault zones that do crop out at the surface separate melange from thick sequences of graywacke; they do not structurally repeat stratigraphic units and do not extend up-section. The ramp-flat geometries observed in classic fold and thrust belts are not present in the Franciscan Complex (Cowan, 1974; Page, 1981) suggesting that the outcrop expression of the Franciscan Complex does not support Namson and Davis'odel for deformation at depth. In fact, without large-scale, layer-parallel competency contrasts, there is no mechanism for the formation of ramp-flat geometries. Shear deformation at depth within the crust is likely to be concentrated within the melange, the entire unit behaving as a fault zone (Cowan, 1985). This distributed strain will likely supersede but not necessarily preclude localized strain, forming relatively narrow fault planes.
Because of this probable widespread distribution of shear strain and a lack of bedded sequences, the Franciscan Complex cannot be modeled as a bedded, brittle medium.
Namson and Davis (1990) also model the Salinian basement of Mesozoic granite east of the Rinconada-Nacimiento fault as a bedded medium, and include it in their fault-propagation fold model for the La Panza anticlinorium (Figure GSG Q10-A.4). Granitic rocks commonly have a low anisotropy and are approximately homogenous and, therefore, it is incorrect to model the deformation of granitic basement as a bedded sedimentary sequence. Also, because of the very different properties of granite and Franciscan Complex melange, we expect a change in deformation Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
I Attachment SG 1 -A M rch 1 0 Pa el style across the Rinconada-Nacimiento fault. Namson and Davis do not account for this possibility, nor do they justify including igneous basement in their fault-propagation fold model. Consequently, their model of the La Panza Range anticlinorium is suspect.
Namson and Davis (1990) postulate that a mid-crustal detachment exists at 11- to 14-kilometers depth, separating the lower crust from the overlying fold and thrust belt. This detachment presumably lies within the Franciscan Complex basement and not at a lithologic boundary, the more common location for the basal detachment in thin-skinned fold and thrust belts. They admit that the depth to the detachment is poorly constrained by surface data and suggest that the base of the seismogenic zone, and hence the brittle-ductile transition, is a likely location for the detachment.
We agree with Namson and Davis'nterpretation that a regional detachment may occur at some depth within the middle to lower crust. Some sort of decollement or zone of ductile shear is presumably accommodating the large magnitude Neogene crustal strains, both translational and rotational, observed in western California. However, the depth to such a detachment and its mechanical behavior are difficult to establish. We do not believe there is adequate evidence to place the detachment at the base of the seismogenic zone within the brittle-ductile transition. There is, however, evidence in the seismic refraction data to identify a more plausible detachment dipping inland from the west at a depth of 14 to 22 kilometers beneath the Los Osos/Santa Maria domain (Figures GSG QIO-A.12 and A.13). This surface is interpreted to be the top of the Mesozoic to early Tertiary subducted oceanic slab (Trehu and Wheeler, 1987). It is likely that crustal shortening within the Los Osos/Santa Maria domain is accommodated by a seismic movement on this surface, or by ductile deformation between the base of the seismogenic zone and the slab surface, rather than by a discrete detachment within a higher lithologic domain as suggested by Namson and Davis.
Assumption of Pre-Compression Horizontality of Tertiary Strata. Namson and Davis (1990) restore their cross sections to a horizontal mid-Cenozoic unconformity and assume that the Tertiary basins have filled with syn-deformational sediments during the thrusting and folding process. This assumption is the source of considerable error in determining fault geometries from the dip of these sediments. These basins were present prior to regional crustal shortening in the form of half-graben, as indicated by the restored geometry of the Orcutt fault and Santa Maria Valley in cross section B-B (Figure GSG Q10-A.4). The dip of strata within the basins and the mid-Cenozoic unconformity were not horizontal, but had an inherited dip of roughly 15 to 20 degrees toward the basin-margin fault. This 15- to 20-degree discrepancy can produce large changes in the geometry of the underlying fault. Cross section A-A should be consistent with B-B in this respect. A good example of the effects of restoring to an improper horizon occurs on cross section A-A where the backlimb of the Point San Luis anticline is defined by the dip of the mid-Cenozoic unconformity on the south limb of the Pismo syncline. Restoration shows that these sediments were not horizontal, but dipping 15 to 40 degrees to the southwest during the mid-Pliocene. This 15- to 40-degree error will change the attitude and depth of the Point San Luis thrust ramp significantly and will produce an unrealistically steep attitude for a fault-bend fold ramp. Namson and Davis'estoration of cross section B-B shows the pre-unconformity basins and related normal faults.
Inconsistencies with Known Geology There are several important discrepancies between the crustal models presented by Namson and Davis (1990) and existing knowledge of the geology in the Los Osos/Santa Maria domain. These discrepancies include an incorrect interpretation of the existence and dimensions of fold structures, and the omission of numerous faults that break the earth's surface and have been characterized by detailed surface and subsurface investigations. These faults include, but are not limited to, the southwestern boundary of the San Luis/Pismo structural block, the Los Osos fault zone, faults along the San Miguelito and Edna faults within the San Luis/Pismo structural block and the Oceanic/West Huasna fault.
Southwestern Boundary of the San Luis/Pismo Structural Block. A complex, diffuse system of late Quaternary faults (Wilmar Avenue, San Luis Bay, Olson, Pecho and Oceano faults) tectonically defines the southwestern boundary of the San Luis-Pismo structural block (Figure GSG Q10-A.14)
(PG&E, 1988). These faults have been characterized by careful investigations involving onshore and Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
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ttachment 10-A March 1 0 Pa e21
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A chm n I -A rch I P 22 CAMBRIA BLOCK 120'30' 35s30'0 0.0 0 mi Estero 0 15 km Bay 00 a
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//////////y~ Southwestern boundary zone Figure GSG QIO-A.14 Map showing fault traces of the Los Osos and southwest boundary fault zones defining the northeast and southwest margins of the San Luis/Pismo structural block. Also shown are uplift rates in millimeters per year determined from marine terrace elevations (modified from PG&E, 1988).
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tt chm nt G 1 -A March 1 Pa e 23 offshore geophysical surveys, field mapping, detailed logging of natural and trench exposures of fault traces, analysis of water- and oil-well data, and an extensive borehole survey of the coastal marine terraces (PG&E, 1988; Lettis and others, in review; Response to Question 43, February 1989, and Response to Question GSG 16). The individual faults are laterally discontinuous, west- to northwest-trending, northeast-dipping, and have predominantly reverse displacement. The late Quaternary slip rates on these faults have been estimated at 0.01 to 0.20 millimeters per year (PG&E, 1988). When the amount of slip on these faults is combined, it accounts for all the observed Quaternary uplift of the San Luis Range. Rather than include these faults in their model, however, Namson and Davis (1990) attribute uplift of the range to a postulated underlying thrust fault ramp.
Los Osos Fault Zone. Namson and Davis (1990) do not include in their model the prominent west-northwest-trending, southwest-dipping Los Osos fault zone. The fault zone borders and tectonically defines the northeastern boundary of the San Luis/Pismo structural block (Figure GSG Q10-A.14)
(PG&E, 1988). Detailed Quaternary mapping, trenching, and well data analyses by PG&E (1988) show that this fault is a late Quaternary reverse fault. A part of the fault bordering the northeastern flank of the Irish Hills is included within an Alquist-Priolo Special Studies Zone by the California Division of Mines and Geology, indicating clear evidence for Holocene activity. Kinematic indicators consisting of slickensided gouge and well-developed mullions indicate dip slip in virtually all trench exposures of the active fault trace. Disruption of dated marine and fluvial terraces and radiocarbon dating of faulted paleosols exposed in trenches across the Los Osos fault indicate slip rates ranging from 0.07 to 0.80 millimeters per year (PG&E 1988; Lettis and others, in review).
These works show that the Los Osos fault accommodates all the observed Quaternary uplift of the San Luis/Pismo structural block along the block's northeastern boundary and is, therefore, a prominent structure in the region. Although the fault is a fundamental structure having excellent geomorphic expression of late Quaternary surface deformation, Namson and Davis (1990) do not include this fault in their model of crustal deformation for the region.
Oceanic/IVest Huasna Fault Zone. The Oceanic/West Huasna fault zone is shown by Hall and others (1979) as a steeply northeast-dipping reverse fault at the base of the Santa Lucia Range. Although studies to assess Quaternary activity have not been conducted, the fault displaces Neogene strata and is a likely candidate for the range-front fault accommodating uplift of the Santa Lucia Range. The fault also separates the north-northwest structural grain of the southern Coast Ranges from the more westerly trending structural grain of the Santa Maria Basin region and is interpreted to be the northeastern boundary structure of the Los Osos/Santa Maria domain (Figure GSG Q10-A.15)
(PG&E, 1988; Lettis and others, 1989). Although the fault zone appears to be a fundamental structure in the tectonic development of coastal south-central California, Namson and Davis (1990) do not include it in the structural framework of their model.
Interpretation of Anticlinal Deformation. The structural model proposed by Namson and Davis (1990) is dependent upon the identification of several major anticlines. The dimensions and geometry of these anticlines govern the locations and geometry of underlying thrust faults according to the theory of fault-bend and fault-propagation folding. These anticlines are the Point San Luis anticline, Santa Lucia Range anticlinorium, and La Panza Range anticlinorium. Based on available geologic and geophysical data, and known geologic relationships observed at the surface, the dimensions and geometry of these anticlines as portrayed by Namson and Davis are highly interpretive and their existence suspect.
Point San Luis Anticline. Definition and characterization of the Point San Luis anticline is critical for assessing the nature and rates of deformation in the San Luis/Pismo region. Namson and Davis (1990) model the anticline as a fault-bend fold and use the anticline to require the existence of a blind thrust ramp beneath the coast from Point Buchon to Point San Luis and extending inland to the southeast beneath the northern Santa Maria Basin. The existence of the Point San Luis anticline as postulated by Namson and Davis (1990) is not supported by geologic or geophysical data. Thus, the inference of deeper crustal structure made from the presence of the anticline is not technically supportable.
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Notice divergence of the Rinconada and Nacimiento faults. Also note the traces of the Oceanic and West Huasna faults (modified from PG&E, 1988).
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tt chment I -A Mrchl 90 Pa e 25 As evidence for the Point San Luis anticline, Namson and Davis (1990) cite (1) the presence of southwest-tilted Tertiary strata directly west of the Hosgri fault zone; (2) the presence of a basement high flanked by Tertiary strata in the Point San Luis region; (3) a basement high in the northern onshore Santa Maria Basin south of the San Rafael Range; (4) moderately southwest dipping Tertiary and Quaternary strata at the northern margin of the onshore Santa Maria Basin; and (5) the northeast-dipping southwest limbs of the Pismo and Huasna synclines.
Namson and Davis (1990) cite the presence of southwest-dipping Tertiary strata directly west of the Hosgri fault zone as primary evidence for the southwest limb (forelimb) of the Point San Luis anticline. For example, their Figure S shows a part of seismic reflection line PG&E-3 that crosses the Hosgri fault zone in San Luis Obispo Bay and illustrates the presence of southwest-dipping reflectors adjacent to and west of the Hosgri fault zone. The use of these deformed strata as evidence for the Point San Luis anticline, however, is incorrect and misleading. The southwest-tilted strata are present along the western margin of the Hosgri fault zone from roughly Point Buchon to Point Sal (PG&E, 1988; see also Response to Question GSG 1). As shown repeatedly in both PG&E and NRC consultant documents (for example, McCulloch's presentation, June 1989 workshop),
this deformation persists subparallel to the Hosgri fault zone and is interpreted to be genetically related to deformation associated with the Hosgri fault zone. However, the postulated Point San Luis anticline trends roughly 25 degrees more westerly than the Hosgri fault zone. The tilted Tertiary strata parallel the Hosgri fault zone and thus diverge from the postulated Point San Luis anticline; they cannot be the southwest limb of the anticline, nor can they be construed in any way as indirect evidence for the anticline. In no instance do these tilted Tertiary strata extend across the Hosgri fault zone and onshore as indicated by Namson and Davis (Figure GSG Q10-A.1). The folded strata on PG&E-3 cited by Namson and Davis are roughly 10 to 12 kilometers southwest of the southwest limb of the postulated anticline.
Namson and Davis (1990) show the crestal trace of the postulated Point San Luis anticline to diverge from the main trace of the Hosgri fault, trend eastward across San Luis Obispo Bay, come onshore near Grover City, and extend across the northern Santa Maria Valley (Figure GSG Q10-A.1).
Although several discontinuous minor antiformal structures are interpreted in the offshore region from geophysical data (PG&E, 1988), they bear no resemblance to the large, broad, flat-crested anticline more than 15 kilometers wide and 80 kilometers long shown by Namson and Davis (1990, Figure 2). Namson and Davis (1990) state that this crest is defined to the southeast by exposure of Franciscan basement rocks, but inspection of Hall's (1982) structure contour maps for the top of pre-
,. Monterey basement and water-well data in the Grover City-Arroyo Grande area (PG&E, 1988) indicate that this basement high is an isolated feature measuring less than 5 kilometers wide and less than 12 kilometers along strike. In addition, this basement high is fault-bounded and flanked by formerly subsiding Tertiary basins. Thus, this basement high is, at least in part, an inherited interbasin region, rather than simply the eroded core of an anticline as interpreted by Namson and Davis (1990).
Namson and Davis (1990) define the forelimb of the Point San Luis anticline on cross section B-B by the moderate homoclinal southward dip of Tertiary sediments at the northern margin of the Santa Maria Basin (Figure GSG Q10-A.4). The southward dip of these sediments is well constrained by borehole data (for example, Hall, 1982) and Vibroseis data (PG&E, 1988) to be less than 10 degrees in Miocene strata and progressively decreasing in Pliocene and younger strata. The gentle dip of Pliocene strata may result from simple depositional infilling of a subsiding basin and tectonic tilting of the basement during basin formation, rather than tectonic folding as suggested by Namson and Davis (1990). Vibroseis data indicate the stratigraphic section thins markedly from south to north, with stratigraphic onlap relationships indicating progressive infilling of the basin. The presence of late Quaternary fluvial deposits at depths greater than 180 meters below sea level in the valley confirms that late Cenozoic subsidence of the basin has occurred (PG&E, 198S). In addition, we have mapped a flight of Quaternary marine terraces that extend inland from Grover City along the northern margin of the Santa Maria Valley (PG&E, 1988). These terraces form a well-preserved sequence of broad wave-cut platforms between the Santa Maria River and Temetate Ridge. The most extensive wave-cut platforms are older than 500,000 years. They are nearly flat-lying, with Diablo Canyon Poorer Plant Pacific Gas and Electric Company Long Term Seismic Program
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ttachm nt 10-A March 1 0 Pa e2 little or no observed folding or tilting by the southwest limb of Namson and Davis'ostulated anticline.
Namson and Davis (1990) show the crest of the Point San Luis anticline in the region of Point San Luis as a broad (10 kilometers wide), flat region disrupted by a single normal fault (Figure GSG Q10-A.3). Onshore field mapping, underwater geologic reconnaissance performed by diver geologists, and offshore side-scan sonar data indicate this region is characterized by close to tight folding and reverse faulting of post-Miocene strata (Response to Question GSG 16). None of this observed Tertiary deformation is accommodated by Namson and Davis'odel.
Namson and Davis (1990) correlate the Point San Luis anticline between their sections A-A and B-B. This correlation presents significant kinematic inconsistencies. They suggest that uplift of the San Rafael Mountains in section B-B is due to the translation of the Huasna syncline over the Point San Luis thrust at depth (Namson and Davis, 1990). They, therefore, indirectly imply the same mechanism of uplift for the Pismo syncline in section A-A, which is the geometric equivalent of the Huasna syncline in section B-B. Data presented by PG8'cE (1988) indicate that all the uplift in the San Luis/Pismo structural block, and hence the Pismo syncline, can be accounted for by slip on reverse faults that flank the northeastern and southwestern margins of the block. Similarly, mapping of marine and fluvial terraces across the axis of the Pismo syncline clearly shows that the syncline is inactive and that there is no internal deformation (folding or faulting) within the block in the contemporary tectonic setting. Therefore, there is no kinematic basis for invoking a major thrust ramp at depth beneath the Pismo syncline to produce continued fold growth within the Santa Lucia Range.
Santa Lucia Range Anticlinorirrm. Namson and Davis (1990) show a major structure, the Santa Lucia Range anticlinorium, along the axis of the Los Osos Valley and model it as a fault-propagation fold (Figure GSG Q10-A.3). Using the anticlinorium as evidence, they postulate a major blind thrust beneath the valley and the town of San Luis Obispo, and within 10 kilometers of the Diablo Canyon Power Plant. There are, however, several fundamental inconsistencies between the geometry, kinematics, and location of this structure and the present knowledge of local geology.
A major inconsistency is Namson and Davis'1990) identification of the south limb of the Santa Lucia Range anticlinorium. In their model, they define the south limb of the anticlinorium as the north limb of the Pismo syncline. North of this syncline, Namson and Davis show the crest of the Santa Lucia Range anticlinorium to be subhorizontal and continuous across strike. Structural data from Hall (1973a) and PG&E (1988) clearly show that the Tertiary bedrock in this area dips approximately 35 to SO degrees northeast into the Los Osos Valley beneath Quaternary deposits. The Mesozoic, Tertiary, and Quaternary units are all disrupted by the northeast-verging Los Osos fault zone. Namson and Davis do not include in their model these observed attitudes in the Tertiary bedrock or the contemporary tectonic behavior of the Los Osos fault zone.
Namson and Davis (1990) further state that this antiformal crest has been "...projected into the section-from along strike," and give no indication as to the distance or direction of the projection.
Inspection of available geologic maps along trend of the postulated anticline to the northwest (Hall, 1973a) and southwest (Hall, 1973b) indicates that there are no structural data to support the existence of a large, broad, flat-crested anticline.
Namson and Davis (1990) model the Santa Lucia Range anticlinorium as an active fault-propagation fold. This postulated fold predicts that active uplift should be occurring in the region of the anticlinorium, the Los Osos Valley. There is no topographic expression anywhere in the valley to support an actively growing anticlinorium. Instead, borehole, gravity, and surface geologic data strongly suggest the valley is locally subsiding, and is not the locus of tectonic uplift (PG8:E, 1988; Lettis and others, in review).
Namson and Davis (1990) cite the "moderately dipping west limb of the Huasna syncline" as the backlimb of the Santa Lucia Range anticlinorium. The west limb of the main Huasna Basin has a dip of 70 degrees, as shown on cross section A-A. The moderate dip to which they refer is the Diablo Canyon Power Plant Pacific Gas and Electric Cain pany Long Term Seismic Program
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ttachmen 1 -A March 1 0 Pa e 27 box-like trough segment of the Huasna syncline. This dip domain is conjectural, for there are no outcrop expressions of these attitudes and no well data to support their inferred existence. Because the dip domain is an unsubstantiated artifact of kink-style section construction, its use for the projection of structures to depth is not valid. Namson and Davis also determine the cutoff angle of their Santa Lucia thrust from the trough dip-domain, an inference that is also not valid.
In addition, Namson and Davis (1990) interpret the Santa Lucia Range anticlinorium to die out along trend to the southeast into a northwest termination of the West Huasna fault (Figure GSG Q10-A.1).
However, the West Huasna fault does not terminate as shown by Namson and Davis, but continues along strike to the northwest as the Oceanic fault (Hall and others, 1979).
Finally, available geophysical data provide no supporting evidence for the continuation of the postulated Santa Lucia Range anticlinorium offshore into the Estero Bay, limiting the length of this proposed anticlinorium to less than 40 kilometers. Thus, it is inappropriate to draw regional structural interpretations from the existence of an anticlinorium that, even if present, is locally restricted. More careful documentation of the possible existence and dimensions of the anticlinorium and integration of the structure with regional structural deformation is required before an assessment of mid-crustal structure and potential seismic sources can be made using the modeling approach employed by Namson and Davis (1990).
La Panza Range Anticlinorium. Namson and Davis (1990) interpret a broad (18 kilometers wide) flat-crested anticline, the La Panza Range anticlinorium, between the Huasna basin and the Carrizo Plain on cross section A-A. They model this structure as a fault-bend fold caused by slip on the blind Black Mountain thrust. The existence and dimensions of this structure are poorly defined by surface data. There are also no known subsurface data to constrain the crustal structure postulated by Namson and Davis in this region.
The dip of the southwest limb (forelimb) of the anticline is derived from the northeast limb of the Huasna syncline. However, the tip of the Black Mountain thrust, which lies beneath this limb, is folded by ramping on the underlying Santa Lucia thrust. The forelimb of the La Panza Range anticlinorium should therefore be folded, because it also partly overlies the Santa Lucia thrust. This is not indicated by Namson and Davis (1990), an inconsistency in the application of fault-bend fold theory that significantly alters the structural model.
Namson and Davis (1990) define the northeast limb (backlimb) dip domain of the anticlinorium and the cut-off angle of the underlying Black Mountain thrust by the northeastward dip of sediments in the Tertiary basin of the Carrizo Plain. These dips can alternatively be explained by northeastward tilting of the basement during basin formation, similar to the basement/cover relationships at the forelimb of the Point San Luis anticline on cross section B-B (Figure GSG Q10-A.4). Also, as mentioned previously, it is extremely doubtful that Salinian basement granite deforms in the manner of a bedded brittle medium to produce a fault-propagation fold under compression.
UpliftRates and Patterns (predicted versus observed). The geometric modeling technique employed by Namson and Davis (1990) is useful for predicting regional rates and patterns of crustal shortening and uplift. These predicted rates can then be compared to actual rates determined from independent techniques (for example, geologic, geodetic, and seismic moment) as a test of the validity and accuracy of the structural model. In our review of their model, we calculated the rate of uplift predicted by the geometry and depth of the faults depicted on cross section A-A and the slip rates given by Namson and Davis (1990, Table 1). We also included in the calculations a very simple compensation for isostatic subsidence caused by instantaneous tectonic thickening of the crust by 3.5 kilometers. More sophisticated calculations of isostatic subsidence that consider loading over time and account for lithospheric flexuring may change the predicted uplift rates by a small amount, but will have little effect on the spatial distribution of the relative values. We then compared this predicted rate and pattern of uplift with the rate and pattern of uplift determined from the age and distribution of marine and fluvial terraces in the region (PG&E, 1988; Hanson and others, in review).
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A t chment S 10-A March 1 Pa e2 The pattern of uplift predicted from Namson and Davis'odel does not agree with the pattern of uplift determined from geologic data, as shown in Figure GSG Q10-A.16. Investigations of Quaternary deformation of the San Luis/Pismo structural block indicate it is undergoing rigid block uplift at a rate of 0.1 to 0.2 millimeter per year without internal deformation parallel to the long axis of the block (Lettis and others, in review). Namson and Davis'odel, however, suggests not only considerably higher uplift rates of up to approximately 0.8 millimeter per year for the backlimb of the Point San Luis anticline, which roughly coincides with the boundaries of the San Luis/Pismo structural block, but also variable uplift rates across the block that are not compatible with geologic observations. Similarly, the age and distribution of Quaternary deposits beneath Morro Bay indicate that the topographically subdued Los Osos Valley is subsiding at a rate greater than 1 millimeter per year (Lettis and others, in review). Calculations of uplift rates from Namson and Davis'odel, however, indicate that the Los Osos Valley should be uplifting at a rate of 0.12 millimeter per year, a rate comparable to that observed for many areas of the topographically well expressed San Luis/Pismo block.
According to the Namson and Davis (1990) model, the backlimb of the Point San Luis anticline in cross section B-B (the San Rafael Range) is being uplifted at a rate of about 0.4 millimeter per year.
Investigations of the elevations of marine terraces (Figure GSG Q10-A.14), however, indicate that this region has experienced a mid- to late Quaternary uplift rate of 0.13 to 0.15 millimeter per year (PG&E, 1988; Hanson and others, in review). In addition, Namson and Davis'odel indicates that the Santa Maria Valley is undergoing Quaternary uplift above a ramp on the Purisima-Soloman thrust fault at a rate of about 0.20 millimeter per year. Their model also predicts localized net subsidence of the southern portion of the basin in the region of the Lompoc-Purisima anticline.
Although we have not conducted investigations of the Quaternary uplift rates throughout the onshore Santa Maria Basin, the absence of elevated marine terraces and the accumulation of thick deposits of the Paso Robles Formation indicates that the basin is generally experiencing net Quaternary subsidence, with regions of localized uplift, not, as Namson and Davis suggest, net uplift with regions of localized subsidence.
These comparisons indicate that the Namson and Davis (1990) model predicts uplift rates much higher than those geologically recorded for the San Luis/Pismo structural block and inferred by geologic relationships for the Santa Maria Basin. This discrepancy might possibly be resolved by (1) decreasing the rate of crustal shortening and thus the slip rates for each of Namson and faults; (2) decreasing the cut-off angles of the fault ramps and splays; (3) more rigorousDavis'redicted modeling of the isostatic subsidence; or (4) a combination of the above. More importantly though, the spatial distribution of uplift rates will remain unchanged by these adjustments. The spatial distribution of uplift rates predicted by Namson and Davis'odel, which does not agree with observed patterns of uplift, can be changed only by altering the lateral position or depth of the thrust faults shown on their cross sections. This, however is not permitted by their representation of the surface geology.
IMPLICATIONS FOR SEISMIC HAZARD ASSESSMENT Namson and Davis (1990) provide an assessment of the seismic hazard in the southern Coast Ranges citing the dimensions of the faults shown on their cross sections and the slip rates calculated for those structures (Namson and Davis, 1990, Table 1). These assessments are inaccurate for two reasons.
First, Namson and Davis attribute all the regional shortening indicated by their model to a single basal detachment. In so doing, they maximize the amount of slip, and hence slip rates, on that fault and on the second-order faults (that is, thrust ramps), thereby maximizing the potential seismic hazard. However, brittle-regime Neogene contractile strains imposed upon the highly anisotropic and chaotic Franciscan Complex are apt to be widely distributed, rather than narrowly confined.
This is suggested by the diffuse pattern and low magnitude of seismic events in the San Luis/Pismo block region (Figure GSG QIO-A.17). It is likely that a significant amount of shortening is accommodated by non-seismogenic mechanisms, such as the development of kink folds and cleavages Diablo Canyon Power Plant Pacific Gas and Electric Cantpany long Tenn Seismic Program
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achm n 10-A March 1990 Pa e2 2.0 2.0 1.5 1.0 f 1.0
'l 0.5 0$
y .'p'::(~rgb s
%7 P A.5 D -1.0 -1.0 Morrow Bay basin projected into section 5-1 mmryr EXPLANATION
-1.5 Prodictod fautt-rotated uptift
-2.0 -2.0 Predicted net uplift/subsidence Catcuhted isostatic subsidence Obsavcd uptiftfsubsidenco Hosgrl Los Osos MidCeezoic fauft fault unconfomrity
?one zone 3.5 km thickening Topography 2 2 0
E 5 -5 O
ur -10 -10 UJ
-15 0 10 Km -15 Figure GSG QIO-A.16 Diagram comparing uplift rates predicted by Namson and Davis'1990) model along cross section A-A with uplift rates estimated from elevations of dated marine and fluvial terraces. Namson and Davis'redicted uplift rates are shown prior to and following simple calculations for isostatic subsidence.
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30'0-A tchm n March 1 1980 - May/1988 Relocations 0
4 0
Magnitude 20' 00 09 o 10 19 o 2.0 2.9 0 3.0 3.9 0
o Oo o ~
farII tg 10' r o 0
/ o o
354 10'21'0km 50'0'0'0'n Diablo Canyon Power Plant NOTE: Base map explanation provided on Plate 3.
Hosgri fault zone A
A'ocho fault ~ Los Osos fault zone B B'
~oo o o 0 o
,,ooo 5 g
-10 0 E
-10 0 o 0 0 P'0 o 0 0
o 0 o O O
-20 -20 10 20 10 20 Distance (km) Distance (km)
Figure GSG QIO-A.17 Map locations and cross sections of relocated earthquakes for the period 1980-1988 in the region of the San Luis/Pismo structural block. The cross sections include all earthquakes projected perpendicular to the section (from PG&E, 1988).
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Attachmen 1 -A March 1 0 Pae 1 and ductile flow of the argillaceous matrix by reactivation of the existing shear fabric. On a crustal-scale model these strains are perceived as layer-parallel shortening and cannot be accounted for by the model of Namson and Davis (1990).
Second, because the potential maximum magnitude earthquake an individual structure can generate is closely related to the surface area of the structure, an accurate assessment of seismic hazard is dependent upon the accurate determination of subsurface crustal geometries. Namson and Davis (1990) admit that the depth to the detachment in their cross sections is poorly constrained, but that the detachment and ramps "...could be moved up or down several kilometers and still be used to resolve the structural geometries with only minor changes in the fault slip or regional shortening."
Although this may be true, the implications for seismic hazard posed by these postulated thrust faults would be significantly affected. Changing the geometry of the underlying faults will change their dimensions and hence have a significant effect on the calculation of the potential magnitude of earthquakes generated by these structures.
We do not agree with Namson and Davis'1990) assessment of the seismic hazard for the southern Coast Ranges because their model does not constrain the dimensions of postulated potentially seismogenic structures, nor does it properly represent the slip rates on known faults. In fact, the geologic relationships upon which they base their model do not agree with known, observed geologic relationships in the Los Osos/Santa Maria domain. For example, their model would consider the Hosgri fault to not be a seismogenic source, despite clear evidence for activity. The dimensions of the structures shown in their model are unsubstantiated, rendering their calculations of earthquake magnitude potential and recurrence intervals invalid.
SUMMARY
Namson and Davis'1990) model for the late Cenozoic structural geometry and kinematics of the earth's crust in south-central coastal California is derived from the construction of two north-northeast-trending retrodeformable cross sections. Applying the theories of fault-bend folding and fault-propagation folding (Suppe, 1983; Suppe and Medwedeff, 1984), they interpret the regional crustal structure as a major southwest-verging blind thrust system above a basal detachment (floor thrust) within the Franciscan Complex basement at a depth of 11 to 14 kilometers. Namson and Davis (1990) use this model to calculate the amount of regional late Cenozoic shortening and slip rates for the individual structures shown on their cross sections. In applying this technique, however, Namson and Davis oversimplify the regional geology and contradict much of the previous surface and subsurface geologic mapping done in this area. Although their model attempts to define the neotectonic development of the region, they fail to include a large portion of the recent investigations of neotectonic deformation performed by PG&E (19SS) and much of the detailed bedrock and subcrop mapping of Hall and Corbato (1967), Hall (1973a, 1973b, 19S2) and Hall and others (1979).
In addition, Namson and Davis'1990) model forces the regional geology to fit a rigidly defined theory of crustal deformation by virtue of a few broadly applied assumptions (for example, strain partitioning, concentric folding, and ramp-flat thrust geometry). The result is a hypothetical model that does not generally conform to observations of local deformation mechanisms, and fails to accommodate many of the observed neotectonic features of south-central California.
The Namson and Davis model, in itself, demonstrates the applicability of the powerful technique of cross-section balancing for understanding crustal-scale deformation in a region where the deep-level geometry and kinematics are difficultto resolve by classical methods. However, we have shown that major aspects of the geometry of Namson and Davis'1990) model are based on invalid assumptions and violate many observations of both regional and local geologic relationships. Because their calculations for regional shortening and rates of slip are derived from these postulated geometries, they also are unsubstantiated. As a result, Namson and Davis'onclusions are not appropriate for assessing seismic hazard in south-central coastal California.
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A chm n 10-A March I P 2 REFERENCES Ballance, P. F., Howell, D. G., and Ort, K., 1983, Late Cenozoic wrench tectonics along the Nacimiento, South Cuyama, and La Panza faults, California, indicated by depositional history of the Simmler Formation; in Anderson, D. W., and Rymer, eds., Tectonics and Sedimentation along Faults of the San Andreas System: Pacific Section, Society of Economic Paleontologists and Mineralogists,
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Bally, A. W., Gordy, P. L., and Stewart, G. A., 1966, Structure, seismic data, and orogenic evolution of southern Canadian Rocky Mountains: Bulletin of Canadian Petroleum Geology, v. 14, p. 337-381.
Boyer, S. E., 1976, Formation of the Grandfather Mountain window, North Carolina, by duplex thrusting (abs.): Geological Society of America Abstracts with Program, v. 8, p. 788-789.
Boyer, S. E., and Elliott D., 1982, Thrust systems: American Association of Petroleum Geologists Bulletin, v. 66, p. 1196-1230.
Clark, D. H., Hall, N. T., Hamilton, D. H., and Heck, R. G., in press, Structural analysis of late Neogene deformation in the central offshore Santa Maria Basin, California.
Cowan, D. S., 1974, Deformation and metamorphism of the Franciscan subduction zone complex northwest of Pacheco Pass, California: Geological Society of America Bulletin, v. 85, 1623-1634.
Cowan D. S., 1985, Structural styles in Mesozoic and Cenozoic melanges in the western cordillera of North America: Geological Society of America Bulletin, v. 96, p. 451-462.
Cox, A., and Engerbretson, D., 1985, Change in motion of Pacific Plate at 5 Myr BP.: Nature,
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Crouch, J., Bachman, S. B., and Shay, J. T., 1984, Post-Miocene compressional tectonics along the central California margin; in Crouch, J., and Bachman, S. B., eds., Tectonics and Sedimentation Along the California Margin: Pacific Section, Society of Economic Paleontologists and Mineralogists,
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Dahlstrom, C. D. A., 1969, Balanced cross sections: Canadian Journal of Earth Sciences, v. 6, p. 743-757.
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Durham, D. L., 1965, Evidence of large strike-slip displacement along a fault in the southern Salinas Valley: U. S. Geological Survey Professional Paper 525-D, p. D106-D111.
Graham, S. A., and Dickinson, W. R., 1978, Evidence for 115 km of right-slip on the San Gregorio-Hosgri fault trend: Science, v. 199, p. 179-181.
Greenhaus, M. R. and Cox, A., 1979, Paleomagnetism of the Morro Rock-Islay Hill Complex as evidence for crustal block rotations in central coastal California: Journal of Geophysical Research,
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Hall, C. A., Jr., 1973a, Geology of the Arroyo Grande Quadrangle, California: California Division of Mines and Geology, Map Sheet 24, scale 1:48,000.
Hall, C. A., Jr., 1973b, Geologic map of the Morro Bay South and Port San Luis Quadrangles, San Luis Obispo County, California: U. S. Geological Survey Miscellaneous Field Studies Map MF-511, scale 1:24,000.
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Attachment 10-A March 1 0 Pa e 33 Hall, C. A., Jr., 1981, San Luis Obispo transform fault and middle Miocene rotation of the western Transverse Ranges, California: Journal of Geophysical Research, v. 86, p. 1015-1031.
Hall, C. A., Jr., 1982, Pre-Monterey subcrop and structure contour maps, western San Luis Obispo and Santa Barbara Counties, south-central California: U.S. Geological Survey Miscellaneous Field Studies Map MF-1384, 6 sheets, scale 1:62,500.
Hall, C. A., and Corbato, C. E., 1967, Stratigraphy and structure of Mesozoic and Cenozoic rocks, Nipomo Quandrangle, southern Coast Ranges, California: Geological Society of America, v. 78,
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Hall, C. A., Jr., Ernst, W. G., Prior, S. W., and Wiese, J. W., 1979, Geologic map of the San Luis Obispo-San Simeon region, California: U. S. Geological Survey Map I-1097, scale 1:48,000.
Hamilton, D. H., and Willingham, C. R., 1978, Evidence for a maximum of 20 km of Neogene right-slip along the San Gregorio fault zone of central California (abs.): Transactions, American Geophysical Union, v. 59, no. 12, p. 1210.
Hanson, K. L., Lettis, W. R., Wesling J. R., Kelson, K. I., Mezger, L., 1989, Correlation and dating of marine terraces, south-central coastal California; implications for late Quaternary crustal deformation: 28th International Geological Congress, v. 2, p. 2-26 to 2-27.
Hanson, K. L., Wesling, J. R., Lettis, W. R., Kelson, K. I., and Mezger, L., in review, Correlation and ages of Quaternary marine terraces, south-central California; in Alterman, I. B., ed.,
Seismotectonics of Central and Coastal California: Geological Society of America Special Paper.
Harbert, W., and Cox, A., 1988, Late Neogene motion of the Pacific Plate: Journal of Geophysical Research, v. 94, no. B3, p. 3052-3064.
Hart, E. W., 1976, Basic geology of the Santa Margarita area, San Luis Obispo County, California:
California Division of Mines and Geology Bulletin 199, 45 p.
Hornafius, J. S., 1985, Neogene tectonic rotation of the Santa Ynez Range, western Transverse Ranges, California, suggested by paleomagnetic investigation of the Monterey Formation: Journal of Geophysical Research, v. 90, p. 12503-12522.
Hornafius, J. S., Luyendyk, B. P., Terres, R. R., and Kamerling, M. J., 1986, Timing and extent of Neogene tectonic rotation in the western Transverse Ranges, California: Geological Society of America Bulletin, v. 97, p. 1476-14S7.
Hsu, K. J., 1969, Preliminary report and geologic guide to Franciscan melanges of the Morro Bay-San Simeon area, California: California Division of Mines and Geology Special Publication 35, 46 P.
Killeen, K. M., 1988, Timing of folding and uplift of the Pismo syncline, San Luis Obispo County, California: M.S. Thesis, University of Nevada, Reno, 75 p. (unpublished).
Lettis, W. R., Hall, N. T., and Hamilton, D. H., 19S9, Quaternary tectonics of south-central coastal California: 28th International Geological Congress, v. 2, p. 2-285 to 2-286.
Lettis, W. R., Hall, N. T., Kelson, K. I., Hanson, K. L., and Wesling, J. R., in review, Evidence for segmentation of the Los Osos fault zone, south-central California; in Alterman, I. B., ed.,
Seismotectonics of Central and Coastal California: Geological Society of America Special Paper.
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I Attachment S 10-A March 1 0 Pae 4 Luyendyk, B. P., and Hornafius, J. S., 1983, Chapter 11 - Neogene crustal rotations, fault slip, and basin development in Southern California; in Ingersoll, R. V., and Ernst, W. G., eds, Cenozoic Development of Coastal California: Rubey Volume, v. 6, Prentice-Hall, Inc., Englewood Cliffs, New Jersey, p. 259-283.
Luyendyk, B. P., Kamerling, M. J., and Terres, R. R., 1980, Geometric model for Neogene crustal rotations in southern California: Geological Society of America Bulletin, v. 91,.p. 211-217.
Luyendyk, B. P., Kamerling, M. J., Terres, R. R., and Hornafius, J. S., 1985, Simple shear of southern California during the Neogene: Journal of Geophysical Research, v. 90, p. 12454-12466.
McCulloch, D. S., 1987, Regional geology and hydrocarbon potential of offshore central California; in Scholl, D. W., Grantz, A., and Vedder, J., eds., Geology and Resource Potential of the Continental Margin of Western North America and Adjacent Ocean Basins, Beaufort Sea to Baja California:
American Association of Petroleum Geologists, Circum Pacific Earth Science, v. 6, p. 353-401.
Mount, V., and Suppe, J., 1987, State of stress near the San Andreas fault; Implications for wrench tectonics: Geology, v. 15, p. 1143-1146.
Namson, J. S., 1987, Structural transect through the Ventura Basin and western Transverse Ranges; in Davis, T. L., and Namson, J. S., eds., Structural Evolution of the Western Transverse Ranges:
Pacific Section, Society of Economic Paleontologists and Mineralogists, 48A, p. 29-41.
Namson, J. S., and Davis, T. L., 1988a, Seismically active fold and thrust belt in the San Joaquin Valley, central California: Geological Society of America Bulletin, v. 100, p. 257-273.
Namson, J. S., and Davis, T. L., 1988b, Structural transect of the western Transverse Ranges, California; implications for lithospheric kinematics and seismic risk evaluation: Geology, v. 16,
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Namson, J. S., and Davis, T. L. 1990, Subsurface study of the late Cenozoic structural geology of the Santa Maria Basin, western Transverse Ranges and southern Coast Ranges, California: American Association of Petroleum Geologists, in press.
Pacific Gas and Electric Company, 1988, Final report of the Diablo Canyon Power Plant Long Term Seismic Program: U.S. Nuclear Regulatory Commission Docket Nos. 50-275 and 50-325.
Page, B. M., 1981, Chapter 13 - The Southern Coast Ranges; in Ernst, W. G., ed., The Geotectonic Development of California: Rubey Volume, v. 1, Prentice-Hall, Inc., Englewood Cliffs, New Jersey,
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Silver, E. A., 1978, The San Gregorio-Hosgri fault zone, an overview; in Silver, E.A., and Normark, W. R., eds., San Gregorio-Hosgri Fault Zone, California: California Division of Mines and Geology Special Report 137, p. 1-2.
Suppe, J., 1980, A retrodeformable cross section of northern Taiwan: Proceedings of the Geological Society of China, v. 23, p. 46-55.
Suppe, J., 1983, Geometry and kinematics of fault-bend folding: American Journal of Science,
- v. 283, p. 684-721.
Suppe, J., 1985, Principles of Structural Geology: Prentice-Hall, Englewood Cliffs, New Jersey.
Suppe, J., and Medwedeff, D. A., 1984, Fault propagation folding (abs.): Geological Society of America Abstracts with Program, v. 16, p. 670.
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Attachment SG 10-A March I Pae 5 Suppe, J., and Namson, J. S., 1979, Fault bend origin of frontal folds of the western Taiwan fold and thrust belt: Petroleum Geology of Taiwan, v. 16, p. 1-18.
Trehu, A., and Wheeler, W. H., 1987, Possible evidence for subducted sedimentary materials beneath central California: Geology, v. 15, p. 254-258.
Weldon, R. J., and Humphreys, E. D., 1989, Comments and replies on "Structural transect of the western Transverse Ranges, California; implications for lithospheric kinematics and seismic risk evaluation": Geology, v. 17, p. 769-770.
Yeats, R. S., and Huftile, G. J., 1989, Comments and replies on "Structural transect of the western Transverse Ranges, California; implications for lithospheric kinematics and seismic risk evaluation":
Geology, v. 17, p. 771-772.
Zoback, M. D., Zoback, M. L., Mount, V. S., Suppe,'J., Eaton, J. P. Healy, J. H., Oppenheimer, D.,
Reasenbery, P., Jones, J., Raleigh, C. B., Wong, L G., Scotti, O., and Wentworth, C., 1987, New evidence on the state of stress of the San Andreas fault system: Science, v. 238, p. 1105-1111.
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ueti n 12 Mrhl P el QUESTION GSG 12 Provide the. new information which was presented at the meeting about the pull-apart basin at the San Simeon-Hosgri stepover.
Understanding the structural relationship between the right-slip San Simeon fault zone and the Hosgri fault zone is essential for describing the contemporary tectonic behavior and seismic potential of the Hosgri fault zone, as well as for assessing the tectonic setting and nature of Quaternary deformation along the south-central margin of California. These two faults approach one another in the near-offshore region between San Simeon Bay and Estero Bay. In an effort to define and document the structural interaction between these two faults, we acquired and interpreted a variety of geophysical and geological data in this near-offshore region. The geophysical data and interpretations are presented in Response to Question GSG 1, Attachment GSG Ql-A (San Simeon/Hosgri pull-apart montage). A thorough discussion of deformation in the San Simeon Bay/Estero Bay region is presented in Attachment GSG Q12-A.
Our analysis of the data in the San Simeon Bay/Estero Bay region indicates that (1) the Hosgri and San Simeon fault zones are prominent, basement-involved structures that terminate in the near-offshore between San Simeon Point and Point Estero; (2) the faults overlap one another for a distance of about 10 to 12 kilometers and form a 5-kilometer-wide en echelon right stepover; and (3) the two faults are structurally related to one another via an actively subsiding basin within the stepover region that is bordered by normal faults and filled with late Pliocene and younger deposits.
Based on kinematic analyses and comparisons with subsiding basins along strike-slip faults worldwide, we interpret the subsiding basin between the San Simeon and Hosgri fault zones to be a tectonic pull-apart basin (Attachment GSG Q12-A), herein called the San Simeon/Hosgri pull-apart structure. Based on the right en echelon fault stepover, the intervening pull-apart structure, and documented right-slip character of the onshore San Simeon fault zone, we conclude that lateral slip is transferred between the San Simeon fault zone and the northern Hosgri fault zone via the pull-apart basin. The pull-apart structure accommodates the lateral slip on the two bordering strike slip faults. Theoretical and experimental modeling and worldwide observations indicate that pull-apart dimensions are related to cumulative lateral slip on the bordering master faults. Based on published relationships (Rodgers, 1980; Woodcock and Fischer, 1986), we estimate minimum slip rates of 1.0 to 4.0 millimeters per year for the southern San Simeon and northern Hosgri fault zones in order to form the subsiding basin that is present today (Attachment GSG Q12-A, Table GSG Q12-A.2).
These slip rates are consistent with time-averaged late Quaternary and Holocene slip rates of 1 to 3 millimeters per year determined independently from paleoseismic investigations and geologic mapping along the onshore reach of the San Simeon fault zone. The rates indicate that the San Simeon and Hosgri fault zones are right-slip faults comparable in order of magnitude of slip rate to the Hayward, Calaveras, San Jacinto, and Elsinore fault zones.
If the Hosgri fault zone is interpreted to be a thrust fault, strike slip on the San Simeon fault zone must progressively or abruptly die out southward, and the lateral strain must be absorbed on splay faults or folds. However, based on our studies of geology and geophysics in the San Simeon Bay-Estero Bay region, no candidate structures of this type are present to accommodate the lateral strain from the San Simeon fault zone.
There is no evidence of Quaternary compressional deformation in the Estero Bay/San Simeon Bay region. Deformation is primarily transtensional or tensional basin subsidence and normal faulting.
In addition, near-surface deformation does not indicate the presence of a deeper crustal underlying thrust fault. The transtensional deformation observed in the region, therefore, is not the result of extension in the crest of a fold within the hanging wall of a thrust fault. Furthermore, we have not found a published example of a thrust fault producing hanging-wall extensional deformation of similar dimensions, geometry, and tectonic setting (where one master fault is a known strike-slip fault) anywhere else in the world.
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ue tion 12 March 1 0 Pa e2 Seismic hazard analyses require that estimates'be made of likely rupture lengths along fault zones.
Theoretical and empirical studies indicate that extensional releasing stepovers or dilatational jogs are effective barriers to coseismic rupture along strike-slip fault systems worldwide (for example, Sibson, 1985, 1986; King, 1986). The San Simeon/Hosgri pull-apart basin is the largest known extensional releasing stepover along the entire San Gregorio/San Simeon/Hosgri fault system. We interpret the basin, therefore, to be a major, long-lived segmentation separating coseismic fault ruptures that occur on the southern San Simeon and northern Hosgri fault zones (see Response to Question GSG 15).
REFERENCES King, G. C. P., 1986, Speculations on the geometry of the initiation and termination processes of earthquake rupture and its relation to morphology and geological structure: PAGEOPH, v. 124,
- p. 567-585.
Rodgers, D. A., 1980, Analysis of pull-apart basin development produced by en echelon strike-slip faults; in Balance, P. F., and Reading, H. G., eds., Sedimentation in oblique-slip mobile zones:
International Association of Sedimentologists, Special Publication 4, p. 27-41.
Sibson, R. H., 1985, Stopping of earthquake ruptures and dilational jogs: Nature, v. 316, p. 248-251.
Sibson, R. H., 1986, Brecciation processes in fault zones; inferences from earthquake rupturing:
PAGEOPH, v. 124, nos. 1/2, p. 159-175.
Woodcock, N. H., and Fischer, M., 1986, Strike-slip duplexes: Journal of Structural Geology, v. 8, no. 7, p. 725-735.
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I' Atachm n 12-A March l 0 Pa el ATTACHMENTGSG Q12-A THE SAN SIMEON/HOSGRI PULL-APART STRUCTURE, IMPLICATIONS FOR LATE QUATERNARY ACTIVITYON THE HOSGRI FAULT ZONE INTRODUCTION The San Simeon and Hosgri fault zones are prominent structural features along the coast of south-central California(Figure GSG Q12-A.1). Detailed geologic mapping and paleoseismic investigations conducted along the onshore reach of the San Simeon fault zone near San Simeon Point document its geologic character as a major Quaternary right-slip fault (Hall, 1975; Weber, 1983; Hall and others, 1987; Hanson and others, 1987; PG&E, 1988; Hanson and Lettis, in review; Hall and Hunt, in review). The Hosgri fault zone, however, lies entirely offshore, where comparable geologic mapping and paleoseismic investigations cannot be performed. Evidence for the style and timing of deformation along this fault is primarily derived from remote'techniques such as seismic reflection profiles, side-scan sonar data, and potential field data, supplemented by analyses of bathymetry and seismicity data. Using a variety of these data sets, the fault has been variously interpreted as an active listric thrust fault (Crouch and others, 1984), an inactive, former basin-margin normal fault (Davis and McIntosh, 1987), and an active right-slip fault (McCulloch, 1987; Hamilton, 1987; PG&E, 1988). Based on regional and detailed geophysical, geologic, stratigraphic, and tectonic analyses, most authors interpret that the strike-slip character of the San Simeon fault zone extends southward onto the Hosgri fault zone, and that these two faults comprise the southern two components of the larger San Gregorio/Hosgri fault system (Silver, 1974, 1978; Graham and Dickinson, 1976, 1978a, 1978b; Hall, 1975, 1978; Greene, 1977; Leslie, 1980, 1981; Clark and others, 1984; PG&E, 1988).
Understanding the structural relationship between the well-documented San Simeon fault zone and the Hosgri fault zone is essential for describing the contemporary tectonic behavior and seismic potential of the Hosgri fault zone, as well as for assessing the tectonic setting and nature of Quaternary deformation along the south-central margin of California. These two faults approach one another in the near-offshore region between San Simeon Bay and Estero Bay (Figure GSG Q12-A.l). The nature of this fault interaction can theoretically take one of several forms: the two faults may merge as one through-going laterally continuous fault as proposed by Leslie (1980, 1981); the faults may extend to the north and south, respectively, as two distinct, separate, subparallel structures having little or no kinematic or structural interaction, as proposed by Hoskins and Griffiths (1971),
PG&E (1975), and Hamilton and Willingham (1977); the two faults may progressively or abruptly die out as they approach one another, in which case, strain along the faults would be consumed by secondary deformation at or along the fault terminations; or the two faults may be en echelon components of the same fault system, in which case, slip is transferred wholly or in part between the two faults across an en echelon stepover as proposed by PG&E (1988).
In an effort to define and document the structural interaction between the San Simeon and Hosgri fault zones, we acquired and interpreted a variety of geophysical and geological data in the San Simeon Bay to Estero Bay region. Our studies show that the San Simeon and Hosgri fault zones are distinct basement-involved faults that terminate in the near-offshore region. Where these faults overlap, they form a 5-kilometer-wide, en echelon right stepover that contains a subsiding late Tertiary and Quaternary basin. The presence of a subsiding basin within a right stepover between two right-slip faults is a widely recognized structural feature referred to as a pull-apart basin (Burchfield and Stewart, 1966; Mann and others, 1983), a releasing step or bend (Crowell, 1974), or a dilatational jog (Sibson, 1985, 1986a, 1986b). Empirical and theoretical kinematic studies of strike-slip fault systems indicate that en echelon steps between offset strands of strike-slip faults produce a characteristic style and pattern of deformation dependent on both the orientation of the stepover (right-stepping or left-stepping) and the sense of slip on the fault system.
Based on the right en echelon fault stepover, the intervening pull-apart structure, and the documented right-slip character of the onshore San Simeon fault zone, we conclude that lateral slip is transferred between the San Simeon fault zone and the northern Hosgri fault zone via the pull-Diablo Canyon Power Plant Pacific Gas and Electrfc Company Long Term Seismic Program
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At chment 12-A M rch l 0 Pa e2 Monterey
.Bay L,
+. 36o30 0 121' X oo x ~o (P~.
20 mi Gr 0 20 km Vp San Simeon
+ 35'30'220 Bay Estero Bay 120'35'30'oint Skier San Luis EXPLANATION
~ Diablo Canyon Power Plant Point Sal Arguello
+ 34'30'21'oint Figure GSG Q12-A.1 Regional map showing the location of the Hosgri and San Simeon fault zones in south-central coastal California.
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Atchmn 12-A March 1 0 Pa apart basin. If the Hosgri fault zone is interpreted to be a thrust fault, strike-slip on the San Simeon fault zone must progressively or abruptly die out southward, and the lateral strain must be absorbed on splay faults or folds. However, based on our studies of geology and geophysics in the San Simeon Bay-Estero Bay region, no candidate structures of this type are present to accommodate the lateral strain from the San Simeon fault zone.
In this attachment, we describe the evidence for the San Simeon/Hosgri pull-apart basin, the dimen-sions and structural history of the basin, and implications drawn from this structure in assessing the sense and rate of slip along the Hosgri fault zone. Based on empirical and theoretical studies of pull-apart basins worldwide that relate dimensions and age of the basins to rates of slip along the master bounding faults, we also estimate the rate of lateral slip on the Hosgri and San Simeon fault zones.
We compare these rates with independently derived rates from the San Simeon fault zone onshore and discuss the implications of the pull-apart basin for seismic rupture segmentation along the Hosgri and San Simeon fault zone.
REGIONAL SETTING The San Gregorio/Hosgri fault system is a complex system of faulting that is subparallel to and lies primarily offshore from the central California coast (Figure GSG Q12-A.1). This system of faults branches from, the San Andreas fault near Bolinas Lagoon on the north (Jennings, 1975) and dies out to the southeast near Point Arguello (Cummings and others, 1987; PG&E, 1988). From the time the Hosgri fault zone was first identified in 1970 (Wolf and Wagner, 1970; Hoskins and Griffiths, 1971; Wagner, 1974), the major focus of research on the San Gregorio/Hosgri fault system was the estimation of the amount and timing of large-scale, right-slip displacement that had occurred along it. Based on interpreted offset stratigraphic units, cumulative Cenozoic lateral offsets of 80 to 150 kilometers have been proposed for all or parts of the fault system (Silver, 1974; Hall, 1975; Graham, 1976; Graham and Dickinson, 1976, 1978a, 1978b; Greene, 1977; Silver and Normark, 1978; Seiders, 1979; Blake and others, 1978; Clark and others, 1984). In contrast to these estimates, several authors have argued for lesser amounts of Neogene lateral offset along the southern part of the fault system, ranging from near zero on the Hosgri fault zone (Crouch and others, 1984) to estimates of less than 30 kilometers on the Hosgri and San Simeon fault zones (Hamilton and Willingham, 1977, 1978, 1979; Seiders, 1979; Hamilton, 1984).
The cumulative lateral displacement occurred over time intervals that likely span several different tectonic settings. The fault system may have evolved in the Oligocene to early Miocene as transform faulting replaced subduction along the North American plate margin (Atwater, 1970; Atwater and Molnar, 1973; Graham, 1978; McCulloch, 1987; Hall, 19S1). The presence of large Miocene basins along the continental shelf of California suggests that early faulting along the fault system was extensional or transtensional (McCulloch,.19S7; McCulloch and Lewis, 1988). In the early Pliocene,.
a change in relative plate motion between the Pacific and North American plates placed the plate margin under a component of compression (Page and Engebretson, 1984; Engebretson and others, 1985; Cox and Engebretson, 1985; Harbert and Cox, 1989). Estimates of subsequent deformation on all or parts of the fault system range from purely thrust (Crouch and others, 1984) to convergent strike slip (for example, Hall, 1975; PG&E, 1988).
Interpretations of the contemporary behavior of the Hosgri fault zone are hindered by two important factors: first, the entire fault system is offshore, where established techniques for assessing Quaternary activity (for example, mapping, trenching, drilling) cannot be performed; and second, the fault system has an imprint of preexisting deformation that spans significant changes in tectonic setting. For example, investigation of the fault zone by geophysical techniques provides images of the upper crust from the sea floor to several kilometers that reflect the composite structural and stratigraphic history of the fault. The assessment of Quaternary activity on the Hosgri fault zone, therefore, must come from an integrated interpretation of offshore geophysical data combined with geologic and tectonic analyses of the fault's structural association with the San Gregorio, Sur, and San Simeon fault zones to the north.
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Attachment 12-A March 1 0 Pa e4 The San Gregorio, Sur, and San Simeon fault zones, although primarily offshore, come onshore at four locations, and investigations of their Quaternary activity have been performed there. Geologic studies of the San Gregorio fault zone at Seal Cove and Ano Nuevo near Santa Cruz (Weber and Lajoie, 1977, 1979a, 1979b), the Sur fault zone in the Point Sur area (Hall, 1989), and the San Simeon fault zone in the Point Piedras Blancas area (PG&E, 1988; Hall, 1975; Envicom, 1977; Weber and others, 1981; Weber, 1983; Manson, 1985; Hanson and Lettis, in review; and Hall and Hunt, in review) indicate that these faults are prominent right-slip fault zones.
The San Simeon fault zone is a prominent structural feature where it is locally exposed onshore from San Simeon Point to north of Ragged Point, a distance of 20 kilometers (Figure GSG Q12-A.1). At the northern end of San Simeon Bay, the fault zone consists of major boundary faults that define a zone about 120 meters wide. The zone splays to the northwest and has several traces that trend more westerly than the N35'W strike of the main eastern trace of the fault zone. Although the dominant sense of displacement along major strands within the zone is right-slip, the more westerly trending fault splays have a component of dip-slip displacement (Hall, 1975; PG&E, 1988; Weber, 1983; Hanson and Lettis, in review).
Several studies have been performed to quantify the rate of lateral slip across the San Simeon fault zone. Based on reconnaissance mapping of marine terraces between Cayucos and Cape San Martin and correlation of marine terraces across the San Simeon fault zone, Weber (1983) interpreted a Pleistocene right-lateral slip rate of between 5 and 19 millimeters per year, with a preferred estimate of 8 to 10 millimeters per year for the past 350 to 400 thousand years. Based on a subsequent detailed analysis of deflected stream drainages and more detailed marine terrace mapping, supple-mented by drilling and soil-profile development studies in the San Simeon Point region directly north of San Simeon Bay, PG&E (1988) and Hanson and Lettis (in review) estimate right-lateral slip rates of 1 to 3 millimeters per year during the last 214 thousand years. Similarly, PG&E (1988) and Hall and Hunt (in review) estimate a minimum Holocene right-lateral slip rate of 1 to 2 millimeters per year, on the basis of offset fluvial deposits exposed in trench excavations along Oak Knoll Creek and Airport Creek. In each of these studies, a minor amount of up-on-the-west vertical separation is also recognized, with the ratio of lateral to vertical separation ranging from 5:1 to 20:1.
Quaternary activity on the Hosgri fault zone is less well-constrained than on the San Simeon fault zone, primarily because it lies entirely offshore where comparable Quaternary mapping and paleoseismic investigations cannot be performed. Crouch and others (1984) cite down-on-the-west basement separation across the Hosgri fault zone, the presence of subparallel folds in the offshore Santa Maria Basin, thrust focal mechanisms of microseismicity in south-central coastal California, and shallow-dipping reflectors on seismic reflection data to infer that the Hosgri fault zone is one of a region-wide system of west-verging, shallow-dipping listric thrust faults. Retrodeformable cross sections prepared by Davis and McIntosh (1987) and Namson and Davis (1990) that are based on limited offshore geophysical data and selected onshore geologic data suggest that the Hosgri fault zone is a steeply northeast dipping, rotated former basin-margin normal fault that is not active in the contemporary tectonic regime. These interpretations, however, do not provide an estimated slip rate for the Hosgri fault zone, nor do they address the structural association of the fault system with the known strike-slip San Simeon fault zone on trend to the north.
Detailed analysis of seismic reflection data, geologic and geomorphic data, seismicity data, and consideration of regional tectonic kinematics have led PG&E (1988) and Lettis and others (1989) to interpret that right slip is the dominant contemporary sense of displacement along the Hosgri fault zone. These authors cite reversals in apparent sense of vertical separation both down-dip and along strike, linearity of the trace, a steeply dipping fault plane, negative and positive flower structures with associated folding and thrusting, strike-slip earthquake focal mechanisms, and local pull-apart basins as indicative of a right-slip fault system. Based on structural association with the San Simeon fault zone, these authors also estimate a lateral slip rate of 1 to 3 millimeters per year that progressively decreases southward as slip is consumed by crustal shortening along bordering, more westerly trending reverse faults within the Los Osos/Santa Maria domain (Lettis and others, 1989).
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Attachment GS 12-A March 1 DATA To evaluate the lateral continuity of the Hosgri and San Simeon fault zones and the nature of their interaction, we conducted a program of geophysical exploration, seafloor sampling, and geomorphic analyses in the near-shore coastal region between San Simeon Bay and Estero Bay. Geophysical data consist of both common depth point (CDP) and shallow high-resolution seismic reflection data, with a line spacing of approximately 2 kilometers and 1 kilometer, respectively (Figure GSG Q12-A.2).
These data are part of an extensive suite of offshore seismic data obtained by PG&E for their evaluation of offshore geologic structure, and are described in PG&E (1988). CDP data acquired for PG&E or purchased from various companies were shot between 1976 and 1986 using sparker, watergun, or airgun sources. In general, the CDP data are deep-penetration (2 seconds or more two-way travel time (TWTT)) and 24-fold or greater. High resolution seismic reflection data were acquired with sparker, boomer, airgun, or sleeve exploder sources, with stratigraphic penetration down to 1 to 2 seconds.
Data quality of both the CDP and high-resolution seismic data is variable depending upon the source and geologic complexity. Both stacked and migrated data sets were used in the interpretation process.
Geologic interpretations beneath about one second TWTT were based primarily on the CDP data.
Seafloor geomorphic features and shallow-crustal deformation above about one second TWTT were identified primarily from the high-resolution data set. The interpreted time sections were converted to depth-corrected sections using a velocity model developed from an integrated analysis of the seismic data and well information from the offshore Santa Maria Basin, and extrapolated into the Estero Bay/San Simeon Bay region. The velocity model was applied to the seismic sections to create depth sections having spatially correct geometric orientations for interpretation of stratigraphic thickness and structural deformation. Regional aeromagnetic data published by McCulloch and Chapman (1977) and Beyer and McCulloch (1988) were used to assist in the interpretation of depth to basement.
In addition to the interpretation of geophysical data, we collected samples of bedrock exposed in seafloor outcrops in shallow coastal areas, primarily to evaluate the offshore continuity of the San Simeon fault zone and to assess the age of Tertiary units interpreted on the seismic data. Lithologic samples and bedding attitudes were acquired from the seafloor by diver geologists in San Simeon Bay and along the coast between Cambria and Point Estero (Figure GSG Q12-A.2). Onshore, the San Simeon fault zone juxtaposes rocks of the Franciscan Complex on the east against rocks of the Miocene Monterey Formation on the west, with an intervening tectonic slice of Pliocene(?) to Pleistocene Careaga(?) Formation (Hall, 1975; Seiders, 1979; PG&E, 1988). We used the distribu-tion of these lithologies to constrain the location of the San Simeon fault zone in San Simeon Bay.
Farther offshore to the south, near-shore lithologic samples and geomorphic features such as scarps, lineaments, and the morphology of the coastline were used to constrain the location of the San Simeon fault zone (PG&E, 1988).
RESULTS Analysis of data from the offshore region between San Simeon Bay and Estero Bay indicate that (1) the Hosgri and San Simeon fault zones are prominent basement-involved structures that terminate in the near-offshore between San Simeon Bay and Point Estero; (2) the faults overlap one another for a distance of about 10 to 12 kilometers and form a S-kilometer-wide, right en echelon stepover; and (3) a subsiding basin bordered by normal faults and containing late Pliocene and younger deposits exists within the stepover region.
The locations of near-surface geologic structures and morphologic features in the San Simeon Bay/Estero Bay region are shown on Figure GSG Q12-A.2 (see also Attachment GSG Ql-A). The vertical distribution of these structures and late Cenozoic deposits in the area are shown on seismic reflection profiles S-l through S-6 (Figures GSG Q12-A.3 through A.8). These profiles were selected from the extensive suite of data examined to provide uniform spatial coverage of the area and to illustrate clearly the shallow stratigraphy and structure of the subsiding basin between the Diablo Canyon Power Plant Pacific Gas and Electric Company long Term Seismic Program
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tt chmen SG 12-A March 1 0 Pa e6 9 seafloor samples of San Franciscan Complex Simeon Point San 33 seafloor samples of Simeon Monterey Formation Bay EXPLANATION s CPS Fault dashed where inferred single tick on downthrown side, teeth on upthrown block of thrust fault Axial trend of syncline S-1 Seismic reflection line shown in yO X GSG Q1-A Area of GSG Q12-9 and GSG Q12-10
'g Point Estero Estero Bay Figure GSG Q12-A.2 Map of geologic structures in the San Simeon Bay to Estero Bay region showing locations of seismic lines.
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HOSGRI FAULT ZONE CGI 19 SHOIPOlhI4 > I 10 20 30 40 60 0 0.0 0.0 IQ n
n CO Vl Sl CL m
EO o
n 'C E5 C Ill 8 0 L5 CO 1.0 c 1.0 0
UJ C.
I-2.0 2.0 0.0 0.0 MPU TMU TMU 4P ~
MPU-
'/ r 4 T7 C
0 4'.0 O
TMU
~ 1.0 Ill Ol e
O O
IU ~c C.
R Cl' I o
g Ol CO ~
e Q '0
'lJ g CO O
R 2.0 2.0 CM-49 Figure GSG Q12-A.3 0
Migrated seismic line S-l (CM-49) (a) and interpretation (b). MPU is the mid-Fliocene unconformity and TMU is the top of Miocene unconformity.
HOSGRI FAULT ZONE CGI 19 SHOTIOINIS > 10 20 30 40 60 0.0 " '- -Ica 0.0
'D fQ n
n C1 IIl CCl III m
CII n C4 A D C
0 lll Cl 0 8 1.0
'zs co 1.0 0 CCI 0
0 a
I 2.0 2.0 0.0
=-=--
'PU,
>= ~---~-MPU 0.0 TMU-D C
0 m 1.0 1.0 0
lll 0 I
CCl CD
à 'a 8 0 0
ED CC '0 CV 2.0 2.0 CM-47 Figure GSG Q12-A.4 0
Migrated seismic line S-2 (CM-47) (a) and interpretation (b). MPU is the mid-Pliocene unconformity and TMU is the top of Miocene unconformity.
I HOSGRI FAULT ZONE I 19 SIIOIPOIN15 a 10 20 30 40 70 0.0 0.0 DI CD CD CD Ch CD CL Ill CD CD CD D
C 0
II
~rG Ill o 1.0 1.0 DI O IU 0 a
I 2.0 2.0 0.0 I ~ v ~, ~ D- 0.0 C
MPU
/
TMU MPU ~< O ~ + -
~ ~A
~
~
C
< ~r
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3 0 m CI o 1.0 1.0 O O
IU 0
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g OI O
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~
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'U CD O
C7 g CD 2.0 2.0 CM-45 Figure GSG Q12-A.5 Migrated seismic line S-3 (CM-45) (a) and interpretation (b). MPU is the mid-Pliocene unconformity and TMU is the top of Miocene unconformity.
k HOSGRI FAULT ZONE CGI ~ 19 SI I0 IMIN1S 5 10 20 30 40 60 70 l
0.0 0.0
'CS n
CO CII CII III CCI e CC O C 0 m C5 9 0 1.0 1.0 0 CO O III 0 E
I 2.0 2.0 0.0 MPU.'-=- --> --=I==t-~=-- -~=-- 0.0 TMU
- MP~U, ~~
'0 0 m 0
0 =. TMU ~~'-
1.0 1.0 O
IU 0 I
D CC I-D' D O 3 co CO D CI Vl 3 '0Cl O
'0 CD D
CD CC CI 2.0 2.0 CM-43 Figure GSG Q12-A.6 Migrated seismic line S-4 (CM-43) (a) and interpretation (b). MPU is the mid-Pliocene unconformity and TMU is the top of Miocene unconformity.
HOSGRI FAULT ZONE CGI ~ 19 SHO1POINTS > 1 10 20 30 40 50
'a 0.0 0.0 hl n
F Chl lh Ol CL m
ED n
o nO C 0 III 8
'CS O fO 1.O 1.0 O
III 0 I
0 2.0 2.0 0.0 0.0 TMO<
MPU e
'5 0 Pl 1.0 TMU r~ rg 1.0 O
III 0 CL R'D I
pg CJ' o
g CO CA co g 0 O
n CD CI CV Q
u 2.0 2.0 CM-39 0 figure GSG Q12-A.7 Migrated seismic line S-S (CM-39) (a) and interpretation (b). MPU is the mid-Pliocene.
unconformity and TMU is the top of Miocene unconformity.
ATTACHI'IENT GBS 912-A NARCH 1990 PAGE 12 THIS PAGE CONTAINS PROPRIETORY INFORNATION AND HAS NOT BEEN REPRODUCED
I' Attachment 12-A March 1 0 Pa el northern Hosgri fault zone and the southern San Simeon fault zone. Profiles S-l through S-5 are roughly normal to the regional structural grain (Figure GSG Q12-A.2) and illustrate the down-dip geometry and sense of vertical separation of faults identified in the area. Profile S-6 is parallel to the regional structural grain and illustrates the lateral continuity and along-strike terminations of the subsiding basin. Below, we describe the locations and nature of terminations of the San Simeon and Hosgri fault zones; this is followed by a description of stratigraphic units identified from the seismic data in the intervening subsiding basin, and discussion of the geometry and dimensions of the intervening subsiding basin.
Location and Terminatlons of the San Simeon and Hosgri Fault Zones The location of the San Simeon and Hosgri fault zones in the near offshore region between Estero Bay and San Simeon Bay is shown on Figure GSG Q12-A.2. The San Simeon fault zone extends S38'E from its well-constrained location onshore at San Simeon Bay for a distance of 38 to 40 kilometers to a southern termination offshore from Point Estero (Figure GSG Q12-A.2). Along this reach, the fault zone is inferred to have one or more prominent traces within a zone up to 1 kilometer wide, similar to the character of the fault zone onshore directly north of San Simeon Bay (PG&E, 1988; Hanson and Lettis, in review). For 5 kilometers southeast of the onshore San Simeon Bay exposure, the location of the fault zone is constrained by (1) the distribution of offshore lithologies (determined from sea-bottom hand-collected samples) that shows the Tertiary Monterey Formation juxtaposed against Franciscan Complex rocks across a narrow linear zone of sandy sea bottom, inferred to represent a slice of the less competent Pliocene(?) and Pleistocene Careaga Formation (Figure GSG Q12-A.2); (2) a series of low, east-facing seafloor steps comparable to the east-facing scarps mapped by Weber (1983) and Hanson and Lettis (in review) onshore to the north (Figure GSG Q12-A.2); and (3) disrupted sea floor imaged on high-resolution Scammon data within San Simeon Bay (PG&E, 1988). South of San Simeon Bay, the fault zone is inferred to lie shoreward of our geophysical data coverage. Because the Cambria coastline is not cut by a fault comparable in size to the fault zone observed onshore at San Simeon Bay (Hall and others, 1979), we interpret the San Simeon fault zone to lie in a narrow linear corridor between the coastline and the shoreward limit of the geophysical data. A shore-parallel syncline observed on the near-shore geophysical data for a distance of 15 kilometers in this area (Figure GSG Q12-A.2) and the straight, possibly structurally controlled, rocky coastline supports the inference that a major shore-parallel structural feature is present near the coast.
We project the fault zone southeast from this straight section of coastline an additional 7 kilometers to a point near Point Estero, based on the presence of southwest-facing seafloor scarps. No geomorphic expression of a fault is observed south of a point near the latitude of Point Estero.
Geophysical data in northern Estero Bay show an unbroken, northeast-dipping stratigraphic section along the projection of the fault zone (Figures GSG Q12-A.6 and A.9), indicating that the San Simeon fault zone does not extend into Estero Bay along the southeastern trend established by the high-resolution geophysical data in San Simeon Bay and the linear Cambria coastline.
The northern Hosgri fault zone extends across the eastern part of Estero Bay and terminates approximately 10 kilometers north of the latitude of Point Estero (Figure GSG Q12-A.2). From south to north, the fault trend bends from N25'W to N45 W. Throughout this reach, the location of the fault is well constrained from the grid of seismic data and is marked locally by disruptions in seafloor bathymetry. The fault zone is imaged primarily as abrupt lateral changes in reflection character or as near-vertical to steeply dipping reflection-free zones of incoherent energy.
Comparable features are observed in seismic sections across the San Andreas fault zone in Cholame Valley (Shedlock and others, in press) and across active and inactive strike-slip faults in both interplate and intraplate settings (Lemiszki and Brown, 1988). The primary fault traces are high-angle to a depth of 1 to 2 seconds (TWTT), below which reduced data quality precludes interpretation. The fault typically separates a thick section of coherent reflectors interpreted to be Tertiary strata on the west from a zone of incoherent energy typical of Franciscan basement on the east (Figures GSG Q12-A.3 through A.8).
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chm n 12-A March I 0 P 14 o II//
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+Op iei ini iui Boundary of San Simeon/Hosgrl pun.apart basin u Thickness cornours of post-Miocene deposits; contour interval 40 meters, dashed lines are supplementary
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contours at 20 meter intervals East. facing seafker step Cretaceous or Jurassic Franciscan Complex rocks at rfiver geobgy bcation Figure GSG Q12-A.10 Isopach map of seafloor to top of Miocene unconformity.
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Attachmen S 12-A March I 0 Pa e I San Simeon Point
+o Estero "0 Gg 5 km g4 Estero Bay EXPLANATION Fault, dashed where inferred, single tick on downthrown side Area of seafloor bedrock outcrop Post-late Wisconsinan sediment thickness > Sm Figure GSG Q12-A.11 Isopach map of post-late-Wisconsinan deposits.
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At achmen 12-A March I 0 Pa e 20 Post.mid Pliocene 0
San Simeon Fault Zone 0
~ ~ ~
-:Pliocene". ':;g):.'-:,
1000 q."::::.:.".-::;:":::;::;:,':-::"'".'-':::-: -'-'c 1-3mm/yr San SimeOn-Hosgrl
+s y X Point Ester'o Stepo ver s = separation o = overlap x = offset y depth <os osos 4/I Diablo Canyon 0 10 km Figure GSG Q12-A.17 Map-view illustration of geometric parameters used in assessing model-driven slip rates.
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Table GSG Q12-A.2
'a n51 ESTIMATED SLIP RATES ON SAN SIMEON AND HOSGRI FAULT ZONES BASED ON GEOMEIRY F OF SAN SIMEON/HOSGRI PULL-APART BASIN Eh SQ CL SLIP RATE fVl Estimated Observed Master-n EO Observed Basin Calculated Subsidence Rate Fault Overlap Estimated Age of cY Depth (assume basin Offset or (calculated from (assume overlap Sediment a depth is 2 observed Basin sediment thickaess ~ offset or basin 9
'n IQ Model sediment thickness) Stretching and sediment age) stretching) 5.3 Ma 2.8 Ma 18 Ka
~od e!!!~1980 y = 0. 10 x (where o ~ 2s) y = 0.2& km x 2 2.8 km (not applicable) z 0.53 z 1.0 (maximum post top mm/yr mm/yr y ~ basin depth of Miocene or post-x offset mid-Pliocene s ~ separation (spacing sediment thickaess) between master faults) o = overlap y 2 0.012 km x a 0.12 km a 6.7 (maximum post-late- mm/yr Wisconsinan sediment thickness) y? 0.006 km x 2 0.08 km 2 3.3 (average post-late- mm/yr Wisconsinan sediment thickness) x ~ 10 J S km 1.9 J 0.9 3,6 J 1,8 mm/yr mmlyr r0 a Woodcock and ischer 1 86 Ol CO O
g subsidence rate
= ~x slip rate y 0.2&km (aot applicable) 0.28 km/5.3 Ma 0.05 mm/yr
= x~ 10+Skm 1.8 k 0.9 mm/yr EA e
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A O y ~ depth y ~ 0.28 km 0.28 km/2.8 Ma ~ x ~ 10 f 5 km 3.6 J 1.8 e cD 0.10 mm/yr mm/yr g Ol x ~ length of basin g PO
I A achm n 12-A March 1 Pa e 30 and as detected on Scammon seismic data), to the Cambria coastline, where it is inferred to trend subparallel to a prominent syncline in Tertiary strata and thus separates Tertiary strata offshore from the Cambria slab basement complex onshore. A limited southeastern extent of the San Simeon fault zone is supported indirectly by the structural geometry of the pull-apart basin. The basin is a half-graben tilted to the west against the Hosgri fault zone. If the San Simeon fault extended along the entire margin of the basin one might expect a rhomb-shaped or full-graben basin geometry. The maximum value of 15 kilometers assumes that the San Simeon fault zone extends southeast to beyond Point Estero and accounts for the length of the observed pull-apart basin, which is about 15 to 18 kilometers long and is an indirect measure of fault overlap.
The equations developed by Hempton and Neher (1986) and Hempton and Dunne (1984) were not used to estimate slip rates in this study. The equation developed by Hempton and Dunne (1984) relates sediment thickness to basin length and does not provide a direct measure of fault displacernent over a period of time. The equation developed by Hempton and Neher (1986) relates basin subsidence to master-fault displacement based on claybox experiments. Their studies show that basin subsidence does not begin until a threshold of cumulative fault displacement is achieved, after which there is a linear relationship between basin subsidence and fault displacement. The threshold of cumulative fault displacement at a crustal scale is likely to be different, because of the rheological properties of the crust, and thus the equation developed by Hempton and Neher (1986) is not valid for this analysis, If we assume that from 1 to 3 kilometers of cumulative fault displacement is required to initiate basin subsidence in the San Simeon/Hosgri pull-apart, Hempton and Neher's equation yields a slip rate greater than 0.5 to 1.0 millimeters per year for the master border faults.
The estimated slip rates presented in Table GSG Q12-A.2 range from about 1 to 4 millimeters per year. We consider these rates to be minimum estimates for the lateral slip rates on the bordering strike slip faults for the following reasons:
Thickness of basin sediment is used as an indirect measure of structural relief and is predicated on the assumption that the basin is closed and has little or no loss of sediment.
Because the Hosgri/San Simeon basin lies offshore, however, the thickness of strata contained within the basin above the lowest mapped unconformity (our interpreted top of Miocene, Figures GSG Q12-A.3 through A.7, or alternatively, the mid-Pliocene) has been reduced by subsequent erosional events. At least two unconformities (for example, the interpreted mid-Pliocene unconformity (Figures GSG Q12-A.3 through A.7) and the late Wisconsinan unconformity), occur stratigraphically above the top of Miocene unconformity. These unconformities represent periods of erosion and/or intervals of non-deposition during periods of climatically controlled low sea level.
At least some lateral slip on the bordering master faults is likely accommodated by local compressional structures north and south of the pull-apart and by inefficient (or non-ideal) transfer of slip and structural evolution of the pull-apart basin.
The late Pliocene(?) and Pleistocene Careaga Formation occurs as a fault sliver within the San Simeon fault zone at San Simeon Bay about 5 to 10 kilometers north of the pull-apart basin. The Careaga Formation at this location is a fossiliferous marine conglomeratic deposit containing abundant angular clasts of locally reworked Monterey Formation shale and black chert. The fossil assemblage and local reworking of Monterey clasts, many of which are pholad-bored, indicates near-shore deposition, perhaps in the late Pliocene San Simeon/Hosgri pull-apart basin. The outcrop of Careaga at San Simeon Bay, therefore, may represent a faulted slice of the San Simeon/Hosgri pull-apart basin transported roughly 5 to 10 kilometers northward in the past 2 million years, or a slip rate of 2.5 to 5 millimeters per year on the San Simeon fault zone.
The age of sediment in the basin is not well constrained and may be considerably younger than the 5.3- and 2.8-million-year estimates used in the slip rate calculations.
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tachm nt 12-A March 1 0 Pa I Despite these limitations, the minimum estimated slip rates compare favorably with those derived independently from paleoseismic investigations conducted along the onshore reach of the San Simeon fault zone. These investigations yielded slip rate estimates of 1 to 3 millimeters per year over the past 214,000 years (PG&E, 1988; Hanson and Lettis, in review) and 1 to 2 millimeters per year during the Holocene (PG&E, 1988; Hall and Hunt, in review). We conclude, therefore, that the lateral slip rate along the northern Hosgri fault zone is a minimum of 1 to 4 millimeters per year, and is likely to be comparable to slip rates reported along the onshore San Simeon fault zone.
Implications of the San Simeon/Hosgri Pull-Apart Basin for Seismic Hazard Assessment Various researchers postulate that releasing stepovers or dilatational jogs are potential segmentation points to the propagation of seismic rupture along a fault zone (for example, Sibson, 1985, 1986b; King, 1986b; Kneupfer and others, 1989). Abrupt changes in the amount of slip accompanying individual earthquakes, for example, are commonly associated with releasing or dilatational stepovers (Clark, 1972; Tchalenko and Berberian, 1975; Sich, 1978; Sibson, 1986b). Pull-apart basins are coincident with the ends of several historical ruptures along the North Anatolian fault zone in Turkey (Barka and Kadinsky-Cade, 1988) and the Cholame Valley along the San Andreas fault zone in central California also appears to have hindered or terminated surface rupture associated with historical earthquakes (Brown and Vedder, 1967; Aki, 1979; Allen, 1968; and Shedlock and others, in press). Similarly, studies of seismicity associated with the 1979 ML 5.9 Coyote Lake earthquake on the Calaveras fault indicate that coseismic rupture associated with the earthquake terminated at a dilatational jog or right-releasing step (Bouchon, 1982; Liu and Helmberger, 1983) and that subsequent, post-seismic slip transferred to an en echelon fault segment about 2 kilometers to the southwest (Reasenberg and Ellsworth, 1982; Sibson, 1986b). In a systematic evaluation of a large number of historical fault ruptures, Knuepfer (1989) and Knuepfer and others (1989) documented very few cases in which coseismic rupture propagated across releasing stepovers 5 kilometers or more wrde.
These observations and the growing body of theoretical and empirical evidence strongly indicate that large releasing stepovers along strike-slip fault systems are effective, long-lived segmentation points to coseismic rupture. The San Simeon/Hosgri pull-apart basin is up to 5 kilometers wide and up to 18 kilometers long. It is the largest known extensional stepover along the entire San Gregorio/San Simeon/Hosgri fault system. The stepover clearly separates the northern Hosgri fault zone from the southern San Simeon fault zone, and likely separates ruptures occurring on these two fault zones.
CONCLUSIONS Analysis of geophysical and geologic data from the near-offshore region between San Simeon Bay and Estero Bay shows that the northern Hosgri fault zone and southern San Simeon fault zone are related to one another via a right, en echelon stepover. An extensional, subsiding basin bordered by and containing normal faults occurs locally within the stepover region. Based on kinematic arguments and comparisons with subsiding basins along strike-slip faults worldwide, we interpret the subsiding basin between the Hosgri and San Simeon fault zones to be a tectonic pull-apart basin.
The San Simeon/Hosgri pull-apart basin accommodates the lateral slip on the two bordering strike-slip faults. Based on empirical, theoretical, and experimental modeling techniques that relate pull-apart dimensions to cumulative lateral slip on the bordering master faults, we estimate minimum slip rates of 1 to 4 millimeters per year for the southern San Simeon and northern Hosgri fault zones.
These slip rates are consistent with time-averaged late Quaternary and Holocene slip rates estimated from paleoseismic investigations and geologic mapping along the southern onshore reach of the San Simeon fault zone.
Seismic hazard analyses require that estimates be made of likely rupture lengths along fault zones.
A growing body of theoretical and empirical data indicates that extensional releasing stepovers or dilatational jogs often are effective segmentation points to coseismic rupture along strike-slip fault systems. The San Simeon/Hosgri pull-apart basin is the largest known extensional releasing stepover Diablo Canyon Power Plant Pacific Gas and Electric Company Lang Term Seismic Program
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chm n 12-A M rch 1 Pae 2 along the entire San Gregorio/San Simeon/Hosgri fault system. Based on its geometry and stratigraphic relationships, the basin appears to be a major, long-lived segmentation point separating the southern San Simeon and northern Hosgri fault zones.
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At achment 12-A March 1 0 Pa Bouchon, M., 1982, The rupture mechanism of the Coyote Lake earthquake of 6 August 1979 inferred from near-field data: Bulletin of the Seismological Society of America, v. 72, p. 745-757.
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Crowell, J. C., 1974, Sedimentation along the San Andreas fault, in Dott, R. H., Modern and Ancient Geosynclinal Special Publication Sedimentation: Society of Economic Paleontologists and Mineralogists, v. 19, p. 292-303.
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Attachment 12-A March 1 Pae 4 Envicom, 1977, Fault investigation, Hearst Ranch environmental data base: unpublished report for the Hearst Corporation.
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A ch men 12-A ar h I Hamilton, D. H., 1984, The tectonic boundary of coastal Central California: Ph.D. dissertation, Stanford University, 290 p.
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Isaacs, C. M., Keller, M. A., Gennai, V. A., Stewart, K. C., and Taggart, J. E., Jr., 1983, Preliminary evaluation of Miocene lithostratigraphy in the Point Conception COST well, OCS-CAL 780164 No.
1, off southern California; jn Isaacs, C. M., Garrison, R. E., Graham, S. A., and Jensky, W. A., II, eds., Petroleum Generation and Occurrence in the Miocene Monterey Formation, California: Pacific Section, Society of Economic Paleontologists and Mineralogists.
Jennings, C. W., 1975, Fault map of California with locations of volcanoes, thermal springs and thermal wells: California Division of Mines and Geology Geologic Data Map No. 1, scale 1:750,000.
Kashai, E. L., and Croker, P. F., 1987, Structural geometry and evolution of the Dead Sea - Jordan rift system as deduced from new subsurface data: Tectonophysics, v. 141, p. 33-60.
King, G. C. P., 1986, Speculations on the geometry of the initiation and termination processes of earthquake rupture and its relation to morphology and geological structure: PAGEOPH, v. 124,
- p. 567-585.
Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
2-A March 1 0 Pae 6 Knuepfer, P. L., 1989, Implications of the characteristics of endpoints of historical surface fault ruptures for the nature of fault segmentation, in Schwartz, D. P., and Sibson, R. H., (eds.), Fault Segmentation and Controls of Rupture Initiation and Termination: U.S. Geological Survey Open-File Report 89-315, p. 193-228.
Knuepfer, P. L. K., Baltzer, E. M., and Turko, J. M., 1989, Repeatability of fault rupture segments and earthquake potential: U.S. Geological Survey, National Earthquake Hazards Reduction Program, Final Technical Report, Contract No. 14-08-0001-G1527.
Lemiszki, P. J., and Brown, L. D., 1988, Variable crustal structure of strike-slip fault zones as observed on deep seismic reflection profiles: Geological Society of America Bulletin, v. 100, p. 665-676.
Leslie, R. B., 1980, Continuity and tectonic implications of the San Simeon-Hosgri fault zone, Central California: unpublished Masters Thesis, University of California at Santa Cruz, California.
Leslie, R. B., 1981, Continuity and tectonic implications of the San Simeon-Hosgri fault zone, central California: U.S. Geological Survey Open-File Report 81-430.
Lettis, W. R., Hall, N. T., and Hamilton, D. H., 1989, Quaternary tectonics of south-central coastal California (abs.): 28th International Geological Congress, Abstracts, v. 2, p. 2-285.
Liu, H.-L. and Helmberger, D. V., 1983, The near-source ground motion of the 6 August 1979 Coyote Lake, California, earthquake: Bulletin of the Seismological Society of America, v. 73, p. 201-.
218.
Mann, P., Hempton, M. R., Bradley, D. C., and Burke, K., 1983, Development of pull-apart basins:
Journal of Geology, v. 91, p. 529-554.
Manson, M., 1985, San Simeon fault zone and Cambria fault, San Luis Obispo County, California:
California Division of Mines and Geology Fault Evaluation Report FER-170, 12 p.
McCulloch, D. S., 1987, Regional geology and hydrocarbon potential of offshore Central California:
U.S. Geological Survey Professional Paper (in press), 127 pp.
McCulloch, D. S., and Chapman, R. H., 1977, Maps showing residual magnetic intensity along the California coast, lat. 37 30'N to 34 30'N: U.S. Geological Survey Open-File Report 77-79, 14 aeromagnetic maps, scale 1:250,000.
McCulloch, D. S., and Lewis, S. D., 1988, Offshore resource geology of central California; in U.S.
Geological Survey Research on Energy Resources - 1988 Program and Abstracts: U.S. Geological Survey Circular 1025, p. 34.
McCulloch, D. S., Vedder, J. G., Wagner, H. C., and Bruns, R. G., 1979, Geologic setting; in Cook, H. E., ed., Geologic Studies of the Point Conception Deep Stratigraphic Test Well, OCS-CAL 78-164 No. 1, Outer Continental Shelf, Southern California, United States: U. S. Geological Survey Open-File Report 79-1218.
Namson, J. S., and Davis, T. L. 1990, in review, Subsurface study of the late Cenozoic structural geology of the Santa Maria Basin, western Transverse Ranges and southern Coast Ranges, California.
Pacific Gas and Electric Company, 1975, Appendix 2.5E, Final safety analysis report for Diablo Canyon Nuclear Power Plant: U.S. Atomic Energy Commission Docket Nos. 50-275 and 50-323.
Pacific Gas and Electric Company, 1988, Final report of the Diablo Canyon Long Term Seismic Program: U.S. Nuclear Regulatory Commission Docket Nos. 50-275 and 50-323.
Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
tachmen GS 12-A March I Pae 7 Page, B. M., and Engebretson, D. C., 1984, Correlations between the geologic record and computed plate motions for central California: Tectonics, v. 3, p. 133-155.
Quennell, A. M., 1958, The structural and geomorphic evolution of the Dead Sea Rift Journal of the Geological Society of London, v. 114, p. 1-24.
Reasenberg, P. and Ellsworth, W. L., 1982, Aftershocks of the Coyote Lake, California earthquake of August 6, 1979; a detailed study: Journal of Geophysical Research, v. 87, p. 10,637-10,665.
Reches, Z., 1987, Mechanical aspects of pull-apart basins and push-up swells with applications to the Dead Sea transform: Tectonophysics, v. 141, p. 75-88.
Rodgers, D. A., 1980, Analysis of pull-apart basin development produced by en echelon strike-slip faults: in Ballance, P. F., and Reading, H. G., eds., Sedimentation in Oblique-Slip Mobile Zones, International Association of Sedimentologists Special Publication 4, p. 27-41.
Schubert, C., 1980, Late-Cenozoic pull-apart basins, Bocono fault zone, Venezuelan Andes: Journal of Structural Geology, v. 2, p. 463-468.
Schubert, C., 1982, Origin of Cariaco basin, southern Caribbean Sea: Marine Geology, v. 47, p. 345-360.
Seiders, V. M., 1979, San Gregorio-Hosgri fault zone south of Monterey Bay, California; A reduced estimate of maximum displacement: U.S. Geological Survey Open-File Report 79-385, 10 p.
Shedlock, K. M., Brocher, T. M., and Harding, S. T., in press, Shallow structure and deformation along the San Andreas fault in Cholame Valley, California, based on high-resolution reflection profiling: Journal of Geophysical Research.
Sibson, R. H., 1985, Stopping of earthquake ruptures and dilational jogs: Nature, v. 316, p. 248-251.
Sibson, R. H., 1986a, Brecciation processes in fault zones: inferences from earthquake rupturing:
PAGEOPH, v. 124, nos. 1/2, p. 159-175.
Sibson, R. H., 1986b, Rupture interaction with fault jogs; in Das, S., Boatwright, J., and Sholz, C.H.,
eds., Earthquake Source Mechanics, Maurice Ewing Series 6, American Geophysical Union Monograph 37, p. 157-168.
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Bulletin of the Seismological Society of America, v. 68, p. 1421-1448.
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253.
Silver, E. A., 1978, The San Gregorio-Hosgri fault zone, an overview; in Silver, E. A., and Normark, W. R., eds., San Gregorio-Hosgri Fault Zone, California: California Division of Mines and Geology Special Report 137, p. 1-2.
Silver, E. A., and Normark, W. R., 1978, San Gregorio-Hosgri Fault Zone, California: California Division of Mines and Geology Special Report 137, 567 p.
Tchalenko, J. S., and Berberian, M., 1975, Dasht-e-Bayaz fault, Iran: earthquake and earlier related structures in bedrock: Geological Society of America Bulletin, v. 86, p. 703-709.
Diablo Canyon Power Plant Pacific Gas and Electric Company long Term Seismic Program
achmen 12-A March 1 0 Pa 8 ten-Brink, U. S., and Ben-Avraham, Z., 1989, The anatomy of a pull-apart basin; seismic reflection observations of the Dead Sea Basin: Tectonics, v. 8.
Wagner, H. C., 1974, Marine geology between Cape San Martin and Point Sal, south-central California offshore: U.S. Geological Survey Open-File Report 74-252, 17 p.
Weber, G. E., 1983, Geologic investigation of the marine terraces of the San Simeon region and Pleistocene activity of the San Simeon fault zone, San Luis Obispo County, Californix U.S.
Geological Survey Technical Report, 66 p.
Weber, G. E., and Lajoie, K. R., 1977, Late Pleistocene and Holocene tectonics of the San Gregorio fault zone between Moss Beach and Point Aiio Nuevo, San Mateo County, California (abs.):
Geological Society of America Abstracts with Program, v. 19, no. 4, p. 524.
Weber, G. E., and Lajoie, K. R., 1979a, Late Pleistocene rates of movement along the San Gregorio fault zone, determined from offset of marine terrace shoreline angles; in Weber, G. E., Lajoie, K.
R., and Griggs, G. B., eds., Coastal Tectonics and Coastal Geologic Hazards in Santa Cruz and San Mateo Counties, California: Field Trip Guide for Cordilleran Section of the Geological Society of America, 75th Annual Meeting, p. 101-111.
Weber, G. E., and Lajoie, K. R., 1979b, Evidence for Holocene movement on the Frijoles fault near Point Aflo Nuevo, San Mateo County, California; in Weber, G. E., Lajoie, K. R., and Griggs, G. B.,
eds., Coastal Tectonics and Coastal Geologic Hazards in Santa Cruz and San Mateo Counties, California: Field Trip Guide for Cordilleran Section of the Geological Society of America, 75th Annual Meeting, p.92-100.
Weber, G. E., Oshiro, L. K., Brown, D. F., McCrory, P. A., 1981, Evidence for Late Pleistocene or Holocene faulting along the San Simeon fault zone at San Simeon Bay, San Luis Obispo County, California (abs.): Geological Society of America Abstracts with Program, v. 12, no. 2, p. 113.
Wolf, S. C., and Wagner, H. C., 1970, Preliminary reconnaissance marine geology of area between Santa Lucia escarpment and Point Buchon, California: unpublished U. S. Geological Survey administrative report, 5 p.
Woodcock, N. H., and Fischer, M., 1986, Strike-slip duplexes: Journal of Structural Geology, v. 8, no. 7, p. 725-735.
Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
ue ti n S 1 March 1 Pa el QUESTION GSG 13 Trenching on the San Simeon fault zone indicates that there is an important strike slip component on the Hosgri fault of about 1 to 3 mm/year contributed by the San Simeon fault. There may also be a contribution from the Piedras Blancas zone. Provide a discussion as to whether there is such a contribution and, ifso, its size and sense of slip.
Question 13 addresses whether there is a contribution to slip along on the Hosgri fault zone from shortening within the Piedras Blancas antiform. The question indirectly considers whether strain on the Hosgri fault zone "partitions" northward into a tangential (strike-slip) component on the San Simeon fault zone and a normal (dip slip or convergent) component from compression in the Piedras Blancas antiform. As described in Response to Question GSG 2, if local strain partitioning occurs, an estimate of slip rate and sense of slip on the Hosgri fault zone should combine the rates and orientations of slip along the San Simeon fault zone and across the Piedras Blancas antiform.
To address this concern, we have evaluated the orientations and rates of crustal shortening in the Piedras Blancas antiform with respect to the orientation and rates of slip along the San Simeon and Hosgri fault zones. In this response, we provide a description of the Piedras Blancas antiform, an assessment of the orientation, rates, and sense of vergence of structures within the antiform, and an evaluation of the role of the antiform in the tectonic setting and regional deformation of south-central coastal California.
PIEDRAS BLANCAS ANTIFORM The Piedras Blancas antiform is a northwest-trending structural and basement high west of the south-central San Simeon fault zone (Figure GSG Q13-1). The antiform consists of a series of thrust faults and asymmetric folds that generally trend N35 W to N45'W, subparallel to the San Simeon fault zone (Figure GSG Q13-2). We interpret these anticlines to be structurally related to underlying thrust faults as fault-propagation folds. The antiform is as'wide as 18 kilometers, just north of Point Piedras Blancas, and is about 40 to 45 kilometers long, from just north of the northern termination of the Hosgri fault zone to north of Ragged Point.
Many of the folds and faults within the Piedras Blancas antiform deform post-mid-Pliocene strata, indicating active late Cenozoic compression in the region. Active compression is supported by the occurrence of microseismicity, principally within the central and northern parts of the antiform (PG&E, 1988), and by late Quaternary uplift of marine terraces along the coast from Ragged Point to Point San Simeon.
Association of structures within the antiform to the San Simeon fault zone is difficult to evaluate.
The antiform is cored by a basement high, where the quality of seismic data is degraded. However, we have observed several pertinent structural relationships:
The antiform consists of both northeast-vergent and southwest-vergent thrust faults beneath the asymmetric folds. Northeast-vergent thrust faults and overlying fault-propagation folds are primarily within the western and southern parts of the antiform.
An example of a northeast-vergent thrust fault and overlying asymmetric anticline is shown on Figure GSG Q13-3. The sense of vergence for these thrust faults indicates they are not associated with the San Simeon fault zone at depth and, thus, are not the result of strain partitioning along a deep-crustal fault.
Southwest-verging thrust faults and overlying fault-propagation folds are primarily within the northern and eastern part of the antiform. An example of southwest-verging thrust faults and overlying asymmetric anticlines is shown on Figure GSG Q13-4. In down-dip projection, most of these thrust faults converge with the high-angle San Simeon fault zone at less than 1 kilometer to greater than 7 kilometers depth and, thus, Diablo Canyon Power Plant Pacffic Gas and Electric Company Long Term Seismic Program
I ue tion SG 13 March 1 0 P e2 d~
~~
~ ~
EXPLANATION San Simeon a Diablo Canyon Power Plant Polflt Fault, dotted where inferred Axial trend of anticline or
% C0 syncline g
Piedras Biancas i ', it Point
', 5 Estefo Point Buchon Point San Luis S outh Basin Compressional L.
Domain+
~ + ~
~ ~
~ ~
I
~ ~
Point Sal Point Argu clio O)
'O o n.'4A 20 mi 1
~
30 km Figure GSG 13-1 Regional map showing the location of the Piedras Blancas antiform.
Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term SeismIc Program
I uestion I March l 0 P e
~
5mi 8 km i Arroyo Del Point San Simeon Projection of Blancas San Simeon Antiform Fault Zone EXPLANATION Fault, relative sense of motion shown, dashed Hosgrl where hcatlon not well constrained, dotted Fault Zone where fault does not deform late Pliocene strata Thrust fault, sawteeth on hanging wall, dashed where location not well constrained, dotted where fault does not deform late Pliocene strata Anticlinal and synclinal fold axis, dashed where location not well constrained, dotted where structure does not deform late Pliocene strata Figure GSG 13-2 Structural trends map of the Piedras Blancas antiform.
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QUESTION GSG i3 MARCH 1990 PAGE 4 THIS PAGE CONTAINS PROPRIETORY INFORNATION AND HAS NOT BEEN REPRODUCED f
1'
m CD CC CD Top of Miocene SW Unconformity 9
0 8 km 0.0 0.0
'Q C
0O 1.0 1.0 5
I C
O I-2.0 2.0 GM-83 (Location shown on Figure GSG 013-2)
C O O CC O
O O 9
~
Figure GSG 13-4 CCC O
Q O CD Interpreted seismic profile showing southwest-vergent thrust faults within the Piedras Blancas O
CO O O antiform.
ue ion 1 March 1 Pa e6 may be the result of local or regional strain partitioning along a deep crustal fault zone beneath the surface trace of the San Simeon fault zone (see Response to Question GSG 2 for discussion).
- 2. The Arroyo del Oso fault is a northwest-trending, northeast-dipping reverse fault (Figure GSG Q13-2). The fault displaces marine terrace deposits along the coast and trends offshore, where it appears to be imaged on seismic data as one of the southwest-vergent thrust faults west of Ragged Point. To the southeast, the Arroyo del Oso fault is mapped by Weber (1983), PG&E (1988), and Hanson and Lettis (in review) to merge with the San Simeon fault zone just north of Point San Simeon. The map pattern and dip of the San Simeon fault zone and Arroyo del Oso fault clearly indicate that they converge down-dip at shallow crustal depths. The Arroyo del Oso clearly branches from and is oblique to the San Simeon fault zone to the south, and becomes subparallel to the San Simeon fault zone to the north. The dip-slip displacement observed on the fault may be due to either its more westerly trend than the San Simeon fault zone and, thus, its more favorable orientation to have dip-slip displacement in the regional stress field, or to local strain partitioning along the San Simeon fault zone, or both.
- 3. The Piedras Blancas antiform is within the apex of a broad, regional left-restraining bend in the San Simeon/Hosgri fault system. The fault system progressively changes trend from about N25'W along the Hosgri fault zone near Point Buchon, to about N35'W to N40'W along the San Simeon fault zone north of Ragged Point. The kinematics of restraining bends along regional strike-slip fault systems predict that localized compression will result within the bend (for example, Barka and Kadinsky-Cade, 1989).
These observations indicate that compression within the Piedras Blancas antiform results, wholly or in part, from one or both of two tectonic processes: (1) strain partitioning from a deep crustal fault, and (2) localized compression due to convergence within a restraining bend of the Hosgri/San Simeon fault system. These two processes are important for evaluating rates of regional deformation. The process of strain partitioning (both local and regional) probably reflects regional plate motion stresses; regional deformation resulting from these stresses must be balanced along the North American/Pacific plate margin. Localized deformation along a fault system reflects the kinematics of moving a specific volume of rock through local compressional or extensional geometries along a fault system and need not directly reflect regional plate motion or deformation. These two processes need not be mutually exclusive and likely both are operable simultaneously.
Regardless of the tectonic process that has produced the post-top of Miocene deformation in the Piedras Blancas antiform, the evidence suggests that at least the northern and eastern parts of the antiform may be related to strain partitioning along the San Simeon fault zone. In these areas, we have combined the rate of shortening within the antiform with the rate of lateral shear along the San Simeon fault zone to fully evaluate and characterize the amount and sense of slip that may be transferred between the San Simeon fault zone and the northern Hosgri fault zone.
RATE OF SLIP FOR PIEDRAS BLANCAS ANTIFORM, SAN SIMEON FAULT ZONE AND HOSGRI FAULT ZONE Post-mid-Pliocene structural trends within the Piedras Blancas antiform are essentially subparallel to the south-central San Simeon fault zone. The orientation of crustal shortening reflected by the trends of structures in the antiform, therefore, is normal to the San Simeon fault zone. In this case, there is little or no resolved tangential shear parallel to the San Simeon fault zone. All the crustal deformation within the antiform is accommodated by folding and dip-slip faulting.
We have estimated the rates of east-vergent crustal shortening and west-vergent crustal shortening by restoring the amount of deformation recorded by the 5.3-million-year-old top of Miocene unconformity and conservatively assuming that all of this shortening occurred in the past 2.8 million Diablo Canyon Power Plant Pacific Gas and Electrfc Company Long Term Seismic Program
I l
I I
ue ti n 1 arch I Pa e7 years. The amount of southwest-vergent crustal shortening is about 0.3 a 0.1 kilometer for a rate of 0.1+ 0.05 millimeter per year. The amount of northeast-vergent crustal shortening is about 0.6
+ 0.1 kilometer for a rate of 0.2+ 0.05 millimeter per year. The combined rate of shortening is on the order of 0.2 to 0.4 millimeter per year (Table GSG Q13-1).
Table GSG Q13-1 SLIP RATE ESTIMATES IN PIEDRAS BLANCAS REGION RATE millimeter er ear Crustal
~Shortanin Resolved om onent Total Normal ~Tan an tat Piedras Blancas Antiform Southwest-Vergent 0.1 + 0.05 0.1 < 0.05 Northeast-Vergent 0.2 + 0.05 0.2 Combined 0.3 + 0.1 0.3 < 0.1 San Simeon fault zone <0.1 I to3 TOTAL <0.4 1 to3 The orientation of shortening in the Piedras Blancas antiform is N40'E + 5'n the southern and central part of the antiform, and N47 E + 5'n the northern part of the antiform. These orientations are within 10 degrees of normal to the general trend of the San Simeon fault zone.
Decomposing the rate of shortening within the Piedras Blancas antiform into rates normal to and tangential to the San Simeon fault zone yield values of about 0.2 to 0.4 millimeter per year normal to the fault and less than 0.1 millimeter per year tangential to the fault. If we restrict our analysis to the rate of shortening produced by southwest-vergent thrust faults beneath the northern part of the antiform, the rate of normal strain is about 0.1 millimeter per year and the rate of tangential shear contributed to the San Simeon fault zone is less than 0.05 millimeter per year. The thrust faults that underlie the folds within the anticline dip about 15 to 45 degrees. Using the dip of these faults, we calculated the rates of vertical uplift to be expected from the horizontal shortening. The rates of vertical uplift are less than or equal to the rate of crustal shortening. To be conservative, therefore, we assume that the rate of shortening is equal to the rate of uplift, and directly apply these values to the assessment of vertical rates on the Hosgri fault zone.
If one considers the San Simeon fault zone and the entire Piedras Blancas antiform to be the result of strain partitioning above a single fault zone at depth, the rates of deformation for each of these structures should be combined. The rate of lateral slip on the San Simeon fault zone is well constrained to be 1 to 3 millimeters per year; the rate of vertical slip on the fault zone is less than 0.1 millimeter per year, down on the east (PG&E, 1988; Hanson and Lettis, in review; Hall and Hunt, in review). The combined rate of deformation on the San Simeon fault zone and the Piedras Blancas antiform is I to 3 millimeters per year lateral slip, and less than 0.4 millimeter per year normal (or vertical) slip.
These combined rates are similar to the rates of slip occurring along the Hosgri fault zone (see Response to Question GSG 4). The analysis of slip contribution from the Piedras Blancas antiform indicates that little or no lateral slip is contributed to the San Simeon fault zone at depth or, by implication, to the Hosgri fault zone to the south. The lateral slip transferred between the San Simeon fault zone and the northern Hosgri fault zone across the San Simeon/Hosgri pull-apart basin is 1 to 3 millimeters per year, as shown in Response to Questions GSG 12 and GSG 4.
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ti n 1 March I 0 The analysis of the Piedras Blancas antiform suggests that the vertical component of slip on the Hosgri fault zone may partition northward and is represented by the antiform, whereas the lateral slip is accommodated by the San Simeon fault zone. The rate of vertical slip on the Hosgri fault zone is 0.1 to 0.44 millimeter per year (see Response to GSG 3), similar to the rate of crustal shortening within the Piedras Blancas antiform. The similarity of rates supports our assessment of vertical slip on the Hosgri fault zone, and suggests that little or no vertical (or normal) deformation is lost (or added) north of the northern termination of the Hosgri fault zone. Therefore, we believe the rate of right-lateral slip of 1 to 3 millimeters per year assigned to the northern Hosgri fault zone is geologically reasonable. The rate is based on our paleoseismic investigations of the southern San Simeon fault zone via trenching and mapping of marine terraces (PG&E, 1988; Hanson and Lettis, in review; Hall and Hunt, in review), by our structural and stratigraphic analyses of the San Simeon/Hosgri pull-apart basin (see Response to GSG 12), and by the structural and geometric analyses of the Piedras Blancas antiform, outlined above.
REFERENCES Barka, A. A., and Kadinsky-Cade, K., 1989, Effects of restraining stepovers on earthquake rupture; in Schwartz, D. P., and Sibson, R. H., eds., Fault Segmentation and Controls of Rupture Initiation and Termination: U. S. Geological Survey Open-File Report 89-315, p. 67-79.
Hall, N. T., and Hunt, T. D., in review, Holocene behavior of the San Simeon fault zone, south-central coastal California; in Alterman, I. B., ed., Seismotectonics of Central and Coastal California:
Geological Society of America Special Paper.
Hanson, K. L., and Lettis, W. R., in review, Estimated Pleistocene slip rate for the San Simeon fault zone, south-central coastal California; in Alterman, I. B., (ed.), Seismotectonics of Central and Coastal California: Geological Society of America Special Paper.
Pacific Gas and Electric Company, 1988, Final report of the Diablo Canyon Long Term Seismic Program: U. S. Nuclear Regulatory Commission Docket Nos. 50-275 and 50-323.
IVeber, G. E., 1983, Geologic investigation of the marine terraces of the San Simeon region and Pleistocene activity of the San Simeon fault zone, San Luis Obispo County, California: U. S.
Geological Survey Technical Report 66.
Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
I u i n 14 March 1 0 Pa I QUESTION GSG 14 Displacements of 2 meters per event on the Los Osos fault would sug gest rupture lengths longer than those presented. Provide the information used to determine the rupture lengths and discuss any inco>rsistency with 2 meter displacements.
The basis for assessing rupture length and maximum displacement per event on the Los Osos fault zone are given in detail in Chapters 2 and 3 of the Final Report (PG&E, 1988, p. 2-38 to 2-45, 2-123 to 2-134 and p. 3-29). We focus here on the compatibility of the assessment of rupture lengths of 19 to 36 kilometers and a maximum displacement per event of 2 meters.
The most appropriate way to assess the compatibility of maximum displacement per event with rupture length is to examine the historical data base of ruptures. We have compiled a data base of historical ruptures (Wells and others, in preparation) that is an update and expansion of similar studies conducted by others (for example, Slemmons, 1982). Figures GSG Q14-1 and GSG Q14-2 show the relationships between surface rupture length and maximum displacement per event for reverse faulting, and for all slip types, respectively. The linear regression lines for maximum displacement regressed on rupture length and rupture length regressed on maximum displacement are also shown. In general, the regression statistics do not show a strong correlation between these parameters, although maximum displacement and rupture length are themselves much better correlated with earthquake magnitude (Wells and others, in preparation). The expected value of maximum displacement and rupture length for these regressions is given below:
Maximum Displacement Regressed on Rupture Length Ru ture Len h km Maximum Di iacemen m Reverse faulting 19 2.1 35 2.5 All slip types 19 1.0 35 1.6 Rupture Length Regressed on Maximum Displacement aximum Dis lacement m Ru ture Len h km Reverse faulting 29 All slip types 44 The empirical data show a large scatter. For example, the range of maximum displacements that has been observed for a given rupture length is up to one order of magnitude (Figure GSG Q14-2).
Therefore, the mean relationship between displacement and rupture length, as well as the range of observed values, leads us to conclude that a 2-meter maximum displacement and the 19- or 36-kilometer rupture lengths for the Los Osos fault are compatible when compared with historical observations of past ruptures.
REFERENCES Slemmons, D. B., 1982, Determination of design earthquake magnitudes for microzonation:
Proceedings of the Third International Earthquake Microzonation Conference, v. 1, p. 119-130.
Wells, D. L., Coppersmith, K. J., Slemmons, D. B., and Zhang, X., in preparation, Earthquake source parameters; updated empirical relationships among magnitude, rupture length, rupture area, and surface displacement: for submittal to Bulletin of the Seismological Society of America.
Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Seismic Program
I I
I I
I I
I I
ue ion 14 March 1 0 Pa e2 10' I ~ ~ ~ ~ I I l
I ~ I All Slip Types, 74 Data Points
~C oo o
0 0 100 0 0 o o 0 C9 o
oo o O 0 0 0
og> ~
0 o cQ o O 80 10 <o~O 0
0 I ~
10 1 10 Maximum Displacement (m)
Figure GSG 14-1 Linear regressions between maximum displacement per event and surface rupture length for reverse fault (from Wells and others, in preparation).
t Diablo Canyon Power Plant Pacific Gas and Electric Company long Term Selsmlc Program
uestion G G 14 March I Pa e 10'everse Eqs., 15 Data Points E
g~ 100
'0 0
0 oo o
10 10 Maximum Displacement (m)
Figure GSG 14-2 Linear regressions between maximum displacement per event and surface rupture length for all slip types (from Wells and others, in preparation).
Diablo Canyon Power Plant Pacific Gas and Electric Company Long Term Selsmtc Program
uestion S I March 1990 Pa e4 PROPRIETAR Y D A-T=A SW 0 10 km 0.0 +'8N'/-Oi'CTAW4iSPSYiCQ~~%~lQ&;'five%~<.P4&R~t~(n+~ wA~Y~ " ~ <'<A4%MC~ 0.0
.-~~~~..5K';~~.~M~~3~tr=~" - ".<"~;-.': .:-'~/'i/ "'/gP~~g'i ~
1.0 1.0 O
E onl i
pe ~ease&
5 I-2.0 2.0 I 1 //V /
g ~It h
3.0 3.0 J-106 (Location shown on Figure GSG 013-2)
Figure GSG 13-3 Interpreted seismic profile showing a northeast-vergent thrust fault and associated asymmetric anticline within the Piedras Blancas antiform.
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~DOCK 05000275 Diablo Canyon Power Plant CDC Pacific Gas and Electric Company Long Term Seismic Program
SHOTPOllITS M4 0 CM47 CM45 CM43 CM.39
~ 5400 SI 00 5000 ~e 4800 4100 4500
- 0. 0 n ~
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O C A
a IS
~~ a 1.0 CO Z E I
2.0 0.0 MPU
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oo 2,0 D o o SSw il 9
,PROPRIETARY DATA Figure GSG Q12-A.S Migrated seismic line S-6 (CGI-19) (a) and interpretation (b). MPU is the mid-Pliocene unconformity and TMU is the top of Miocene unconformity.
\
Pacific Gas and Electric Company 77 Beale Streel James D. Shiffer San Francisco, CA 94106 Senior Vice President and General Hanager 415I972.7000 Nuclear Power Generation 415/973-4684 April 2, 1990 PGhE Letter No. DCL-90-090 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C. 20555 Re: Docket No. 50-275, OL-DPR-80 Docket No. 50-323, OL-DPR-82 Diablo Canyon Units 1 and 2 Response to NRC Staff Questions on Geology/Seismology/Geophysics/Tectonics Long Term Seismic Program Final Report Gentlemen:
In letters to PGhE dated August 1 and October 13, 1989, and February 23, 1990, the NRC Staff requested additional information to support its review of geoscience topics presented in the Long Term Seismic Program (LTSP) Final Report. Enclosed are PGhE's responses to these requests.
The August 1, 1989, letter summarized a dune 12-16, 1989, LTSP public meeting and included 16 NRC Staff comments and questions regarding geology/seismology/geophysics (GSG). PGhE responses to GSG Questions 2, 3, 4, 5, 7, 8, 9, 10, 12, 13, 14, and the 29 montages that accompany Question GSG 1 are provided herewith. The responses to GSG Questions 1, 12, and 13 contain proprietary geophysical data that are exempt from public disclosure in accordance with 10 CFR 2.790(a)(9). The proprietary information is appropriately marked and has been transmitted only to the NRC (2 copies) and to NRC consultants (1 copy to R. D. Brown and 1 copy to D. B. Slemmons). It is requested that this material be handled in accordance with NRC procedures for proprietary material.
The October 13, 1989, letter contained six questions on seismic source characterization (SSC). PG&E's responses to SSC Questions 1, 2, 3 and 4 are enclosed.
Responses to the remaining questions from both letters currently are in the review process. We anticipate that these responses will be transmitted to the NRC in the coming week.
~ With regard to the letter of February 23, 1990, which contained 13 questions requesting additional data and analysis, Questions 2, 4, 6, and 9 have been addressed in our responses to other questions.
Namely, Questions 2, 4, and 6 will be addressed in our current response to Question GSG 1; Question 9 was addressed in the response to ground motions Question 5 that was submitted to the NRC in August 1989.
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Document Control Desk April 2, 1990 PGhE Letter No. DCL-90-090 Questions 10, 11, and 12 are related to the Lorna Prieta earthquake. He have recently received abundant preliminary results of some investigations of the Lorna Pri eta earthquake, and expect the results of other studies shortly. He will be prepared to address these questions during the NRC/PGhE Seismic Source Characterization meeting scheduled for April 17-20, 1990, in San Francisco.
He plan to devote more than one full day to the Lorna Prieta earthquake, including a field trip to the epi central region. In addition to addressing these questions orally at the meeting, we will follow up with written summaries of our presentations.
The remaining questions in the letter of February 23, 1990, Questions 1, 3, 5, 7, 8, and 13, wi 11 be addressed orally at either the Seismic Source Characterization meeting or the NRC/PGhE Ground Hotions meeting scheduled for April 30 and Hay 1, 1990, in San Francisco. In addition, we will submit a written summary of these presentations.
Kindly acknowledge receipt of this material on the enclosed copy of this letter and return it in the enclosed addressed envelope.
incerely,
. D. Shif cc w/encs.: R. D. Brown, Dr.
D. Clark J. B. Hartin H. Rood (2)
D. B. Slemmons cc w/o encs.: A. P. Hodgdon H. H. Hendonca P. P. Narbut B. Norton CPUC Diablo Distribution Enclosures 3122S/0081K/DHO/1587