ML20212P642

Long-Term Seismic Program Rept on Nrc/Pg&E Geology/Seismology/Geophysics Workshop Held on 861021 & 22, San Francisco,Ca

ML20212P642
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Site:	Diablo Canyon
Issue date:	02/28/1987
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'I LONG TERM SEISMIC PROGRAM REPORT ON NRC/PGandE GEOLOGY / SEISMOLOGY / GEOPHYSICS WORKSHOP HELD ON OCTOBER 21 AND 22,1986 SAN FRANCISCO, CALIFORNIA PACIFIC GAS AND ELECTRIC COMPANY Diablo Canyon Power Plant Docket Nos. 50-275 and 50-323

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PGandE Letter No.:

DCL-87-046 I

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DIABLO CANYON LONG TERM SEISMIC PROGRAM REPORT ON NRC/PGandE GEOLOGY / SEISMOLOGY / GEOPHYSICS WORKSHOP HELD ON OCTOBER 21 and 22, 1986 SAN FRANCISCO, CALIFORNIA I

I Pacific Gas and Electric Company Diablo Canyon Power Plant I

Docket Nos. 50-275 and 50-323 February 1987 I

I CONTENTS Pace EXECUTIVE

SUMMARY

1 1

INTRODUCTION 3

Purpose of the Workshop 3

Scope of Workshop Report 3

2 CHARACTERIZATION OF THE HOSGRI FAULT ZONE 6

Regional Overview 6

Neotectonic and Quaternary Studies in the I

San Simeon Area 9

Offshore Studies of the Hosgri Fault Zone 47 Plans for Future Work 57 3

CHARACTERIZATION OF THE SAN LUIS/PISMO SYNFORM 58 Regional Overview 58 Neotectonic and Quaternary Studies in the I

Pismo Beach Area 60 Plans for Future Work 71 4

SEISMICITY AND REGIONAL TECTONICS 72 Crustal Velocity Structure 72 Regional Seismicity 82 1927 Lompoc Earthquake Studies 87 Tectonic Framework 96 5

IMPLICATIONS FOR SEISMIC SOURCE CHARACTERIZATION 102 6

REFERENCES 104 APPENDIX A GEOLOGY / SEISMOLOGY / GEOPHYSICS WORK PLAN 106 I

EXECUTIVE

SUMMARY

On October 21 and 22, 1986, a workshop was held to review and discuss the technical progress and interim results of Phase III of the Diablo Canyon Long Term Seismic Program in the areas of geology, seismology, and geophysics.

The Geology / Seismology /

I Geophysics Workshop was held under the auspices of the Nuclear 9egulatory Commission (NRC) and Pacific Gas and Electric Company (PGandE).

The Workshop was publicly announced by the NRC and was attended by about 30 participants from the NRC, PGandE, the U.

S.

Geological Survey (USGS), the California Division of Mines and Geology, and the University of Nevada at Reno.

Conducting the Workshop at this point in the Program was timely in that the summer 1986 phase of field activities and data acquisition had been completed and the activities to be conducted during the subsequent 3 to 6 months were more clearly defined.

Thus, the Workshop served to communicate to the NRC, te other interested state and federal agencies, and to those having I

technical interest or involvement the current status of the geology, seismology, and geophysics tasks of the Program.

The objective of these tasks is to update the identification and characterization of potential earthquake sources in the region of interest in coastal central California.

The technical presentations and discussions covered by the Workshop are organized in this report into four topics, as summarized below.

The results are interim; they represent progress toward the objective described above.

CHARACTERIZATION OF THE HOSGRI FAULT ZONE e

The Hosgri and San Simeon fault zones are each major segments I

of the overall San Gregorio-Hosgri fault system, so that information on the rate of deformation on the San Simeon zone will have relevance to an assessment of the Hosgri zone.

The I

San Simeon fault zone in the vicinity of San Simeon has been studied in detail because it is the most southerly part of this fault system that is accessible to land-based geologic studies.

The sense of slip of the onshore San Simeon fault zone is primarily right-lateral along near-vertical shear planes.

Data obtained to date constrain the rate of slip to a range of about 1 to 10 millimeters per year, with studies now under way expected to allow a more specific estimation.

At least one recent slip event appears to be younger than 8000 years and to have a right-slip component of slightly more than 1 meter.

o Offshore studies using 2-to 5-second geophysical data suggest that the Hosgri fault zone in the vicinity of the Diablo I

Canyon site contains two elements:

a thrust-fault component that does not appear to displace a prominent late Miocene unconformity, and a near-vertical component that penetrates post-Pliocene strata.

Analysis of offshore high-resolution seismic data suggests e

that the Hosgri fault zone has been active during late Quaternary time.

The linearity and steep dips of near-surface traces are consistent with primarily strike slip.

CHARACTERIZATION OF THE SAN LUIS/PISMO/SYNFORM Studies in the San Luis Obispo/Pismo/ Santa Maria Valley region have focused on the late Quaternary deformational history onshore and offshore in the Pismo Beach area.

The Wilmar Avenue fault is characterized by pure dip-slip e

reverse displacement at Pismo Beach.

The fault does not appear to connect with the San Miguelito fault; its extent and structural relationship with the San Luis/Pismo synform are being studied, e

Reflection seismic data have revealed a monoclinal flexure in the subsurface along the northern side of the Santa Maria Valley.

No large-scale deformation of late Quaternary I

landforms or deposits has been observed in association with this structure; the feature is being studied further to address its tectonic significance.

SEISMICITY AND REGIONAL TECTONICS e

The source mechanism of the 1927 Lompoc earthquake has been analyzed by comparing teleseismic records of the event with similar records from recent earthquakes, such as the 1983 Coalinga earthquake.

Preliminary findings are that the 1927 I

event involved nearly pure reverse faulting on a plane 0

0 striking N20 W and dipping 66 NE, had a focal depth of about 10 km, and had a seismic moment of 1 x 1026, corresponding to a moment magnitude of 6.6.

Recent instrumental seismicity reported by the USGS and the e

California Institute of Technology since 1980 in the coastal California region reveals a pattern of deformation consistent with the regional right-oblique and reverse-slip pattern observed geologically.

IMPLICATIONS FOR SEISMIC SOURCE CHARACTERI2ATION The field studies and analyses carried out during Phase III are providing data useful for reducing the uncertainties in seismic source characterization for the Program.

No " surprises" or significant findings have been identified that were not effectively included for source characterization during the Phase II study (PGandE, 1986).

I --

I I

INTRODUCTION PURPOSE OF THE WORKSHOP This report documents the presentations and discussions that were provided by Pacific Gas and Electric Company (PGandE) staff and consultants at the Geology / Seismology / Geophysics Workshop held I

under the auspices of the Nuclear Regulatory Commission (NRC) and PGandE.

The Workshop was in session for two full days and was attended by about 30 participants from the NRC, the U.

S.

Geological Survey (USGS), the University of Nevada at Reno (UNR),

I the California Division of Mines and Geology, and PGandC.

Table 1 lists the attendees and their affiliations.

I The objective of the Workshop was to review and discuss in a working setting the technical progress and results of the Diablo Canyon Long Term Seismic Program in the areas of geology, seismology, and geophysics.

Conducting the Workshop at this time I

in the Program was timely in that the summer phase of field activities and data acquisition for 1986 had been completed and the activities to be conducted during the remainder of 1986 and in 1987 were more clearly defined.

Thus, the Workshop served to communicate to the NRC and to other interested agencies and individuals the current technical status of the part of the I

Program that updates the identification and characterization of potential earthquake sources in the region of interest in coastal central California.

SCOPE OF WORKSHOP REPORT The technical discussions in the Workshop covered field work and data analyses that addressed the group of tasks that constitute the Geology / Seismology / Geophysics Program element.

This Program element was described in the detailed scope of work presented in the Phase II report (PGandE, 1986).

In this Program element, the work plan is divided into nine tasks organized to address significant technical topics in ar efficient and structured manner.

An outline of the scope of work is presented in Appendix A of this report.

As reported during the Workshop, progress was accomplished on portions of the scope of work for each of the nine work tasks. In this report, the Workshop discussions are organized to address four technical areas, using input from the indicated tasks: the characterization of the Hosgri fault zone (Tasks 1 and 2), the I

characterization of the San Luis Obispo/Pismo/ Santa Maria Valley region (Tasks 2, 4,

and 5), the tectonic framework of the region (Tasks 3, 5,

6, 7,

and 8), and implications for seismic sov"ce characterizations (Task 9).

Because this report discusses -

TABLE 1 DIABLO CANYON LONG TE'1M SEISMIC PROGRAM GEOLOGY / SEISMOLOGY / GEOPHYSICS NRC/PGandE WORKSHOP OCTOBER 21-22, 1986 NAME OF ATTENDEE AFFILIATION George A. Thompson ACRS Consultant Earl W.

Hart Calif. Div. of Mines & Geology Robert H.

Sydnor I

Frank W.

Brady Pacific Gas and Electric Co.

Donald A. Brand Lloyd S.

Cluff Roland Madsen William U.

Savage Y.

Ben Tsai Clarence R. Allen PGandE Consultant I

Kevin J.

Coppersmith a

N. Timothy Hall Douglas H. Hamilton Kathryn Hanson William R.

Lettis Marcia K. McLaren Jan D. Rietman I

Bimal E. Sarkar Paul G.

Somerville F.

H.

(Bert) Swan I

C. Richard Willingham Gus Giese-Koch Nuclear Regulatory Commission Richard McMullen Leon Reiter I

Hans E.

Schierling Katheryn Killeen University of Nevada, Reno Barbara Matz I

Steven Nitchman Richard Schweickert David B.

(Burt) Slommons I

Xiaoyi Zhang Robert D.

Brown U.

S. Geological Survey Walter Mooney Anne Trehu I

David P. Schwartz Jean Savy Lawrence Livermore National Laboratory I

progress in the Program as of shortly before the date of the Workshop, many investigations and activities in the scope of work remain to be completed.

For the areas addressed in this report, the investigations scheduled in the time period following the Workshop have been identified.

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I CHARACTERIZATION OF THE HOSGRI FAULT ZONE I

REGIONAL OVERVIEW I

The Geology / Seismology / Geophysics investigations for the Long Term Seismic Program are centered on the Diablo Canyon site, along the coast of central California.

The major features that define the geologic and tectonic setting of this region are shown l

on Figure 1.

The indicated region extends between Monterey Bay on,the north and the Santa Barbara Channel on the south, and between the San Andreas fault zone on the east and the offshore I

continental slope on the west.

The region is traversed diagonally by segments of the San Gregorio/Hosgri fault system (Hamilton, 1984).

The Hosgri fault zone, which is the I

southernmost member of this system, is recognized as the closest seismically significant source to the Diablo Canyon site and as the dominant contributor to the seismic hazard at that location i

(PGandE, 1975).

The initial field and geophysical studies for the Program have therefore concentrated on delineating the key parameters of the Hosgri fault zone necessary to update the characterization of its seismic potential.

The San Gregorio/Hosgri fault system lies between the junction of the San Gregorio and San Andreas fault zones, just northwest of San Francisco (northwest of the map area of Figure 1), and an I

area offshore from the coastline between Point Arguello and Point Conception.

The fault system comprises a variety of structural features that, in aggregate, define a tectonic boundary structure I

that separates the uplifted onshore region from the submerged offshore basins.

The onshore region has a variety of geomorphic, stratigraphic and structural features that reflect a complex geologic history and widespread exposure due to uplift.

In the southerly part of this region, near Diablo Canyon, the onshore and near-offshore geology is dominated by a series of alternating synformal and antiformal structures that trend more westerly than I

the adjacent Hosgri fault zone.

Across the Hosgri zone to the west, the offshore Santa Maria Basin exhibits little deformation younger than a largely buried mid-Pliocene age unconformity.

I Structural trends within the southerly part of the offshore basin, however, are well displayed in the lower part of the Neogene stratigraphic section as north-northwest-trending folds I

and associated thrust faults that cut the lower part of this section.

The major components of the San Gregorio/Hosgri fault system are, from north to south, the San Gregorio fault zone, the Serra Hill fault and the Point Sur antiform, the San Simeon fault zone and the Piedras Blancas antiform, and the Hosgri fault zone.

The San I

Gregorio fault has been regarded as a major branch within the San Andreas fault system, and has well-established evidence of late Quaternary right slip.

East of the Point Sur antiform, this fault branches into several splays.

At least two splays extend _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

GEOLOGIC AND TECTONIC SETTING,

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into the northern Santa Lucia Range east of the Serra Hill fault, each of which has geomorphic expression suggestive of right-slip faulting.

The Serra Hill fault, its southeasterly extension known as the Sur fault, and the Bixby Creek lineament (the latter two are not shown on Figure 1) are splays that extend across the i

eastern margin of the Point Sur antiform.

Available data (Hamilton, 1984) suggest that displacement along these faults is predominantly east side up, reverse dip slip.

Between the Point the shoreline is essentially a sur and Piedras Blancas areas, t

fault-line scarp along the offshore part of the San Simeon fault I

The dip separation across the San Simeon fault zone in zone.

this area amounts to several kilometers.

Both the Point sur and the Piedras Blancas antiforms are basement-rock-cored antiformal folds that appear to form major interruptions in the continuity The shoreward of faulting along the San Gregorio-Hosgri system.

extention of the offshore Santa Maria Basin is also bounded by these local upwarp features, as shown on Figure 1.

The complex of structural features that forms the eastern margin of the offshore Santa Maria Basin southward from the vicinity of I

the Piedras Blancas area has generally been referred to as the As seen in deeper (2-second-plus) seismic Hosgri fault zone.

records, this structure appears as a faulted monoclinal upwarp.

I High resolution seismic records show its trace to be marked locally by scarps and sags, at least in the area offshore of the Diablo Canyon site. Offshore of the Santa Maria Valley, the Hosgri fault zone has little or no expression on the sea floor.

The role played by the San Gregorio-Hosgri fault system is crucial to understanding and defining the tectonic model for the I

study region.

Several contrasting models have been proposed during the past 10 years, and all depend heavily upon the interpretation of sense and rate of slip for the faults of this Several early models portray the system as a long, system.

through-going strike-slip fault along which approximately 100 km of right slip has occurred during Neogene time (the past approximately 24 million years).

A model proposed by Weldon and Humphreys (1986) assigned 2.2 contimeters per year of right slip to unspecified faults in the This is the' I

offshore region at the latitude of Diablo Canyon.

difference between 5.6 centimeters per year of interplateand the transform motion determined by Minster and Jordan (1978) 3.4 centimeters per year of slip documented along the onshore San Andreas fault system (Sieh and Jahns, 1984).

Contrasting with this view is the model proposed by Crouch and others (1984),

which characterizes all larger faults in the region as listric This I

thrusts related to an underlying aseismic detachment zone.

compressional system is interpreted as a response to a component of plate convergence that results from deviation of the strike of the San Andreas fault from the ideal direction of transform plate boundary motion in central California.

Another tectonic model (for examplo, Hornafius, 1985) stems from palcomagnetic studies indicating that approximately 90 degrees of clockwise rotation of the Western Transverse Ranges terrane has occurred during Neogene I

Such rotation appears to require large-scale right slip time.

along the faults of the coastal system, especially along the southerly part of the Hosgri fault zone.

Although these earlier I

regional tectonic models have not been formulated with the it is evaluation of earthquake hazard as a primary consideration, an objective of the Long Term Seismic Program that the characterization of both individual faults and of the overall tectonic regime of coastal central California be undertaken for This in turn has indicated a need to this specific purpose.

focus the initial studies on the feature shown in the Phase II report (PGandE, 1986) to be the most significant contributor to the seismic hazard at Diablo Canyon, namely the Hosgri fault Appendix A outlines the important considerations and the zone.

I subtasks contained in Task 1 for characterization of the Hosgri The studies reported during the October 21-22 fault zone.

Workshop focus on the Hosgri fault zone as it is being evaluated (subtask 1.4), and on the San Simeon fault I

by geophysical methods zone (subtasks 1.3, 2.1 and 2.3), which is the closest part of the San Gregorio-Hosgri fault system accessible to on-land investigation.

NEOTECTONIC AND QUATERNARY STUDIES IN THE SAN SIMEON AREA Neotectonic and Quaternary studies have been conducted in the San Simeon area to characterize the behavior of the San Simeon fault Because of the probable lateral continuity of or zone.

transferral of slip from the San Simeon fault zone to the I

characterization of the San Simeon offshore Hosgri fault zone, zone will likely provide a better understanding of behavior of the Hosgri fault zone.

The objectives of these neotectonic and I

Quaternary studies are to evaluate the:

Location of active fault traces and other tectonic deformation e

Relationship of Quaternary tectonics to preexisting structure e

Sense of slip and style of deformation e

Timing of deformation along various parts of the zone e

Most recent rates of slip (lateral and vertical components) e Field activities conducted in the San Simeon area to address these objectives are summarized in the following paragraphs and I

consisted of:

Geologic mapping of marine and fluvial terraces e

Submarine geologic mapping of offshore bedrock e

Topographic profiling of marine and fluvial terraces f4 e -

Drilling and seismic refraction surveying to assess depth to bedrock and to identify major fault strands; about 3000 feet e

in 79 boreholes and about 11,000 feet in 22 refraction lines I

have been completed 39 pits Exploratory pits to assess soil profile development; e

have been completed Trenching across major fault strands; 16 trenches totaling i

e 1300 feet have been completed Detailed logging of natural exposures I

e Correlation and dating studies e

Previous mapping by PGandE (1975), Hall (1976), Weber (1980),

Envicom (1977), and Manson (1985) indicates that the San Simeon I

fault zone onshore widens progressively northward from a relatively narrow zone of deformation near San Simeon Point to a (Figure 2).

wide zone of deformation near Piedras Blancas Point I

The San Simeon fault is the easternmost member of the fault zone, Initial field studies were concentrated as shown in Figure 2.

along the southern portion of the fault zone where the potential i

is greatest for characterizing slip across the entire fault zone (Figure 3).

Quaternary deposits in the area were mapped.

Offshore submarine geologic mapping and bedrock sampling were conducted around San Simeon Point and in San Simeon Bay (Figure I

Drilling, seismic refraction, soil test pits, and dating 3).

studies were conducted at specific localities throughout the Trenching was conducted along the primary fault trace at area.

Oak Knoll Creek, Airport Creek, and the Borrow I

three localities:

Natural exposures have been logged in Airport Creek Pit area.

and along the sea cliff overlooking San Simeon Bay.

Color infrared aerial photographs (1:40,000-scale) were examined The to study the geomorphic expression of the fault zone.

photographs show that the fault zone has strong geomorphic expression and widens northwestward.

The primary fault strands I

displace small drainage courses in a consistent right-lateral direction.

The creeks are incised into marine terraces that are The drainages are typically 350,000 years old or younger.

These values provide a displaced on the order of 300 meters.

rough approximation of slip rate on individual fault strands and indicate that the San Simeon fault has experienced recurrent late-Quaternary right-lateral displacements.

Figure 4 is an interpretation of one of the aerial photographs and illustrates both the prominent right-lateral deflection of correlative I

features in several drainages and the increasing width of deformation to the northwest.

Figures 5 and 6 diagrammatically illustrate the features of I

marine terraces and how marine terraces can be used for assessing fault behavior.

A marine terrace consists of a wave-cut-platform typically veneered by a sequence of marine and continental The continental deposits include stream deposits, I

deposits.

I GEOLOGIC AND TECTONIC SETTING I

OFTHE SAN SIMEON/PIEDRAS BLANCAS POINT REGION KJJ lo l

I Ragged Point

\\

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I s

N\\

i

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=

Tb 9s Point Sierra Neseda Jo E,

KJf 444 O

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Piedras Blancas Point' KIf I

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I San Simeon Point k

Pd I

0 S mi I

I I

I O

5km EXPLANATION I

Mapped fault; dashed where approximately l Tm l Monterey and Snquoc/

located; modified from Hamilton (1984; Pismo Formations l Tb l SMimentary breccia of

--- Geologic contact I

Point Sierra Nevada Serpentine l KJf l Franciscan Formation l

f, l Jurassic sedimentary rocks

[ Jo l Ophiolite Assemblage FIGURE 2

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SAN SIMEON FAULT ZONE TRENO.HING LOCALITI'.!S

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EXPLANATION e*

Submarine Rock Samples Monterey Formation Approximate trend of the $4n Simeon Fa.M e

I e Franciscan Formation FIGURE 3.

E E M M

M M

.g g

g g

0

,i DEFLECTED DRAINAGES ALONG THE SAN SIMEON FAULT ZONE

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1 EXPLANATION

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.3 7-

[

'a-a' Correlative feature w

Shoreline angle

.f *'

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i I

=-

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SKETCH OF TYPICAL MARINE TERR ACE SHOWING ELEVATED PLATFORM AND OVERLYING TERRACE DEPOSITS l

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SKETCH OF PROGRESSIVELY DISPLACED SHORELINE ANGLES ACROSS AN ACTIVE FAULT

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E Q

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Modem Shoreline Angle N

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l FIGURE 6

[

wind-blown sand and silt, and colluvium derived from the eroding

(

former sea cliff.

The wave-cut platform intersects the former sea cliff at the shoreline angle.

The shoreline angle was cut at mean high tide and thus was essentially horizontal at the time of its formation. The shoreline angle, therefore, provides a unique

[.

horizontal line in space from which to assess subsequent tilting, folding, and fault displacement in both vertical and horizontal components.

The shoreline angle, however, is typically veneered h

by younger deposits.

Detailed mapping of isolated exposures, supplemented by drilling and seismic refraction surveys, is generally used to identify its location and elevation.

Figure 7 is a preliminary map of the marine terrace shoreline angles in the San Simeon area.

A sequence of marine terraces has been identified, and the shoreline angles of the youngest six of p

L these terraces are shown in Figure 7.

Two of the terraces, the Tripod and the oso, have been mapped and correlated across the San Simeon fault zone.

These terraces are interpreted to be

{

124,000 years and 214,000 years old on the bases of correlation with paleo-sea-level curves and on lateral continuity of the lower terrace with a dated locality 20 miles to the southeast r

near Cayucos. Further work is in progress to document the L

correlation of marine terraces across the fault zone and to provide better age control on the terrace sequence.

(

Figure 8 is a longitudinal profile subparallel to the coast line showing the relative elevations of marine terrace shoreline angles and their relationship to the San Simeon fault zone.

The location of the profile is shown on Figure 7.

The profile indicates that the marine terraces have been vertically displaced along the San Simeon fault zone and that the terraces have been regionally tilted or warped into the fault zone.

As part of the studies to map and correlate the Quaternary marine terrace stratigraphy and to characterize fault activity, an

{

extensive program of drilling and shallow seismic refraction profiling was conducted.

To date, 79 boreholes have been drilled and 22 seismic refraction lines have been surveyed.

These

[

activities provided depth-to-bedrock information from which a top-of-bedrock structure contour map was prepared as a working L

document at a scale of 1:5333.

The contour map serves to identify the location and orientation of faults, to assess

[

paleogeography during former high sea level stands, and to document the distribution and correlation of marine terraces.

{

Figures 9 and 10 illustrate the results of drilling and trenching in the Borrow Pit locality.

Figure 9 is a topographic map of the borrow pit showing the primary strands of the San Simeon fault zone as observed in the Borrow Pit exposure.

Trenches were

[-

excavated across the fault to assess fault activity, and a series of exploratory pits and drill holes were emplaced to evaluate the width and orientation of the fault zone.

A geologic cross E -

1 F7 n

n n

n F7 n

n n

n n

M W

n n

C

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s s

MAP OF MARINE TERRACE SHORELINE ANGLES

^ %'Ti! ::h

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ALONG THE SAN SIMEON FAULT ZONE NEAR SAN SIMEON, CALIFORNIA

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TRIPOD (2s>.,

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'8 EXPLANATlON

'., g( sa (s,.,..P

' "... ;.S/MEON'Y, 's (

elevation of shoreline angle in feet

-.$3.'

/

/f Marine terrace shoreline angle, showing f.

.s gjy s,

s* F S/MEON

.s

\\g,,,

iso,,,

Piercing point - Tripod shoreline angle

,". +y.:

a an s-(124,000 yr)

. * "i Simeon Point

..s (20-25'i

'A' Piercing point - Oso shoreline angle a

(214,000 yr)

-.s.

.n 0

4000 ft 3

a i

7--

Fault; dashed where approximately 10'00 m C

• D located, queried where uncertain 0

rn Zone of most recent deformation j

3; along the San Simeon fault l

l l

)

t m

m rm rm rm rm rm m

rm rm rm rm rm rm rm rm rm rm r-SHORELINE ANGLE ELEVATIONS OF MARINE TERRACES IN SAN SIMEON AREA EXPLANATION A'

A

[

Elevation range for shoreline angle 300_

A Correlation of 124,000 yr (Tripod) shoreline San Simeon Fault Zone a

Correlation of 214,000 yr (Oso) shoreline angle

- 75 WCP Wave cut platform stripped Location of intersection of San Simeon fault trace and shoreline argte.

i 200-T,

}e

_300J00 yr _ _ _ - _ _ _ ___ _ _ _ _._ _ _ _ _ _._

stripped WCP

- 50 z.

7 z

9 9

l a---F-a j

co 3

r

' [^

l ' - " ' I a 2 ig0g0,yr, __,_, _ __ _ ___ _,_ _ _ p _ _ {

y 124.0

- 2s 4'

.-r I -

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-p-IA-1 l

1 - - - -

-I -IE I

s0-

....... _I_ _ _105,000 yr I'

3._3 g.

)

0 Little (MSL)0 Adobe Oak Knoll Airport Arroyo Broken Pico Creek Creek Creek del Bridge Puerto Creek Creek 2!

O 5000ft 3

O I

I San Simeon C

i i

T 0

1000 m Point m

CD 40X Vertical Exaggeration Fw hm d S& M's FW 7

I BORROW PIT LOCALITY ON I

SAN SIMEON FAULT b

X 149.2

@ DH-18 DH44 I

DH 43 k

DH 2 DH 17 0

100ft 6

DH 25 3

b0 I

pit s

O

\\

50kn SO

\\ v's o

+

oi#e DH-164 u*

DH 13

/

DH-30 I

j T4 C,

T2A4 N36W I

E P-1 i

I DH-32 4D O EP 5 8

116.6 Stecnds within the 9s San Simeon Fauft Zone cS E P-3

\\

q, 0,0 EP-2\\

d EXPLANATION I

30"-wide backhoe trench DH 16b 5"-diameter auger hole EP-1 0 30"-wide backhoe exploratory pit

- 120 Topographic contour showing etevation in feet I

...smarmas Trace of faults exposed in trenches; dotted where concealed FIGURE 9 M

M M

M M

CROSS SECTION ALONG BORROW PIT ROAD, SAN SIMEON FAULT N64E i

9

~9 h9 9

Otm Qtm g$

g 7

O'

- 50 g

M9 5

O'"

\\

150 -

g k_ ='= = = Ap

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- 40 E

Ge Ge l

- 30 *E, 100

--- -=

g 7

____._7.____

g, K

KJf KJf

- 20 $

~

- Q ~Oc io

.x Tc

~

H

]lf All

-10 f

( 50-Tm A T gg 1

l Klf AlT l

l g

C d

0 g

i e

i I

I I

0 100 200 300 400 500 600 700 800 HORIZONTAL DISTANCE, feet i

EXPLANATION lGiml Quaternary marine terrace units Fault; dashed where approximately located, i

dotted where concealed, queried where uncertain, l Ge l Quaternary eolian deposit ll A/T (away, toward) and arrows indicate apparent relative direction of displacement

~

lOcl Quaternary colluvium A.l T l Tc l Careaga Fm.

Q l Tm l Monterey Fm.

h Drill hole For location of Section B B'see Figure 9 m

l KJf l Franciscan Fm.

I section through the Borrow Pit locality based on drill hole data is shown on Figure 10.

The drilling results indicate that the fault zone is several hundred feet wide and that individual fault I

strands juxtapose late Quaternary deposits against bedrock.

}

Several techniques were used to correlate and date the Quaternary These I

marine and fluvial stratigraphy in the San Simeon area.

techniques are:

Correlation to paleo-sea-level curves o

e Soil stratigraphy e

Thermoluminescence e

Radiocarbon dating Fossil analyses including paleoclimatic indicators, U-series e

dating, and amino acid dating Paleo-sea-level curves are commonly used to correlate marine terrace sequences from region to region and to approximate the ages of marint terraces (Bull, in press).

The technique is based on an assumption of constant uplift and the correlation of marine I

terrace elevations to former high-sea-level stands.

The correlation of marine terraces northeast of the San Simeon fault zone to the paleo-sea-level curve is shown on Figure 11.

I Predicted terrace elevations are shown using an assumed constant uplift rate of 0.17 meter per 1000 years.

The uplift rate was estimated by assuming (based on correlation to the dated locality at Cayucos) that the terrace at elevation 85 feet is correlative to the 124,000-year high-sea-level stand.

Ages of other marine terraces are estimated by correlating the flight of terraces to the well-dated paleo-sea-level curve.

The relative degree of soil-profile development has also been used to map and correlate marine terraces in the San Simeon area.

I With increasing age, the thickness and percent clay content of the textural B horizon increase.

Details of the soil investigation are presented later in this report.

Several dating techniques are being used in the San Simeon area to date the marine and fluvial stratigraphy.

Locations of samples are shown on Figure 12.

Five samples of silty clay were I

collected and submitted for thermoluminescence analysis.

More than 30 samples of charcoal and wood were collected and submitted for radiocarbon analysis.

Five samples of invertebrate fossils I

were identified and collected.

Faunal assemblages will be determined for interpretation of paleoclimate.

Suitable shell material will be submitted for amino acid racemization analysis, and coral fragments will be submitted for uranium series dating.

I At this time, one radiocarbon date has been reported for a fluvial terrace along Oak Knoll Creek, giving an age of 3780 radiocarbon years before present.

I m

rm rm rm m

rm ra rm m

rm rm rm rm rm ra rm rm rm r

ESTIMATED UPLIFT RATE FOR STRUCTURAL BLOCK NORTHEAST OF SAN SIMEON FAULT ZONE 280 -

Emergent EXPLANATION

- 80 4.l1 Terraces 100' Elevation predicted y,l 240 -

using uplift rate g,;

i)' ',

(100') Observed elevation

- 60 200 -

//.

Airport U"Ygen isotope stage

?, / 7 193'(175-100')

5a

.6 fr m Shakleton and M, IN;Il'[gI i

Opdyke (1973) 160-r,..

-40

sf, N'.,U.

j'/ja % :',f-Q 92', f'/,'y 108' (110') Oso

/'

120 -

/Wz.u',./.,,,,.,

.#fr 6.C/

'a,.' J O, -

5 ' ' ' ' f'I7 86' (70 80')

80 --

- 20

/ 'F ' '

'/.

Tripod (IN g

E.

I

@.!,ff'2s' (20-2s') San Simeon j

4I4-

'd.

kI14 Q,hM.lbfM_

e' o 3!i Y $ 3o I

zp 5 ~m

'T T

J

~<

}

3 w

Uplift d

~4 -

0.17/1000yr

-20 c

1 3

5 7

9 a gl tI' j

{

.{

{ j}g

_ 9

\\l l

[

w

-120 -

{

l Paleo sea.tevel curve

-160-(Bull,in press) g

\\

l I d

()

Sea level curve

\\

/

y '

-200 i

_5, 0

40 80 120 160 200 240 280 320 h

AGE,1000 years a

m, -

[

MAP SHOWING LOCATION OF SAMPLES FOR DATING IN THE SAN SIMEON AREA

{

hz

( k

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fx

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x

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, %fp(rj 3;

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3

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ag=;,

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1

(

. Nx Na'

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hv

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o.:

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p..e-p-

[

i, O

N lp

,o,.g,3 orin tai.ac. aoc.

x a_

O Sa n.,Simeon 9

+8

'Poinh **

f EXPLANATION

,o,

e Invertebrate megafossils E Thermoluminesence A Radiocarbon

[

[

[

FIGURE 12 l

Figure 13a shows the distribution of marine terrace shoreline angles and locations of fault strands in the Airport Creek / Oak Two shoreline angles have been correlated Knoll Creek area.

Solid triangles represent piercing points across the fault zone.

for the 124,000-year (Tripod) marine terrace.

Open triangles represent piercing points for the 214,000-year (Oso) marine Calculation of displacement across the zone requires an terrace.

Because understanding of the original shoreline paleogeography.

the shoreline angles are in part subparallel to the fault trace, uncertainty is introduced by projection of the shoreline angles I

across the fault trace.

Figures 13b and 13c show the estimation of minimum and maximum displacement based on assumed projections of the shoreline Field studies are continuing to further constrain the angles.

paleogeography and thus to reduce the uncertainty in the estimation of minimum and maximum displacements.

Table 2 shows preliminary estimates of displacement and slip rate across the San Simeon fault zone.

Lateral displacement I

constitutes more than 90 percent of the net displacement and clearly indicates that the fault is a strike-slip structure.

Values for net slip rate range from a minimum of 1.5 millimeters I

per year on the primary trace to nearly 10 millimeters per year across the entire onshore fault zone.

Preferred values of 2.8 millimeters per year on the primary trace and 5.8 millimeters per year across the entire zone were estimated based on the most I

likely age of the Tripod terrace and the central value of the displacement.

Field studies in progress are expected to further constrain this rate.

I TABLE 2 1 OF SLIP RATE ACROSS SAN SIMEON FAULT ZONE PRELIMINARY ESTIMATES Net I

Displacement 2(m)

Net Slio Rate (mm/vri Total Total Age Primary Fault Primary Fault Feature (1000 vr)

Fault Zone Fault Zone Tripod

+ 2.1

+ 3.9 shoreline

+9 350 i 150 725 1 275 2.8 - 1.3 5.8 - 2.4 angle 124 - 21 Oso shoreline 4.6 i 1 I

990 1 230 angle 214 These estimates are preliminary pending dating and final mapping 1

I results.

Horizontal component is more than 90 percent of the net 2

I displacement.

[

k.

! z.

\\. f j

GEOLOGIC MAP AND

.,{

ALTERNATIVE INTERPRETATIONS OF DISPLACED MARINE TERRACE

'"y f \\. /

SHORELINE ANGLES, AIRPORT CREEK /

m..

OAK KNOLLCREEK AREA f

VA!

Nof s S

I L

f

\\

/

i o

EXPLANATION

p.. %.,,..

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a. MARINE SHORELINE ANGLES IN THE AIRPORT CREEK /

OAK KNOLLCREEK AREA

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b. MINIMUM DISPLACEMENT (450ml

c. MAXIMUM DISPLACEMENT (1000m)

OF TRIPOD SHORELINE OF TRIPOD SHORELINE ANGl.E (124Yix1000 yr).

ANGLE (12473 x 1000 yr) FIGURE 13

I Soils Chronoseauence in the San Simeon Area Thirty-nine pits were excavated into the terrace deposits on I

elevated marine abrasion (wave-cut) platforms in the San Simeon area to expose the degree to which soil horizons have developed on terraces of various ages.

According to the most widely I

accepted soil model, soil formation (S) is a function (f) of five soil environment factors:

climate (Cl), organic matter or biota (O), topography or relief (R), parent material (P), and time (T).

The relations of these factors to soil formation can be expressed by the following equation:

S = f(C1,0,R,P,T)

In a geographically restricted area such as the flight of marine terraces near San Simeon, it is possible, through careful selection of excavation sites, to minimize the effects of I

climate, organic matter, relief, and parent material, and assess the pattern of soil development chiefly as a function of age.

Higher and therefore older marine terraces should, according to the soil formation equation, have progressively more strongly developed soil horizons.

Field observations at San Simeon, as well as initial laboratory results, demonstrate that this is the case.

Furthermore, the soils analysis indicates that at least the youngest four terraces in the San Simeon flight have sufficiently distinctive soil profiles that the degree of soil development can be used to correlate terrace surfaces across the San Simeon fault zone.

Such correlations are essential for reconstructing the paleogeography of the region and for estimating fault slip rates.

Figure 14 is a diagrammatic sketch of an elevated marine abrasion platform with a variety of terrace materials deposited on top.

l Shown is the typical setting selected for excavating pits in the area to evaluate the relict soils.

Relict soils are surface soils (as opposed to buried soils) that have formed on a preexisting landscape.

Sites where the soils have been buried or are constantly being added to by younger materials (such as near the base of an ancient sea cliff, which acts as a source of colluvial materials as it degrades) or sites near the top of the l

modern sea cliff or the margins of incised stream canyons where j

the soils are being stripped away by erosion have been carefully avoided in order to establish a soils chronosequence that is based upon the relative age of the relict land surfaces.

Most soils described for use in the chronosequence are on stable surfaces.

Horizon thickness is thought, therefore, to reflect I

in situ soil formation and not colluvial additions or erosional losses that would tend to thicken or thin the profile.

Table 3 outlines the sequence of soil horizons that develop under typical pedogenic conditions. <

SCHEMATIC RELATIONSHIP OF RELICT SOILS I

ON MARINE ABRASION PLATFORMS I

Ancient sea cliff I

Colluvium I

Best location for Relict soils soil pits

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= -

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abrasion platform I

I FIGURE 14 -

ll TABLE 3 l

HYPOTHETICAL PEDOGENIC SOIL HORIZONS Ol*

Original form of most vegetative matter visible to eye 0

Organic debris Original form of most vegetative I

02 matter cannot be recognized with eye A

Dark-colored horizon having high A

content of organic matter mixed with mineral matter l

Horizons of E

Light-colored horizon of maximum maximum eluviation typified by loss of THE SOLUM biological iron, aluminum, or clay, and

activity, concentration of resistant materlais such as quartz l

d eluviation Genetic soil or both I

a formed by the AB Transitional to B but more like A than B E

soil-forming processes 8

BA Transitional to B but more like B B

than A Horizons of Accumulation of clay, iron, aluminum, illuviation, humus or in combination; residual residual concentration of sesquioxides or

!3 concentration, Bt mixed; sesquioxide coatings giving

'l coloring and darker, stronger red colors; or certain having granular, blocky, or structure prismatic structure BC Transitional to C I

c g

Gleyed layer having base colors near neutral Parent material from which soil is presumed to have formed; lacks Beta horizon, accumulation of clay, soil structure; weathered; may be gleyed, cemented, and have BETA iron, sesquioxides above bedrock; accumulation of soluble salts oxidized or reduced I

Unweathered parent material R

(typically bedrock)

• All these horizons will not be present in any profile, but every profile has some of them. The pedologic soil consists of the 0, A, B, and C horizons. I

The o horizon is not used to judge soil age because it reflects only the most recent additions to the surface, most of which have not been integrated into the soil.

The thickness of A horizons (on stable surfaces) is limited by the maximum depth of animal burrowing (approximately 30 cm in the San Simeon area).

There is little cultural influence on A horizons at San Simeon, with the exception of localized Indian middens.

The A horizons at San Simeon are highly bioturbated and the material within them may be completely turned over in as few as 200 years.

Elevated silt contents in San Simeon A horizons suggest addition of a minor eolian component blown in from the coast.

The E horizons form at Gan Simeon due to lateral flow of water across the top of

~ clay-rich B horizons.

The B horizons are so clay-rich that they effectively act as aquitards.

When storms occur, vertical penetration of water is inhibited, and it generally flows laterally on top of the horizon.

This strips the uppermost parc of the B horizon of previously accumulated iron oxides and some clay, giving the E horizon a bleached and mottled appearance.

The increasing development of the B horizon with increasing age / elevation of the terrace surface is one of the most diagnostic soil parameters observed at San Simeon.

The B horizons at Gan Simeon form extremely rapidly due to the influence of sodium ions at the coast.

Sodium enters the soil system primarily through wave spray.

The wave-spray sodium is incorporated into the atmosphere (especially in fogs) and carried inland, where fog drip from the vegetation brings the sodium into the soil profile.

Sodium forms a particularly large cation.

When it attaches to clay particles, its large ionic radius inhibits ionic attraction among clay particles and thus actually disperses clay throughout the profile.

The result is a clay-rich and possibly thickened B horizon.

The accumulation of clay in the B horizon promotes the development of the soil structure (peds).

The thickness of the B horizon also is strongly influenced by the average wetting depth. Thus, soils that formed during the Pleistocene, for example, generally have thicker profile development than Holocene soils, because the older soils originated in a comparatively wetter climatic regime.

The C horizons have little evidence of pedogenesis; however, their properties can be used to evaluate how much pedogenesis has occurred in the overlying horizons.

By comparing the soil morphological properties of the C to overlying horizons (which presumably were similar to the C horizon upon initial deposition), it is possible to evaluate statistically the amount of soil development in a particular profile.

After the statistical evaluation, the data can be compared to other California coastal sites that have good age control and, assuming similar pedogenic factors between the sites, soil ages at San Simeon can be estimated.

Figure 15 shows field estimates of clay distribution in soils formed on four different marine terraces in the San Simeon region and shows that clay development generally increases with age, although at a non-linear rate.

The clay distribution is one of the key soil-development properties used to generate the time-dependent soil-development model illustrated in Figure 16.

This model assumes that the soil-forming factors of organic matter (O), parent material (P), climate (Cl), and topography (R) are held constant and that time (T) is the primary variable.

Figure 16 is another way of showing that visually estimated clay volume increases with age in the San Simeon chronosequence.

When laboratory grain-size analyses are completed, a clay development curve can be established for the' area.

Then, it will be possible to compare the observed clay development in the San Simeon I

chronoseguence with that of other well-dated marine terrace sequences, for purposes of correlation.

Figure 17 presents initial laboratory analysis of grain-size data on the San Simeon terrace soils.

Shown is a plot of the percent of clay versus depth for a typical soil on both the Airport and San Simeon terraces.

Characteristics of the San Simeon Fault Zone Assessed by Offshore Samplina and by Mappinc and Locaina of Natural and Trench-wall Excosures Figure 3 provides a map of the sites of detailed geologic sampling and logging that were conducted during summer 1986 I

within and adjacent to the San Simeon fault zone.

In the following discussions, sites of detailed investigations on the l

San Simeon fault zone are described beginning at San Simeon Bay l

and progessing northward to Oak Knoll Creek.

Figure 3 shows that in San Simeon Bay, slip along the fault has juxtaposed chert of the Monterey Formation against the varied lithologies within the Franciscan Formation.

The submarine rock samples collected during about fifty dives show that these two formations are separated on the floor of San Simeon Bay by a strip of sandy bottom, probably developed on the less resistant fault slice, of shallow, marine, fossiliferous sandstone exposed onshore and tentatively identified by Hall (1976) as part of the Careaga Formation.

The distribution of submarine rock outcrops 0

suggests that the approximate N35 W onshore trend of the San Simeon fault zone extends offshore for at least a mile southeast of San Simeon Point.

Figure 18 is a diagrammatic northwest-looking oblique view of the sea cliffs at San Simeon Cove that are cut by major strands of the San Simeon fault zone.

Although sand and colluvium derived from the dunes cover much of the cliffs in this area, wave erosion and limited hand excavations have revealed or constrained several key geologic relationships.

On the southwest side of the _.

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Model subject to change pending completion of laboratory analyses.

RELATIVE VOLUME OF CLAY IN Bt HORIZON

FIGURE 16

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PERCENT CLAY IN TWO SAN SIMEON SOIL PROFILES AS A FUNCTION OF DEPTH i

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(younger terrace)

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A Airport 1 Soil Profile (older terrace)

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20 40 60 80 100 PERCENT CLAY I

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GULL'S EYE VIEW OF SAN SIMEON FAULT ZONE AT SAN SIMEON COVE SW NE

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area shown en Figure 18, sheared and contorted Monterey chert has been juxtaposed next to a fossiliferous, shallow marine pebbly sandstone that, as noted above, Hall (1976) tentatively I

correlated with the Pleistocene Careaga Formation.

Although this fault is not exposed at present, it can be constrained to lie within a zone that is about one-half meter wide.

Both the Careaga(?) and the Monterey formations appear to be truncated by a marine abrasion platform that might be the San Simeon terrace (as originally defined by Weber, 1980) or a younger (marine isotope stage SA?) terrace.

This abrasion platform appears to be elevated about 2 meters on the

' southwestern side of the fault.

Alternatively, the difference in elevation may reflect contrasts in erodability between the Monterey and Careaga(?) lithologies.

Prominent shearing is also present within the tilted beds of the Careaga(?) Formation.

The shears exposed in a small peninsula trend N28 W and appear to be truncated by the marine abrasion 0

platform.

Shears on the northeastern side cut the deformed estuarine deposits, basal marine terrace gravels, and the Careaga(?) Formation.

Along its eastern boundary, the Careaga(?)

Formation is in fault contact with the Franciscan Formation, but this contact is not exposed along San Simeon beach.

The location of this latter fault has been well constrained, however, by both drilling and trenching some 2000 feet to the northwest at the Borrow Pit locality.

Figure 9 shows the extent and distribution of backhoe exploratory trenching in a Hearst Ranch sand quarry, here named the Borrow Pit locality.

Removal of sand from this pit had exposed clay fault gouge from one of the San Simeon fault zone strands prior to the current subsurface work.

The trenching at this site revealed two major fault strands, both of which have an apparent down-to-the-west dip-slip component of displacement.

These two faults appear to correspond to the northeastern contact of the Careaga(?) Formation shown in Figure 18.

Figure 19 is based upon the geologic and soils logs of the northwestern wall of trench T-3, which was excavated on the hill east of the borrow pit, where the geologic section had not been disturbed.

Faulting is marked by a prominent vertical to subvertical sheet of fault gouge separating dune deposits and older terrace materials.

The strike of the major shear is 0

0 rotated clockwise from N36 W at the base of the trench to N22 W near the surface.

This upward rotation of strike, coupled with well-developed subhorizontal slickensides and grooves on the fault plane, establish that this strand is essentially a pure right-slip fault.

Northeast of the fault, a marine abrasion platform tentatively identified as the Tripod terrace, as defined by Weber (1980), is exposed in the uplifted block along with deeply weathered remnants of overlying marine terrace deposits.

As seen in Figure 19, the soil horizons overlying the Bt horizon

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BORROW PIT TRENCH T-3 SAN SIMEON FAULT ZONE I

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2*. 5* SE L

EXPLANATION

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1m

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/ Fault; dashed where approximately Location of Trench T-3

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located shown on Figure 9

--- Geologic contact; dashed where

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gradational

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FIGURE 19 -

are preferentially developed over the fault.

This suggests that a relatively recent slip event fractured the topsoil and promoted downward migration of organic materials (A horizon) and soil water into the Bt horizon.

Figure 20 is a diagrammatic cross section based upon trenching at I

the Borrow Pit locality and upon logging of nearby drill holes.

The down-dropped block of Careaga(?) Formation between Figure 19 the Monterey Formation on the west and the Franciscan Formation on the east is very evident.

The easternmost fault shown in this section is the western one shown in the map of the Borrow Pit locality (Figure 9).

Figure 21 shows the extensive trenching performed on either side of Airport Creek by both Envicom, a geotechnical consulting firm hired by the Hearst Corporation in 1977 to perform an evaluation of the San Simeon fault zone, and by PGandE in 1986.

The Airport Creek locality lies about 3000 feet northwest of the Borrow Pit locality and is marked by two physiographic features indicative of recently active faulting.

These are a right deflection in the channel of Airport Creek where it crosses a major strand of the San Simeon fault zone, and a trough bounded by fault scarps on the southeastern side of the creek.

PGandE trenches T-1, T-2, T-4, and T-5 were. excavated across the fault trace, trenches T-3, T-6, and T-7 were excavated to expose the stratigraphic relationships within a section of partially ponded sediments on the northeast side of the fault.

Figure 22 shows a detailed geologic / soils log of a fault strand of the San Simeon fault zone as exposed in the northwest wall of Airport Creek trench T-1.

Here, as at the borrow pit, this strand of the San Simeon fault zone is vertical to subvertical, has subhorizontal slickensides that plunge 6 to 7 degrees to the southeast, and is characterized by right slip.

Detailed logging of this trench revealed three distinct soils, including the modern soil (X) and two buried soils (Y and Z).

Multiple slip events can be identified in this trench-wall exposur,.

The results of the radiocarbon dating and detailed analysis will help reconstruct the sequence and timing of prehistoric slip events and may lead to an improved estimate of the recurrence intervals for faulting along this strand of the San Simeon fault zone.

Figure 23 is based upon a detailed plane-table map of the right-deflected active channel of Airport Creek between trenches T-1 and T-4.

Although the creek changes direction near the fault, both channel walls display a consistent 6 feet of right-lateral displacement.

Detailed logging of the channel walls I

suggests that the channel deflection is tectonically, not stratigraphically, controlled.

The age of channel wall displacement is not constrained at present, but is probably late Holocene.

This inference is based on the observation that the channel is incised into fluvial terrace deposits that were deposited during the latest Pleistocene-Holocene rise in sea level.

M M

r CROSS SECTION ACRGSS BORROW PIT TRENCH SITE 1

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0 100 200 300 400 500 600 700 800 HORIZONTAL DISTANCE, feet EXPLANATION lQtml Quaternary marine terrace units

. Fault; dashed where approximately located, (l

dotted where concealed, queried where uncertain, l Ge l Quaternary dune sand Ig' A/T (away, toward) and arrows indicate apparent relative direction of displacement

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l Tc l Careaga Fm.

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l Tm l Monterey Fm.

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[ KJf l Franciscan Fm.

For location of Section C-C'see Figure 9 2

Drill hole Oc 33 m

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MAP OF TRENCH LOCATIONS AT AIRPORT CREEK LOCALITY SAN SIMEON FAULT ZONE Spring c

r t

i 4 Env.4 EP 80 0-Deflected Channel 3g

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tectonic origin gg 1

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30"-wide backhoe trench excavated by PG&E Backhoe trench excavated by Envicom I

EP-1 l

0 30"-wide backhoe exploratory pit excavated by PG&E

' FIGURE 21

LOG OF SAN SIMEON FAULT STRAND EXPOSED IN NORTHWEST WALL AIRPORT CREEK TRENCH T-1 Zon;h of '

N66E Fractured Peds Y

Y

/

(

r Topsoil (A)

(A)

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EXPLANATION Sandy Clay X

Modern soil 0

2 ft Y, Z Buried paleosols I

i

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Fault; dashed where approximately.

0 1m located, queried where uncertain.

A/T (away, toward) indicate apparent I-relative direction of displacement.

FIGURE 22 __

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MAP OF I

DEFLECTED CH ANNEL O F AIRPORT CREEK I

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Edge of Channel ll N

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I;;i Two separate areas near Oak Knoll Creek were explored for faults

(,

within the San Simeon zone (Figure 24).

Trenches T-1, T-2, and T-3 were excavated to depths of 15 or more feet in the h

semiconsolidated Holocene fluvial terrace deposits on the south margin of Oak Knoll Creek.

An active strand was intersected in trenches T-1 and T-2 and will be described in detail below.

Initially, trench T-3 was planned to extend an additional several hundred feet to the northeast to detect any active, more easterly strands within the fault zone, but unstable trench walls and rather homogeneous, poorly stratified deposits made it necessary to pursue an alternate approach.

Trenches T-4 and T-5 were then excavated in an area of shallow bedrock overlain by a thin veneer of alluvial and colluvial materials.

A buried fault scarp step in the bedrock was unearthed in trench T-5.

Figure 25 is a diagrammatic oblique view showing the approximate relative positions of trenches T-1, T-2, and T-3, the active strand found in T-1 and T-2, and the approximate projected y

position of the more easterly strand encountered in T-5.

The north wall of trench T-2 (Figure 26) was logged.in detail and J

revealed a vertical fault having a strike of N26 W at the bottom of the trench (15 feet below the surface).

At a depth of 5 feet, R

0 the fault rotates clockwise to N14 W.

A sandy marker horizon located at a depth of 5 to 7 feet was exposed in the walls of trench T-2A, which was excavated to locate " piercing points" on opposite sides of the fault.

A map showing contours of equal thickness on the sandy horizon was prepared and the horizontal separation of originally continuous features was measured (Figure 27).

The most recent slip on this fault strand has the following characteristics:

1)

Horizontal right slip = 3.8 1 0.1 feet 2)

Vertical slip (east side up) = 0.7 to 0.8 foot 3)

Net slip about 3.9 feet 4)

Poorly defined slickensides and grooves indicate that the slip vector plunges 10 to 15 degrees to the north Whether this slip occurred during one or more than one events cannot be conclusively resolved with t?.e present data.

However, a consistent amount of vertical sendtst on measured across the fault to the bottom of the trenc'i G aw that the upper 15 feet of the Oak Knoll terrace section ary Jir;,;ced the same amount.

Radiocarbon dates on samples collected from trenches T-1, T-2, and T-3 are currently being obtained and will help to assess the timing of last slip on this active strand.

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SKETCH OF BACKHOE TRENCHES AND FAULT STRANDS AT OAK KNOLL CREEK Hearst Castle y

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Oak Knoll Creek Terrace

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FIGURE 25

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w Table 4 outlines the major preliminary conclusions that have been reached to date about the San Simeon fault zone in the vicinity

~

of San Simeon.

These conclusions represent a significant advance in the understanding of this important fault zone, which may experience all or a significant part of the slip occurring on the Hosgri fault zone.

Radiocarbon dates in progress should, when ages have been estimated, provide further insight into rates of slip along the system of northwest-trending strike-slip faults along the coast of central California.

TABLE 4

~

PRELIMINARY CONCLUSIONS FROM THE NEOTECTONIC AND QUATERNARY STUDIES IN THE SAN SIMEON AREA The sense of slip on the San Simeon fault zone in the San e

Simeon area is primarily right lateral along steeply dipping to vertical shear planes.

The zone of late Quaternary faulting increases in width e

northward from several hundred feet to more than one mile.

The zone consists of several discrete traces and numerous subsidiary splays.

Contrasts in bedrock lithologies across large shear zones e

suggest continuing displacement along preexisting faults.

Primary fault traces show multiple displacements of

~

o Holocene deposits.

Some subsidiary fault splays displace late Pleistocene to Holocene deposits.

~

Preliminary estimates of late Pleistocene rates of slip e

range from 1.5 to 9.7 millimeters per year, with a preferred range of 2.8 to 5.8 millimeters per year.

Holocene alluvium containing datable charcoal is displaced e

I

~1.2 meters right laterally.

OFFSHORE STUDIES OF THE HOSGRI FAULT ZONE The offshore Hosgri fault zone is being studied by means of interpretation of two general sets of geophysical data, one a high-resolution set emphasizing the surface and shallow subsur-I face aspects of the fault, and one concerned with its geophysical expression in the time-depth range between 1 and 5 seconds.

Effectively, the latter data set provides evidence of the struc-tural form of the Hosgri zone and related adjacent structural and seismostratigraphic features to a maximum of around 2 seconds time-depth.

In highly deformed, structurally complex areas, 35 however, details of the faulting are commonly obscured even at g

relatively shallow depth, and seismic records may be essentially uninterpretable below about 1.5 seconds time-depth.

I g

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Offshore 2-to 5-Second Seismic Study The study of the deeper expression of the Hosgri fault zone through the time of the October 21-22 Workshop was concentrated r

along its reach between Estero Bay and Point Arguello.

The L

primary data set utilized for this study was the OPI-GSI suite of 5-second seismic records, which was acquired for the Long Term Seismic Program in 1985.

This set was supplemented by data

(

already held by PGandE, mainly represented by 5-second lines purchased from Western Geophysical and 2-second lines purchased from Consolidated Geotechnics Inc.

Additional seismic data held

{

by Ogle Petroleum Inc. also have been available for reference.

The initial effort in interpreting the structure of the Hosgri r

fault zone resulted in development of a structural trend map and L

annotated seismic profiles that cross the fault at intervals between Estero Bay and Point Arguello.

This work showed that along most of this reach, the Hosgri fault zone consists of two

{

structural components.

Above about 1 second time-depth, the Hosgri zone is represented by one or more vertical to steeply east-dipping breaks.

Although the gross vertical separation p

across the Hosgri zone is everywhere east-up, the sense of L

apparent displacement across these breaks is not always consistent in the uppermost part of the section.

Locally, it is east side down along the reach opposite the Sanra Maria Valley

[

and Point Sal. Splays from the high-angle breaks locally appear to form " flower structures", a structural pattern commonly associated with strike-slip faulting.

Near the base of the

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Neogene section in the offshore Santa Maria Basin along the west side of the Hosgri zone, a second component of faulting is evident, typically at a time depth between 1.5 and 2 seconds.

This component is represented by shallow, east-dipping breaks having an apparent thrust component of displacement.

The latter features correspond to the thrust aspect of the Hosgri fault zone first described in the Diablo Canyon FSAR, Appendix 2.5E (PGandE, I

1975), and later interpreted by Crouch and others (1984) as L

evidence for an active compressional tectonic regime characterized by widespread listric thrusting in the central coastal region of California.

The available GSI, Wectern, and P

i other seismic records have not shown clear evidence allowing interpretation of the structural relationship between the high angle and low angle breaks associated with the Hosgri zone, y

although the data indicate that only the high angle breaks reach or approach the present sea floor.

To resolve this important uncertainty, additional intermediate-to high-resolution seismic

{

data are being purchased and a program of reprocessing selected GSI lines has also been undertaken.

None of the higher resolution data and only a small portion of the reprocessed data were available for interpretation at the time of the October Workshop.

Following the preliminary structural interpretation of the 2-to 5-second seismic records, a program of developing structural contour maps on key horizons within the offshore Santa Maria I

Basin was initiated to provide a basis for evaluating the pattern f 1

and timing of deformation in the region adjacent to the Hosgri fault zone.

Three horizons have been contoured to date:

the top of the mid-Miocene Monterey Formation, the top-of-Miocene unconformity within the Sisquoc Formation, and a horizon of probable late Pliocene or Plio-Pleistocene age.

The salient characteristics of these horizons are as follows:

Top of Monterey Horizon -- This mid-Miocer.e formational e

horizon is deformed by north-northwest-trending folds west of the Hosgri fault zone.

Some of these folds are apparently associated with subparallel-striking thrust faults that break the top of Monterey horizon.

Folding and uplift of this horizon is locally evident east of the Hosgri fault zone, and additional mapping in this region is currently underway.

Top of Miocene Horizon -- This horizon is an unconformity in e

the Sisquoc Formation deformed by the same folding that I

affected the top of Monterey horizon; however, in general, the amplitude of folding is diminished at this depth compared with that at the mid-Miocene top of Monterey horizon.

Fewer faults penetrate this horizon than penetrate the top of Monterey horizon.

It is noteworthy that the thrust faults thus far observed do not penetrate this horizon, including those directly associated with the Hosgri fault zone.

This horizon is generally warped up to the east along the Hosgri fault zone, and appears in most areas to terminate against older uplifted terrane east of the Hosgri fault zone.

In some areas, the horizon may be displaced by faults within the Hosgri zone, although alternative interpretations are also plausable and are currently being investigated.

O Plio-Pleistocene Horizon -- This horizon is recognized as an unconformity at the eastern margin of the offshore Santa Maria Basin and is inferred to be a disconformity in I

the interior of the basin.

The horizon is deformed along the Hosgri zone by locally complex warping and by offset along high angle breaks.

The horizon appears to be recognizable on both sides of the Hosgri in the area of Estero Bay, but not farther south, where it has been uplifted and removed by erosion along the eastern margin of the offshore Santa Maria Basin.

Work is continuing to extend the structure contour mapping farther north along the Hosgri trend, using data recently acquired from Western Geophysical, and east of the Hosgri zone in San Luis Obispo Bay, using new and reprocessed data from several sources, including lines being acquired from Nekton, Inc.

Offshore Mich-Resolution Survey of Hoscri Fault Zone As discussed previously, a major focus of the Long Term Seismic Program is to assess the nature of the structural association between the Hosgri and the San Simeon fault zones.

If a direct association or connection does in fact exist, then key parameters of the San Simeon fault zone, as evaluated by conventional I

onshore studies and outlined previously in this report, can be applied to the Hosgri zone.

Until the additional offshore high-resolution seismic reflection data are acquired and become available for interpretation, the nature of this association will remain uncertain.

I Abundant high-resolution seismic reflection data exist within the PGandE data set along much of the Hosgri trend between San Simeon Point on the north and Point Conception on the south. The most extensive and instructive data set is the 1979 Fairfield Industries, Inc. survey prepared as part of a geohazards study for Lease Sale 53 for the Minerals Management Service (MMS) of the U.S. Geological Survey, as shown in Figure 28.

This data set consists of four components:

1)

Side-scan sonar (analog) 2)

3.5-kHz sparker lines (analog) 3)

1-kHz Fairflex 1/4-millisecond sparker lines (analog) 4)

1-second 12-fold CDP data (digital)

Side-scan sonar shows certain textural characteristics and the distribution of sediment on the ocean floor, as well as the position and some megascopic structural characteristics of submarine rock outcrops.

The 3.5-kHz data depict with considerable accuracy the configuration of the sea floor and reveal important details of stratification and its disruption by sliding, folding, and faulting within the upper 50 to 100+ feet of the sedimentary column, which is typically of latest Quaternary age. The 1-kHz Fairflex lines provide data on the geometry of acoustic reflectors to depths of 500 or more feet l

within the ocean floor sediments.

These lines provide the penetration and resolution necessary to link the surficial 3.'5-kHz data and the 1-second CDP lines.

The CDP lines image the Hosgri zone along with other faults and structures that have deformed the Neogene sedimentary basins adjacent to Diablo Canyon.

The 1-second CDP lines also provide the necessary connection between the high-resolution MMS data set and I

conventional 5-second petroleum industry seismic reflection lines, which show the deeper structure of the Hosgri fault zone and major offshore folds.

About two-thirds of the CDP data (that portion closest to shore) was processed by the MMS.

A prominent reflection thought to be of Plio-Pleistocene age was mapped in draft form throughout the region.

The Fairfield 12-fold CDP data was primarily used for the interprctation, due to its signal quality and resolving power at the depths of the Plio-Pleistocene reflection.

The location of faults and other structural features was corrobcrated by using of the analog Fairfield data, as well as the 2-second CDP lines shot by Consolidated Geotechnics, Inc.

in 1976.

Figure 29 displays a portion of a time contour map of the Plio-Pleistocene reflection, together with intersecting fault zones.

The Hosgri fault zone may be seen to uplift and warp the horizon into a pattern suggestive of a positive flower structure, a feature frequently I

associated with compressive strike-slip faults. --

E

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I Figures 30 and 31 show the typical surface and near-surface expression of the Hosgri fault zone as imaged by the 1-kHz Fairflex 1/4-millisecond analog sparker data.

Four time sections I

with a vertical exaggeration of approximately 18x are presented.

Lines 242 and 218 lie in Estero Bay; lines 170 and 120 lie between Point San Luis and Point Sal.

MMS line 242 shows a characteristic topographic expression of the Hosgri fault zone in Estero Bay (Figure 30).

Here the Hosgri fault zone consists of a series of vertical to steeply dipping I

fault slices that form the upper part of a west-facing escarpment on the sea floor.

West of the Hosgri fault zone, well-bedded marine sediments of late Cenozoic age dip gently to the west I

toward deeper water.

East of the Hosgri, a shallow, faulted basin contains young marine sediments that have been ponded against an elevated fault slice.

On this line, the fault slice is topographically expressed as a rock outcrop on the sea floor.

The sediments east of the fault thin to the east and are deposited on an elevated block of acoustic basement.

The uplifted fault slice appears on the side-scan sonar records to I

consist of folded Tertiary sedimentary rocks.

MMS line 218 lies off Point Buchon, a few kilometers north and west of the Diablo Canyon site (Figure 30).

Here the Hosgri zone is clearly expressed in association with a west-facing scarp that cuts the present sea floor.

Gas is apparently leaking up one of the fault planes and discharging as a plume of bubbles in the water column.

The west-facing shelf-slope break on the sea floor lies west of the near-surface traces of the Hosgri fault zone.

The upper edge of the slope has experienced a submarine slope failure that can be traced for tens of kilometers along the western side of the central Hosgri fault zone.

MMS line 170 (Figure 31) is one of the more important and interesting of the Fairfield Industries Lease Sale 53 traverses in that it extends several kilometers east of the Hosgri fault zone into shallow water.

South of Point San Luis, the Hosgri is no longer expressed as a west-facing zone of inflection on the sea floor.

As is characteristic of its more northerly reaches, the Hosgri fault zone has elevated acoustic basement on its eastern side and is currently ponding young marine sediment in a half-graben.

This half-graben has experienced recurrent down-to-the-west tilting, which is particularly well shown in the 3.5-kHz record for this line (Figure 32).

MMS line 120 lies just south of Point Sal (Figure 31).

Faults within the Hosgri zone bound a graben filled with young marine sediments.

The possibility of ongoing deformation across this zone is indicated by a flattening of the sea floor gradient across the graben.

Acoustic basement lies close to the ocean floor on the east side of the graben.

West of the graben, the Purisima fault zone separates the Hosgri fault zone from the thick sequence of largely undeformed, west-dipping marine sediments of late Cenozoic age in the offshore Santa Maria Basin. _

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e-

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FIGygg 3t E

INTERPRETATION OF MMS LINE 170,3.5-kHz DATA MMS Line 170 190 195 200 205 210

'85 Sea Level 50 milliseconds Possible disruption of Angular unconformity 5000 feet Holocene i reflector (latest Plestocene ?)

/

Hosgri Feuft

'. lS ' "*'W Strand m

\\

1. L y

\\

' e s

3 s

/ s Acoustic Besement y. A. '...

.. '... gir. ':#

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f

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y.. g -

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. ~

(late Plentocene ?)

• • s Half groben with deformed

• . Marine Sediment sediments and multiple engular unconformities Approximate Vertical Exaggeration 12 x ri C

33 m

f.J t4

l l

l PLANS FOR FUTURE WORK l

Substantial progress has been made in the characterization of the onshore part of the San Simeon fault zone and of the Hosgri fault j

zone between Estero Bay and Point Arguello.

At San Simeon, however, additional work is needed to assess the distribution of slip among different strands within the fault zone, and to further constrain the overall late Quaternary slip rate, which is l

l presently thought to be about 3 to 6 millimeters per year.

The structural relationship of the San Simeon fault zone to the Hosgri fault zone within the San Gregorio-Hosgri fault system must then be evaluated; this is a significant consideration in relating the slip rate and style of displacement documented at San Simeon to the Hosgri fault zone offshore.

l Further work on the Hosgri fault zone itself will be directed toward confirming the structural interpretation of the main zone, performing further investigations to document structural l

relationships at the north and south ends of the fault zone, carrying the structural interpretation of the faulting across its traces from west to east, and working out the evidence for and timing of major changes in style of faulting.

Extension and l

refinement of the structural contouring of various horizons in the offshore Santa Maria Basin stratigraphic section will provide the primary basis for the latter evaluation.

l Previously identified subtasks of Task 1 (Appendix A) will address the topic of along-strike variations in rate and style of l

displacement along major segments within the San Gregorio-Hosgri fault system.

This topic is important both for relating the Hosgri fault zone near Diablo Canyon to the overall San Gregorio-Hosgri system, and for evaluating the significance of the overall system in the regional tectonic model.

l g

-sv-

3 CHARACTERIZATION OF THE SAN LUIS/PISMO SYNFORM I

REGIONAL OVERVIEW l

The Diablo Canyon site is located along the western flank of the l

San Luis/Pismo synform, which lies within the San Luis Obispo/

l Pismo/ Santa Maria Valley region, as shown on Figure 33. This I

region encompasses the mountain and valley coastal tract and adjoining offshore area that lie between the Los Osos antiform and Huasna synform on the north and east, respectively, and the Santa Maria Valley synform on the south.

The offshore Santa Maria Basin underlies the sea floor to the west of the Hosgri fault zone, which forms the offshore western structural boundary of the region.

Four generalized geologic units are shown on Figure 33.

These consist of 1) an undivided basement rock unit, mainly represented by the Franciscan Formation but also including the Point Sal and Cuesta Ridge ophiolite complex outcrops, 2) the Oligocene age intrusive volcanic rocks of the Morro Rock /Islay Hill Complex, 3) undivided mid to late Tertiary sedimentary and volcanic rocks, and 4) undivided Quaternary deposits.

The structural pattern in the part of this region that lies between the Hosgri zone and the West Huasna fault is dominated by west-northwest-aligned I

synformal fold belts and intervening antiformal highs.

The synformal fold belts involve stratiform rocks ranging in age from early Miocene to late Pliocene, in a sedimentary sequence that reaches a maximum thickness of more than 3 kilometers.

The structural grain in the Franciscan basement rocks exposed in the Los Osos antiform parallels the trend of the Tertiary synforms and is well-defined by internal contacts, tectonic fabric, and by the aligned series of sill-like intrusive bodies of the Morro Rock /Islay Hill Complex.

The internal structure of the San Luis/Pismo synform is complex and involves many tight folds and crenulations.

This contrasts with the relatively broad, structurally simple downfolds of the adjacent Huasna and Santa Maria Valley synforms.

The two boundary faults of the San Luis Obispo/Pismo/ Santa Maria Valley region, the Hosgri and West Huasna, each extend well beyond the local region and are the subject of other l

investigations within the scope of the Program's geology /

seismology / geophysics studies.

Within this region, the recognized faults of interest all lie along the flanks of the major fold structures.

The Los Osos and Edna/ Indian Knob faults are along the northeast flank of the San Luis/Pismo synform; the San Miguelito, Pismo, and Wilmar Avenue faults are along its southwest flank.

The Casmalia and Lions Head faults define the I

northeast and southwest flanks of the Purisima Hills antiform. -

I ND TECTONIC SETTlkG '

GEOLOGIC OF THE SAN LUIS OBISPO/PISMO/

I SANTA MARI A VALLEY REGION I

Xn

<O s.

s.

N Los Osos * *O d.

'N s-s J. '""

1. ', T San.

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o g

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& ions N'

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10 mi I

I O

10 km E XPL AN ATION GEOLOGIC UNITS

/

Fault; dashed where approximately located l,

.l ousternary deposits

/

Buried reverse / thrust fault Mad to late Tertiary sedimentary

/

and volcanic rocks I

/

8d r o Roc sa Co em 4

eu,,- -emi.ee ueso,.c a,d e. re,t a.,

m.,a_,p.c a-se.mee,a,.

,, FIGURE 33

I

'I The Little Pine and Foxen Canyon faults, which lie southeast of the area of Figure 33, define a trend that may extend to the northwest along the hypothesized Santa Maria River fault.

The I

Oceano monocline, identified as a subsurface feature in seismic reflection lines recently completed by Seisdata Services, Inc.,

is the only structural feature identified to date along the I

general projection of the Little Pine /Foxen Canyon trend.

The inland projection of this trend will be the focus of Quaternary geology studies, and the offshore projection will be examined using high-resolution seismic reflection data.

The onshore network of 5-second seismic reflection lines recently shot by Seisdata Services, Inc., constitutes one of the principal elements of new data acquired for the current studies.

The location of the seisdata seismic' lines and their relationship to geologic features in the San Luis Obispo/Pismo/ Santa Maria Valley region are shown on Figure 34.

Studies now being initiated in the San Luis Obispo/Pismo/ Santa Maria Valley region relevant to the Long Term Seismic Program are organized among Tasks 4 and 5, as identified in the Phase II work plan (PGandE, 1986).

The significant considerations and subtasks are listed in Appendix A.

NEOTECTONIC AND QUATERNARY STUDIES IN THE PISMO BEACH AREA Neotectonic and Quaternary studies have been initiated in the I

Pismo Beach /Nipomo area as part of the Program's subtasks 2.1, 2.3, 4.2 and 5.2.

Field studies to date have included:

Geologic mapping of marine terraces in the Avila Beach /Pismo e

Beach area.

Bedrock reconnaissance mapping in the Pismo Beach /Nipomo I

Mesa / northern Santa Maria Basin area.

e s

Offshore submarine geologic mapping in the San Luis Bay area.

e Detailed logging of the Wilmar Avenue sea cliff exposure and e

Farm Boy Quarry exposure.

Dating studies on marine terrace deposits, including e

j invertebrate faunal assemblage identification, amino acid l

l racemization, and uranium series dating.

l These studies are in their initial stages and are currently in progress.

Additional field activities, including topographic I

profiling, additional field mapping, drilling, seismic refraction surveying, and trenching, are planned in the Santa Maria Basin, the Los Osos Valley, and the Montana De Oro to Point San Luis regions. --.

c-.

... 2 2.-

m m_

~

SAN LUIS OBISPO/PISMO/

I

SANTA MARIA VALLEY REGION !.-

SHOWING SEISDATA LINES

?

l X*-

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o.

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lg O,O N

Los Osos %'

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' Valley i

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/

Fault; dashed where approximately 13cated l'.

. l Quaternary deposits

//

Buried reverse / thrust favi:

Mid te Tertwy sedimentary l

N

^ "' "' " '"'*

l > < l "r":XJ ";':V!3e'.

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FIGURE 34

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L

[

(

The objectives of the neotectonic and Quaternary studies in the region are to:

Locate active fault traces and other potential seismogenic e

[

features Evaluate the lateral continuity and spatial relationship of e

(

observed faults Assess recency of fault displacement e

'e Assess late Pleistocene and Holocene rates of slip (vertical j

and lateral components)

(

Assess sense of slip and style of deformation e

The primary emphasis of the field program is to locate active

{

geologic structures (both known structures and previously unrecognized structures) and to characterize fault / fold behavior, including geometry, activity, slip rate, and lateral continuity.

The results of these studies will be incorporated into the

[.

Seismic Source Characterization Task (Task 9) of the Program.

The planned and initial areas of Quaternary investigation in the

(

Avila Beach /Pismo Beach region are shown in Figure 35.

Quaternary mapping will be conducted in the entire study area to assess fault activity along the hypothesized Santa Maria River fault trend, the San Miguelito fault, the Pismo fault, and the

[

Wilmar Avenue fault.

Initial mapping has been conducted along the coast between Avila Beach and Pismo Beach. Several fossil localities have been identified and samples have been collected.

[

Samples suitable for U-series or amino acid racemization analyses have been collected from localities shown on Figure 35, and submitted for analysis.

A preliminary map of marine terraces in the Avila Beach /Shell Beach area is shown on Figure 36.

A sequence of five marine terraces has been recognized at Mallagh Landing.

Preliminary

[

mapping of the terraces at Mallagh Landing indicates that the terraces are not displaced vertically by the San Miguelito fault.

These terraces, however, are veneered by a thick mantle of

(

colluvium and wind-blown sand to the southeast.

Drilling and seismic refraction studies are planned in this area to identify and map the marine terrace sequence.

Acquisition of property access rights has been initiated.

Correlation of the terrace

[

sequence to the southeast will provide important constraints on the activity and geometry of the Wilmar Avenue fault.

[

Wilmar Avenue Fault The wave-cut platform exposed in the sea cliff near the

{

southwestern end of Wilmar Avenue at Pismo Beach is dislocated by a zone of at least three fault strands (Figure 37).

The Tertiary bedrock units juxtaposed by the Wilmar Avenue fault zone were E F

i

[

[

MAP OF LOCATIONS OF

[.

INITIAL INVESTIG ATIONS IN THE PISMO BEACH STUDY AREA

[

4m

[

• %g,,O F AULT i

US AREA OF

(

-AviLA.

QUATERNARY

.m vertebrate b

"#~

.:1 INVESTIGATION b"

' S. U serN

• Amino acid.

fossil

[

2 POINT SAM LUIS SHELL SEACH.

U-eerles, amino acid Farmboy p[ +o Quarry Exposure PISMO BEACH,

?

WILMAR AVE. FAULT INITIAL AREA

[

OF DETAILED PACIFIC OCEAN MAPPING

[-

j:

NIPOMO

1:.

[

~ %,,

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p 4

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i P'

=

0 Skm

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[ FIGURE 35

~

m m

m m

m GEOLOGIC MAP OF MARINE TERRACES BETWEEN AVILA BEACH AND SHELL BEACH Cree j

Hwy.

Ob\\s 0. p.),D g 101

-~~

t.uts

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1

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Fault

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Beach

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EXPLANATION

/'N

[D \\ Landslide deposit Otf Fluvial terrace 0

1 mi

""' Drainage Q1 Marine terrace i

i I

"'~" Marine terrace shoreline angie (higher numbers older) n 0

1km 3

Ound Quaternary terrace

.c

/ Fault; dashed where approximately (undifferentiated)

• ,,,e located, dotted where concealed, queried where uncertain M

WCM Tilt of wave cut Terrace surface disturbed by platform

}

cultural activity i

4

I I

GENERALIZED SKETCH OF BEDROCK AND FAULT RELATIONSHIPS I

AT PISMO BEACH I

A,[g g-y 0

200 f t

'I

[

l I

4&

O 50 m I

Approx. Scale g

Tmot y

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k I

s N \\82 4".

Wilmar Ave.

EXPLANATION o

bg

)

Fault I

6 Tpps SEDIMENTARY AND VOLCANIC ROCKS 69 Squire Member of Pismo TPPJ Formation o,

g Tuff of Obispo Approximate Line I

Tmot Formation Aof Cross Section K (Figure 38)

Tmr Rincon Formation g

\\

INTRUSIVE VOLCANIC ROCK Tmod Diabase (Obispo Formation ?)

, TfD Strike and dip of beds 69 Fault; arrow shows direction of dip; dotted where concealed

//,)

Landsiide deposit I

FIGURE 37

k

(

originally mapped by Hall (1973), and recently were noted as potentially significant structures by Steve Nitchman of the

(

{.

1986).

University of Nevada, Reno (oral communication, August 15, A diagrammatic cross section of the Wilmar Avenue fault is shown

'in Figure 38.

A sketch log of the southern strand of the Wilmar Avenue fault zone shows an approximate 5-foot vertical separation of the marine abrasion platform (Figure 39).

Progressively smaller

(

displacements may be seen in the overlying terrace deposits, indicating multiple faulting events and suggesting deposition and faulting occurred contemporaneously.

Some of the decreasing

{

vertical separation seen upsection may be due to fault splaying.

The age of the most recent displacement is not known.

0 Slickensides on this fault plunge 56 degrees towards S25 W,

[

indicating essentially pure dip slip, parallel to bedding.

The northern strand of the Wilmar Avenue fault zone is characterized by a highly sheared gouge zone that is 7 to 8 feet

{

wide where exposed at the base of the sea cliff (Figure 40).

Rocks of the Rincon, Obispo, and Pismo formations are juxtaposed within and across this zone.

The northern fault branch appears to have produced a vertical separation of about 21 feet on the abrasion platform (Figure 38).

It is not yet known whether the marine platforms on opposite sides of this fault are of the Because the sea cliff along the trend of this fault is

(.

same age.

covered by both vegetation and colluvial debris, the effects of fault slip on the overlying terrace deposits are not known at present.

This fault does not appear to displace the geomorphic

{

terrace surface, even though slickensides indicate that displacement on this fault is dip slip.

(

The possible extensions of the Wilmar Avenue fault to the northwest and to the southeast are being investigated.

Field mapping and a detailed analysis of low-altitude oblique aerial photographs of the sea cliff between Pismo Beach and Avila Beach

{

indicate that this is the only sea-cliff exposure of the Wilmar Avenue fault.

The pattern of resistant submarine outcrops and sea stacks shown in Figure 41 suggests that the Wilmar Avenue

[-

fault may extend offshore in a westerly direction.

It is unlikely that the rocks of the Squire Member of the Pismo Formation, which lie south of the fault, are resistant enough to

(

form outcrops in the shallow offshore waters.

Thus, the southerly limit of the submarine rock exposures off Pismo Beach, as shown in Figure 41, likely marks the offshore trace of the Wilmar Avenue fault.

Identifying and tracing this fault

[

offshore will require the additional high-resolution seismic reflection data that are currently being collected.

A program of detailed mapping and analysis of stratigraphic sections along the

[

trend of the Wilmar Avenue fault to the southeast is also in progress and will soon be followed by drilling and trenching studies.

E -

a

m m

m m

m m

m m

m rm rm rm rm.

m rm rm r

r r

m DIAGRAMMATIC CROSS SECTION OF WILMAR AVENUE FAULT PISMO BEACH, CALIFORNIA l

l NE Location of-Location of i

j Figure 40 Figure 39 Wave-Cut Platform M

i i

Estimated Age -100,000 years Obispo (?) Diabase Terrace Upper Surface f Rincon Fm.

Y' T Eh I

p Terrace

-/

Sea (Beach)

S-

{ ?

Deposits

?

Level k

'

o n

a o

e o

o L

• 1.5' 4'

h\\. "' *' * * ~ - a

/, / /l/

6/, "

d 21' N

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.Q.\\..'.\\'...

~~'

' /

I'/ /

a, i g

Tuff of Obispo Fm.

Rincon Fm.

Squire Member of Pismo Formation (Late Pliocene) k (Early Miocene)

(Late Oligocene)

(Fold Interpretation from Hall,1973) 23 OC 2

0 100 f t m-1 I

O m

v

~

i SKETCH OF SOUTHERN STRAND OF WILMAR AVENUE FAULT

~

I N

S 5

Top of Step in Sea Cliff Exposure

~

l 5

//.I

.::::$[& s

~

, / -

/, / /

O p,,.s #

N g.:'.

N54W,51SW,

/'/

N w

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(

/

f__ >

N'

/

(/

/,

. Partially Covered

/

m'

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,/. -

/ / N, '.

W._ '(I)\\ / // /

/. /

0'

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/(

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wj Partially.

.i I

/-

O Covered

/

N u.-f

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k'

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,/ l '/.

t N

Marine Abrasion Platform N.

/,' '/)

/

N-;=.Q

/-

g m

~~

~ M- ~

~S

\\

Marine Abrasion Platform

\\

~

Tpps Squire Member of N72W,69SW Pismo Formation N

\\.

\\

N

\\

- ~. _

/

/

/

< Partially Covered /

/. / /

/

/

/

.m K

j

/~

i

/

/

o o

Modern Beach Sand 0

fft EXPLANATION 0

1m Corritative deposit Fmilt; dashed where approximately located.

pl/ cueried where uncertain, strows indicate

[/

apparent relative <! recison of displace-nent t

~

~

FIGURE 39

I ll SKETCH LOG OF NORTHERN STRAND OF WILMAR AVENUE FAULT N15E Tmr N88E,46N (bedding)

Unconsolidated surficial deposits, predominantly colluvium l

.f f. - l l \\

' NEO-89E, _

e 45-60N lj Tmr j l fault slice 7 N70-75W, ~ ~.

'/

Osd I

[

54N l

f l

l'/

/

Tmot(?)

Tpps

/m

}

N65W,52N N65W,'45N Sheared, weathered tuff:

r EXPLAtlATION neans !idated surficial Gsd

.&p) sits.

' Sqd.e Membar of Pismo Tpps Formation Tuff of Obis o

24 f 7 Fc,rmation i

0 1m Tmr Rincon Format:on Fault gnage (pervasively sheared clay)

Ftult l

FIGURE 40

I I

SUBMARINE GEOLOGIC MAPPING IN AVILA/PlSMO BEACH STUDY AREA I

s i

MIGUELITO

1. 4%

1 i

i 7

0 1km Tmo /

Q

'I Tmm 4 Avila Beach

'o, I

e

• e e

Point San Luis e

Tmm I

.Tmo e"Te;. -

O g

4.,_

1. A Pacific Ocean Q

Tpps Y

~'

WILMAR AVENUE FAULT I

EXPLANATION

- Fault Tpps Squire Member of Pismo Formation Monterey Formttion (Tmm) e

--- Inferred fault, Diabase member of Otnspo Formation (?)

approximate I

location

• Obispo Formation. (Tmo)

Contact Rincon Formatio.i (Tmr) o I

I FIGURE 41 7

[

(

PLANS FOR FUTURE WORK The San Luis Obispo/Pismo/ Santa Maria Valley region studies were

(

in an early stage at the time of the Workshop.

Most work had been concentrated in the Avila/Pismo Beach area.

The significant considerations that were recognized had been anticipated in the work program developed during Phase II (PGandE, 1986).

[

Clhrification of the Wilmar Avenue fault zone will serve to further focus the studies of structural relations within and

[

along the flanks of the San Luis/Pismo synform.

Onshore studies, including mapping, test pits, drilling, seismic refraction, and possibly trenching will be performed in the Wilmar Avenue /Pismo Beach area, and along the Nipomo Mesa, located along trend

[

farther to the southeast, as well as along the Edna and Los Osos faults across the fold to the east.

Understanding the relationships among these faults as structures associated with

(-

the San Luis/Pismo synform will be important in assessing their capability and earthquake generation potential.

Studies along the Little Pine /Foxen Canyon trend and in the Santa

[

Maria Valley region have so far consisted mainly of identification of the Oceano monocline as a subsurface structure coincident with the trace of the Santa Maria River fault, as

(

hypothesized by Hall (1978), and verification (R. Schweikert, UNR, oral communication, October 22, 1986) that the Little Pine fault is a reverse fault having defined end points, rather than a

{'

segment of a long strike-slip fault as proposed by Hall.

Data now available do not constrain the extent, structural relationships or capability of the Oceano monocline; however its geographic location and structural form indicate there is a need to establish these parameters.

To this end, a specific program of geophysical and geologic studies are planned for this feature.

The program includes tracing the feature in the subsurface by

{

means of geophysical techniques, then characterizing it by detailed surface mapping and, if required, drilling and trenching.

The trend of an offshore northwestward extension of

[

the Oceano monocline will be evaluated by aeromagnetic anomalies and by data from the scheduled high-resolution offshore survey.

(

In addition to the established importance of the Little Pine /Foxen Canyon / Santa Maria River trend, the trend of faults and folds along the southwest margin of the Santa Maria Valley is

{.

seen as having significance, at least to understanding the regional tectonic model. Detailed mapping and structural analysis are planned to be carried out along this fault trend.

Additional geophysical data to study the Casmalia fault segment of the trend include existing offshore seismic lines to be purchased from Nekton, Inc., and additional onshore lines by Seisdata (Figure 34).

Interpretation of these lines together with structural analyses

[

from well loqs will define a structural framework for mapping and detailed field studies.

4 SEISMICITY AND REGIONAL TECTONICS The locations, geologic associations, and source characteristics c

.I of past earthquake activity in the central coastal California region are a fundamental component of the earthquake hazard l,_

assessment for the Long Term Seismic Program.

Of particular L

importance is the regional earthquake activity that has been instrumentally recorded.

Analysis of this data set can provide quantitative earthquake characteristics to complement the geologic and geophysical investigations, leading to improved seismic source characterizations for the Program.

The initial

['

studies hate focused on three areas, which are discussed in this

(_

section:

crustal velocity structure, recent instrumental I

seismicity, and analysis of the 1927 Lompoc earthquake.

The results of these studies directly address primary objectives of the Program:

e Assess the relationship between observed earthquake activity I

and areas of recent geologic deformation (tectonic model).

e Identify the source characteristics of potential earthquakes using features of accurately recorded and well-studied recent earthquakes.

CRUSTAL VELOCITY STRUCTURE An accurate understanding of the crustal velocity structure in the region of interest is needed for four purposes:

e Accurate earthquake locations depend upon an accurate velocity model that extends into the upper mantle at depths in excess of 25 kilometers and that incorporates lateral velocity variations.

e In the complex geologic setting west of the San Andreas fault in central California, an accurate crustal velocity model that includes the evaluation of lateral velocity variations will provide constraints on possible patterns of tectonic deformation.

e Major structural changes, such as may be associated with the Hosgri fault zone, may be revealed by geophysical studies used to assess the velocity structure, thus allowing the imaging of I

such structures at depth within the crust.

e The crustal velocity structure and attenuation are necessary inputs to ground-motion modeling.

An important new geophysical interpretation was recently released in preprint form by Trehu and Wheeler (in press).

In their study, the crustal velocity along a line from Morro Bay to Cholame Valley was interpreted from a combined refraction and reflection

The location of the geophysical lines is shown in data set.

Figure 42.

This profile crosses the eastern part of the and traverses the granitic Salinian block Franciscan terrane, I

between the Nacimiento and San Andreas faults.

The Trehu-Wheeler interpretation is shown in Figure 43; reflection events are indicated by dotted lines, and traced rays are indicated for shotpoint 1.

Three prominent features of the crustal model are noted:

I The San Andreas fault is prominently seen as a narrow I

region of lower velocity rock e

The granitic Salinian block is distinguished from the l

e Franciscan rocks to the west by its higher velocity (6.0 kilometers per second versus 5.5 kilometers per second).

A low-velocity wedge is interpreted to lie beneath the I

Franciscan assemblage and to extend well to the east of the e

western edge of the granitic basement.

The Trehu-Wheeler crustal model shown in Figure 43 has significant implications to understanding earthquake potential in the Diablo Canyon site vicinity.

These implications suggest a series of questions:

What is the lateral and westward extent of the interpreted e

low-velocity zone?

What are the material properties of the low-velocity zone?

e Is it capable of accumulating and releasing elastic strain energy?

how would Depending on the extent of the low-velocity zone, e

determinations of earthquake locations and focal mechanisms I

be affected by the presence of the zone?

How was the zone created or emplaced in its present location?

e I

What are the tectonic implications of the location and nature of the zone?

To address these questions, two studies have been initiated.

In the first of these, additional geophysical data will be collected both onshore and offshore, as illustrated in Figure 44.

The planned geophysical survey is scheduled for late October and I

early November, and consists of marine reflection profiling along lines 1, 2,

and 3, seismic refraction profiling using onshore explosions recorded by offshore sonobuoys and ocean-bottom seismometers, and seismic refraction profiling using onshore recording with dense instrumental arrays of offshore large-volume airgun shooting.

This geophysical survey is expected to extend the Trehu-Wheeler model and to allow the development of answers I

to the questions posed above.

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Along with the planned geophysical survey, existing data sets may be analyzed to help address questions about the crustal velocity model.

Of particular value is a set of earthquake recordings made by the U. S.

Geological Survey in 1980-1981 using portable three-component analog recorders at numerous temporarily occupied sites along the central California coast.

Five of these stations are shown in Figure 45.

As an initial evaluation of the Trehu-Wheeler model, the USGS record set from the temporary recording period was examined for well-recorded teleseismic events that had subcrustal focal depths.

Teleseismic P-wave arrivals from such events would be sensitive to the strong velocity constrasts, and phase conversions and multiple arrivals should be generated.

The observations for one such event are shown in Figure 46.

This event occurred at a reported depth of 122 kilometers in the subduction zone beneath Western Argentina, and had a magnitude (mb) of 5.1.

To evaluate this data set, synthetic seismograms were computed for a teleseismic P-wave passing up through two crustal models, which are shown in Figures 47 and 48. These two models are the same except for layer 5:

in Figure 47, layer 5 is that in a typical layered crustal model with a 6.8 kilometer-per-second intermediate layer; Figure 48 has a low-velocity zone above the mantle velocity of 8.0 kilometers per second.

Synthetic seismograms representing these two models are shown at the bottom of the two figures.

The most prominent difference between the two synthetic records is seen in the phase that arrives about 7 seconds after the direct P-wave.

This secondary arrival is a P-wave that reflects off the free surface and then reflects again off the top of the mantle (PMP).

This arrival is much larger in amplitude for the model containing a low-velocity zone.

When the synthetics of Figures 47 and 48 are compared with the observations at coastal stations shown in Figure 46, several observations may be noted:

Large secondary arrivals may be seen at stations TTB, TCC, and e

TSS, with arrival times consistent with the low-velocity crustal model shown in Figure 48.

Earlier and later secondary arrivals may be seen at stations e

TPB and TPR, respectively, suggesting that large velocity contrasts may exist at the top of the mantle, but the velocity structure beneath these two stations may differ from that beneath the other three stations.

These results are not conclusive regarding the presence and characteristics of possible low-velocity regions in the lower I

crust, but they do suggest that such large velocity contrasts are possible and that, if present, they may have substantial lateral variations.

The planned geophysical survey described above is expected to provide the data necessary to resolve the crustal velocity model appropriate for the study area.

MAP SHOWING LOCATIONS OF FIVE PORTABLE USGS STATIONS

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[

i c

TPBB Pt. Piedras Blancas TSS A TCC

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A TPR M

DI ABLO CANYON

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EXPLANATION

^TCC station and station code l

FIGURE 45

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VERTICAL-COMPONENT RECORDS FROM DEEP FOCUS TELESEISMIC EVENT Event location:

San Juan, Argentina

[-

31.5 S.,

67.1 W.

25 Dec.

1980, 06:32

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L Velocity Model adapted from Trehu and Wheeler (1986)

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[

Number (km/sec)(km/sec) (gm/cc) Thickness

[

1 4.375 2.525 2.0 1.8 I

2 5.325 3.074 2.5 3.2 r

3 5.825 3.363 2.6 5.0 L

4 6.100 3.522 2.7 4.5 5

6.800 3.900 2.8 6.5 7

6 8.050 4.648 3.5 0.0

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VERTICAL COMPONENT SYNTHETIC SEISMOGR AM

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FIGURE 47

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CENTRAL CALIFORNIA COASTAL MODEL WITH LOW-VELOCITY ZONE Velocity Model adapted from Trehu and Wheeler (1986)

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-I 1

4.375 2.525 2.0 1.8 2

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.I

REGIONAL SEISMICITY The evaluation of earthquake activity in the central coastal California region is an important complement to geologic studies in characterizing regional earthquake potential.

During the initial seismological studies of the Program, emphasis was placed on using the recent seismicity data collected by the USGS seismic network operations in central and southern California.

In Figure 45 addition, available teleseismic recordings were studied to improve the understanding of the source characteristics of the 1927 Lompoc earthquake, the largest historical event in the region.

Figure 49 shows the geologic setting of coastal central California, taken from Figure 1, with the instrumental seismicity since January 1980 superposed.

The seismicity data were obtained from the USGS and CalTech network operations in central and southern California; duplicate events have been removed. No location quality criteria were used to filter the data set, so some relatively poorly located events may be included in the figure.

Features of interest seen in Figure 49 include:

J e

In the northeast corner of the figure, a lineation of microearthquakes is aligned with the surface trace of the San Andreas fault zone.

To the northeast of the San Andreas, a portion of the aftershock activity of the Coalinga earthquake is shown.

e In the central and northwestern areas of the figure, recent earthquake activity appears to be predominantly distributed along the general trends of the Nacimiento and San Simeon faults.

However, the seismicity does not form narrowly defined linear zones, but suggests that deformation is occurring in a zone perhaps several tens of kilometers wide along the structural strike of the regional faults.

In the region from about the Diablo Canyon site to the e

northwest, seismicity is generally bounded on the western side by the Hosgri fault zone.

In the area of the Piedras Blancas antiform, where the Hosgri zone broadens to the west in a series of faults and folds, the seismicity also extends westward.

The observed seismicity does not define any planar features that may serve to define a fault plane along the Hosgri trend.

e The interior of the Salinian block, which lies between the San Andreas and Nacimiento faults, is much less seismic than the crustal blocks to the west and east.

e The interior of the northern offshore Santa Maria Basin has very low levels of seismicity, consistent with the low degree of geologically recent deformation.

l 1

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USGS AND cal. TECH SEISMICITY DATA 1980 - 1985 121*

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FIGURE 49,

r L

e In the central and southern portions of the offshore Santa

{

Maria Basin, south of latitude 35 degrees north, recent seismic activity extends westward across the Santa Lucia Bank to the continental margin.

Seismicity appears to concentrate in the locales of offshore folds, such as the Lompoc and e

L oueenie structures, and broadly within the Santa Lucia structural block.

[

e In the onshore area of the Santa Maria Basin and along the western Transverse Ranges, seismic activity has a broadly scattered pattern.

Several small centers of recent activity F

are apparent, such as the group of events near the western end L

of the Casmalia fault trend.

More detailed assessments have begun of the seismicity recorded r

L in the past 6 years in the areas currently being investigated geologically, the San Simeon and San Luis/Pismo regions, as discussed in Sections 2 and 3 of this report.

Figures 50 and I

51 present seismicity data from the locality of the San Simeon L

geologic studies.

The USGS seismicity data set was selected to eliminate earthquakes since 1980 for which the vertical and r

horizontal standard errors in location exceed 5 and 20 L

kilometers, respectively.

In comparing Figure 50 with Figure 49, some of the events scattered to the west of the Hosgri Zone are no longer shown due to the poor quality of the locations.

The

~[

epicentral pattern shown in Figure 50 illustrates the observations made about Figure 49:

that the earthquake activity does not define clear lineations, and is bounded on the west by E

the margin of the Piedras Blancas antiform and on the east by the L

Nacimiento fault.

These patterns are similar to and typical of patterns observed in seismicity in southern California.

I The cross sections identified in Figure 50 are shown in Figure 51 L

for the San Simeon region.

Several important observations may be made:

e The seismicity in profile A-A', which is perpendicular to the strike of the regional structures, appears to be truncated at p

a depth of about 12 kilometers.

There is a slight suggestion L

of an increase in depth of the seismogenic zone increasing from west to east, but this may be an artifact produced by the focal depth scatter.

r l"

There is no clear vertical or dipping alignment of hypocenters e

beneath the surface traces of the Hosgri, San Simeon, or Nacimiento faults.

Microearthquake activity appears to be

{

occurring within volumes of crustal rock rather than on well-defined planar structures.

[

The largest event located during the recording period in the e

area of Figure 50 is seen in both cross sections of Figure 51.

This event had a magnitude of 5.4, occurred on August 29,

{

1983, and had an oblique reverse focal mechanism with a right-slip component.

The nodal planes dip at angles of 55 and 58 degrees, consistent with the occurrence of high-angle EL -

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PROFILES OF RECENT SEISMICITY IN THE SAN SIMEON REGION i

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l FIGURE 51 l

E reverse faulting in the area, but not suggestive of

{

shallow-angle faulting in the lower portion of the seismogenic zone.

p An epicentral plot and cross sections have also been prepared for L

the San Luis/Pismo area, as shown in Figures 52 and 53.

The same selection criteria were used for this region as for the San Simeon area.

In Figures 52 and 53, several features of the 1

r

[

seismicity pattern are noted:

Earthquake activity occurs in close proximity to the e

F Rinconada, Nacimiento, and San Andreas faults in the northern L

half of Figure 52.

In the cross sections, the seismicity appears to be aligned near-vertically, not suggestive of flattening of the dip of the faults with depth in the seismogenic zone.

Seismicity appears to be bounded on the west by the Hosgri e

[

fault zone and to underlie the region between the Hosgri zone and the West Huasna fault.

The magnitude 5.1 Point Sal earthquake of June 29, 1980, seen p

e in the lower portion of Figure 52, has a predominantly u

reverse-slip focal mechanism (Eaton, 1984).

The location and focal mechanism cf the event suggest that it may be associated

[

with the southwest-dipping Casmalia fault.

The depth of seismicity appears to be limited to about 12 e

E kilometers along the coast beneath the Hosgri Zone (Figure 53, L

C-C'),

and to extend to 16 kilometers or more along the San Andreas fault (Figure 53, A-A').

During the next several months, additional analysis will be carried cut using the seismicity data sets.

Primary crphasic will be placed on using updated crustal velocity models to revise

{

earthquake locations and focal mechanisms.

1927 LOMPOC EARTHQUAKE STUDIES I

The 1927 Lompoc earthquake has been the subject of many studies, u

but some important features of the event remain uncertain.

In an initial effort to reduce some of these uncertainties, analyses

[

have been carried out to use long-period body-wave recordings of the earthquake made at teleseismic distances to refine the estimation of the seismic moment, focal depth, and focal mechanism of the event.

If these parameters can be constrained,

{

then further analysis may be made of the location and tectonic association of the event.

[

During the past 5 or 6 years, major advances have been made in techniques for analyzing teleseismic earthquake recordings using the generation of synthetic seismograms.

Due to the

{

available body of analysis experience, teleseismic travel paths crossing North America are reasonably well understood, allowing confidence in the comparisan of detailed features of earthquakes W -

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RECENT SEISMICITY IN THE SAN LUIS/PISMO REGION J

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5 km vertically and 20 km hori-m

zontally, en" Data from USGS 1/1/80 -

7/23/86 inclusive.

I PROFILES OF RECENT SEISMICITY IN THE SAN LUIS/PISMO REGION g==

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o -20 w

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5 5

-30

,,,n...i...............n,i.........,.......ni...n....i.n....nin.......i..........

O 10 20 30 40 50 60 70 80 90 I

DISTANCE (KM)

I !

l L

IL recorded teleseismically from varied source areas.

This general method has been applied to the 1927 earthquake, using comparisons r

L with recent earthquakes in the Santa Lucia Bank and Coalinga areas.

r L

After inspecting several dozen teleseismic recordings of the 1927 earthquake, the station at De Bilt, Netherlands, was selected for the comparison, for the following reasons:

l The station has been in operation using high-quality, e

well-calibrated long-period instruments since 1922.

The r

instrument information is presented in Figure 54.

L The pass-band of the instrument is adequate for good e

teleseismic recording of long-period body waves.

c 1

The instrumentation has rema.ined in operation since it was e

installed, thus providing recordings of recent earthquakes on

[

the same instruments that recorded the 1927 event.

L The events selected for initial comparison, for which data have been obtained from the De Bilt station, are the 1969 magnitude c

L 5.6 Santa Lucia Bank earthquake and the 1983 magnitude 6.7 Coalinga earthquake.

These records are shown in Figure 55.

The P to S amplitude relations for these three events suggest I

that all three have similar focal mechanisms.

The large P amplitude relative to S is consistent with the reverse-slip mechanisms previously obtained for the 1969 and 1983 events.

I Forward computations were performed using the published source L

models for the Coalinga and Santa Lucia Bank events and a well-studied coalinga aftershock to generate synthetic

-[

seismograms for comparison with the De Bilt recordings.

As seen in Figure 56, these comparisons are very good.

To generate synthetic records for the Lompoc earthquake, advantage was taken I

of the observed similarity in seismograms noted in Figure 55, and L

the Coalinga event was selected to begin to model the Lompoc earthquake.

During the initial modeling process, judgments were made to alter some parameters to improve the fit to the data, as r

L seen in the bottom panel of Figure 56.

These alterations were to rotate the strike of the mechanism to 340 degrees from the 305-degree strike determined by Eaton (1985) for the Coalinga

}

event, to extend the source-time function to 2-2-2 to account for the larger size of the Lompoc earthquake, and to increase the seismic moment by a factor of about two to match the observed E

P-wave amplitude.

As can be seen, the agreement between the L

Coalinga-based synthetic record and the observed Lompoc recording is excellent.

The relative timing of P, pP, and sP strongly constrain the Lompoc focal depth to be very similar to that of

[-

the Coalinga earthquake, about 10 kilometers.

Using this source model for the Lompoc earthquake P-waves, its

{

adequacy was evaluated using the De Bilt S-wave recordings.

Figure 57 shows the Lompoc and Coalinga S-wave recordings at De Bilt, with the data digitized and components rotated to radial -

E SEISMOGRAPH PARAMETERS OF THE

[

DE BILT, NETHERLANDS, STATION F

I Sta. Instrument Comp. Ts Tg Vmax at T Recording Drumspeed Since code (s)

(s)

(s) type (sun / min)

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ =

r L

DBN Galitzin I

12 12 740 7

optical 30 1922 Galitzin NE 25 25 310 14 optical 30 1914 Press-Ewing Z N,E 20 90 1000 20 optical 15 1966 L

10'

[ MAcwscaraos-i l

p"*

fa

. so a.

W

[

8 10 I OlW MSS-O'rNG z/wt f l

2 Des cautzw z h

3 DON cAUT2W wt lil WIT e r

L

• < u 5. -

lill gs oa *

w w

[

T

g WTS

'j l' /

/'

M N

6 eg..i g'. //

h

\\

llill ggs

,a

/

1

\\l s

..ll /

\\ l' 1

'.R.Carmony 3

Y Y

~

F 10' ge ggi, MfE 16n PDuoO (.54 o.

i.

y

1. 3 1

90' 10' ic' E

[

E FIGURE 54

-91_

ll>

l I

ll

/f, l

1

=

~

7

~

w:%_

E

=

K A

s

=

U

^ s Q

S E==

H K

E

=

o T

A K

R A

U A

U Q=

E o

Q H

h' H

TR~

K T

A ~-

N R

o S

M A

A E

=

DA ER B

E VG A =

o RO G

EM A

C N~

S S O

BI I

O E S

C F

P I

o T

L O

U M

+-

N SE L

A N N O

O OO L

S P o

IRM A

6-C AO T

7 3

i

• eI

'PC s=

i ME N

2 OE 8

CR A

9 9

i H

S 1

1 4-T I

I c

=

e c

e 9

e e

=

w s

6 s

s

=

0 9

0

=

1 0

2 2

i 2

1 1

=

I 1

w'I &w, 1

I o

(

w,

.' l iil,

'h m

=

m a

N zgE z

N E

z g [

,@xm $

,7 sus lllllIllIl

\\!

I~

OBSERVED AND SYNTHETIC P-WAVE VERTICAL SEISMOGR AMS l

83/07/22 COALI NG A AFTERSHOCK 25 Mo

.5 x 10 ergs 8ts ; I,

.5, I sec SANTA LUCIA BANK l

26 Mo =.15 x 10 ergs I

h = 8 km 8t s;I,!,I sec COALING A MAINSHOCK l

3 26 Mo =.4 5 x 10 ergs l

h=

10 km 8t s; I,3,Isec

(

E LO M POC fl I

30 sec E

I 3

26 1.0 x 10 ergs M

=

o h = 10 km l

8ts ; 2,2,2 sec A

Y I

"oua' 88

-e2-I

g.

ROTATED (RADIAL AND TANGENTIAL)

OBSERVED S-WAVE SEISMOGR AMS 1927 LOMPOC EARTHQUAKE g

74gogg713t MAX. AMP. (IN) - O. 2931 k,M1N% W kff W l

I RADIAL MAX. AMP. (IN) = 0. 4687 f

0 I

1983 COALING A EARTHQUAKE l-73nceg714t MAX. AMP. (IN) - O.1350 1

M,h-*c4FerVtww+^1W

~~

I RADIAL I

MAX. AMP. (IN) - O. 3019 b

g I

FIGURE 57 -

I OBSERVED AND SYNTHETIC S-WAVE SEISMOGRAMS I

l COALI NG A LOMPOC

.13

.29 l

Tangential (S H) l 340*

Strike = 30 5

• Strike

=

Dip = 66

• Dip = 66*

l Slip = 90*

Slip = 90*

.30

.62 l

Radial (SV) l l

UOsec l i

l Mo

=.4 5 E 2 6 Mo = l. E 2 6 I

lI I

l FIGURE 58 i

.a l"

and transverse.

Again, the visual comparisons of S, SS, and SSS

{

amplitudes suggest the similarity in focal mechanisms.

The small

~

SH amplitude relative to SV suggests the dominance of reverse slip.

Figure 58 shows the comparison between observed recordings of SV and SH and the synthetics calculated using the source r

L models developed froniP-waves.

The closeness of fit for the Lompoc comparison confirms the selected model.

If the mechanism of the Lompoc earthquake is purely dip slip, then the strike is 1

[

constrained within 5 degrees.

Allowing some component of strike '

slip would allow a possible rotation of the strike to be more nearly north-south.

EL The source model for the 1927 Lompoc earthquake developed from the above comparison with the Coalinga and other recent earthquakes has the following parameters:

7 e

Focal depth of 10 kilometers.

e Seismic moment 1026, giving a moment magnitude of 6.~6 0

Focal mechanism purely dip slip on a plane striking N20 W e

0 and dipping 66 NE L

These results are not yet fully confirmed by comparisons with other teleseismic recordings, but serve as a very useful starting I

point for further analyses.

Specific studiesito be carried out include additional teleseismic modeling, analysis of relative travel times for the 1927 and modern events to help constrain the

~

event location, and review of other data for the 1927 earthquake, including intensity patterns and tsunami records, for consistency with the revised source and location results.

TECTONIC FRAMEWORK

~

The development of a tectonic framework during the course of the Long Term Seismic Program is a fundamental prccess of synthesizing data relevent to earthquake generation processes in the coastal region of central California.

Although many tectonic 7

models have been proposed based on many different 'typcis of data, no unique synthesis has been developed that fully addresses the nature of present-day tectonic activity as that activity is related to seismogenic potential.

At the present time in the L

study, the necessary data sets are being collected, verified, and integrated.

These data include patterns and rates of late Quaternary geologic deformation, geodetic data, and information l

derived from regional earthquake studies, lI Two important regional earthquake data sets are the patterns of compressional axes and fault slip' vectors derived from focal mechanisms.

The available data obtained primarily from the literature (Eaton, 1984; Dehlinger and, Bolt, in press) are shown in Figures 59 and 60, plotted on the base map shown in Figure 1.

Figure 59 shows the horizontal projection of the maximum I

.I

.W

.,.n.

,i

.I

'/ :

• EARTHQUAKE P-AXIS ORIENTATIONS s

w'h.ht:}

h SUPERIMPOSED ON THE 1

3p, ;)f, I GEOLOGIC AND TECTONIC SETTING, as i p g.'

COASTAL CENTR AL CALIFORNI A

~

1

,.4 l

? *. i

},

r PPJ w

Point S}ur-(.b Antifor

'C.

(

S R - w',gf,','t,{ p $O, NTg.l.,

O.

r d,ob. '.

o i

os g

M. g v.,

s r,'f/p, %?l_, +, * } {

i hh 0,4 t

I

.g' i

'fk,-

l

[

A'

• c,~&

e g.

i

>,I. t 4.

f3 n

+

l

4 y

PieBras

%~

a% a o

,h*if'.3h \\h,

t B/ancas s

t Antiform ~

"I '. **U'\\. %

IQ. N T

T e

Nf.,

o_.,

.i o

o s

.t n%

%' t. '%'L'j!)b,y %*#'. J ".1

% 'l.

g o,

s

.. ~.h,:

u N. u.

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t YS.

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} %.I.A, g[.'.[ ' j(l.4:.,h

.*s, ?

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h

1. CA 4

s y n.

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+.

g t%s g

j

~ b o h ",j

' '* +'

Q o-Diablo '

a l

'g f ~.

Canyon

.p

• w*

EXPLANATION %

I

? \\,h,-

J; j

C w

' Q:

, 'f ig.:.

g

-p,,

q

-1 P. Axis T

Oricattation g,' 3i g

2.

"E-t pJ.,. p -

g.,.

I

-m ;

g

-n p

-s <

g Santa

y,. y-4'- nc 1 q.3 :s (t.,

/

M<5 e

o u

- js'44.i yg

\\.*.

1. ,D C

Maria ~ 5.

Mf;,

.I i tp s

\\

N

og'#g

.\\.

g 3

'y @&i I*

Q, % g 4..

x

.y, \\

o S

M25 5

.h.

c

- g'

?: '

1 Fault: dashed where 9

b.

.P approximately located; d'

• ~

4 _.

h

' N r (=ff~/:

arrows indicate appar.

ent relative direction of f t

===. % -

of displacement.

. ;Westerni.

.Ev - ? Ynez F.

~

• *

• Reverse / thrust fault g ers e.

=

g m.

" Buried reverse / thrust t

0 20 km Geologic Units in the Coastal and Offshore Region R Ouaternary Mid to late 1 ertiary FIGURE 59

" Mesozoic to early Tertiary f

n.e...

lj FAULT PLANE STRIKE AND SLIP T '6 VECTOR ORIENTATIONS

=

. :'t

'l 9

tl.

, t.

gy k91,

So

- I

?

t, N

Point Sur%

t Antiform.Q '$.

-',r,

.h p -.-

k,E.

te,,[h.d.!'".[h.9

,g, [r

\\

n.

• e, ".

.l,',

re (0

1. 4.E h

/

t, it 0,4 50,

'i 3

Y

.i

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q;. % p.

,-4 1

I c.

j g

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Blancas t

I

~~'._ %.&*.

h Antiform

. \\s o

=**

s a

s

%st.'y\\j, I

N 4

> h..

Q.'. h.

ye g

o%

-g

. (V. A(.?.

G

4

.. 'q.g: *Y't}.tkig i

t.

S%

f%

. Qg :~h,,7,

%,, r s

01

&.n2 9

.t

.s 5,

wg?

g

,nyo,,n y

e c

I EXPLANATION $

hf

^

,\\

Strike direction h

h y,['#

.~

]

ervi shp vector

} }

o$

y yb

~

-r,

\\ M..

h,*

E*,

M<5 I. khh.. Q k

Np,}\\

L nta g

( 3't%g

?;

4 Maria ' T..

-o'),g,

y

^.Q ,g(

T Ik %

i3. #4

}.

~ 'y, s.., %

M 2:5 lley c g

L ' y%y-qs f'

\\

y; ;.R,

'M-Q; f~

?

3, y,:. x L;:

Fault; dashed where b-5 Y # I V'

+

l

[

approximately located; \\;?

M T3; n ( th arrows indicate appar.

9 ent relative direction of,,

g.j

.y7, y

g f.,,,,l,,g-

g I

of displacement.

• % a.u

-~.ne,ste.rnd.

d Ynet'F.

. !*tTransverse r\\

^ ^ Reverse / thrust fault

^

" Buried reverse / thrust f

f "I y%

[

E Geologic Units in the 0

20 km Coastal and Offshore Region I 7 Quaternary Mid to late Tertiary Mesozoic to early Tertiary FIGURE 60

compressive stress direction (lower hemisphere). Larger bars These represent earthquakes having magnitudes of 5 or greater.

data lead to several observations relevant to the regional tectonic framework:

P-axes along the San Andreas fault are oriented nearly e

north-south, which is consistent with the observed regional stress regime observed farther north along the San Andreas To the west of the San Andreas, between Monterey Bay and the e

I Transverse Ranges, the P-axes are generally rotated 10 to 20 degrees to the east of north.

There is some suggestion that the P-axes may rotate farther to the northeast near the latitude of the northern Transverse Ranges.

One implication of this rotation is that active faults in the region that strike parallel to the San Andreas fault zone will I

tend to have a reverse-oblique component.

Faults having a strike perpendicular to the P-axes will tend to respond to the regional stress regime by dominantly reverse slip.

These implications appear to be consistent with the fault strike and slip directions observed in focal mechanisms within the region.

These are shown in Figure 60 as strike lines and lower-hemisphere slip vectors

,E inside circles centered at the earthquake epicenter.

The

-E-earthquakes of magnitude 5 and larger are shown as larger circles.

As seen in the figure, events along the Nacimiento and San Simeon faults have reverse-oblique slip, with a reduction in

,g the reverse component near Monterey Bay.

Events in the southern

'g portion of the figure have dominantly reverse faulting mechanisms.

The preliminary synthesis of geologic deformation in the study region ic shown in Figure 61.

The deformation rates are classified in three categories and are based upon known fault I

slip rates when available, inferred fault slip rates, or upon rates of folding estimated from fold amplitudes expressed in Quaternary-age rocks.

The San Andreas fault is the only I

structure that appears to be deforming at a rate substantially faster than about 10 millimeters per year.

In the range of 1 to 10 millimeters per year, the San Simeon and northern and central portions of the Hosgri fault zones along the coastal margin and a I'

portion of the Rinconada fault appear to have this deformation rate. Other structures in the region appear to have deformation rates less than about 1 millimeter per year.

these deformation rates may be slightly higher or In some cases, the lower than the selected classification boundaries; however, overall pattern appears to be coherent.

It is noteworthy that along the structural trend of the region from Monterey Bay to the the total amount of deformation accommodated Transverse Ranges, between the relatively undeformed offshore basins and the San I

Andreas fault zone is roughly constant.

In the north, most of in the deformation occurs on faults in the Santa Lucia Mountains; the southern portion of the region, the deformation is

,I distributed over a broader zone.

1 l

73-LATE QUATERNARY DEFORidlATION RATES

, '; 1 g]1..

WITHIN COASTAL CENTRAL CALIFORNIA s-

,x 2 i, S..

. y 7,

-=--

s 5 '1

~~ ~

'h. 9 pt 't.

• 3:, if..

~ R~ 4.o c:'

l t4

,f p

l 4

t

~~~

Point Sur

• n'.!

.." tf}:l5b. G

~~d Antiform d

y l

C "~ 'y

h1

-1 7.w, o'4*,

y.*,\\,7Q..

s s*

~

. e..h :,..

\\

a-

. q...'

&e,,,e,,h., *,6["..]l-(y,4,

%i

\\

_.7 _

l

~#

Q,.

~

--...v,

%,b m c&c,*.

E.'.. -

.o.:.

. :7 3, t J.-

3

.d,,

e-

% =ds,: <v.

o i:,

. ',' n%:

O.,

. - 1.t,.

T ?iN.qi;,-

r x C'ig.syQ "h;p'j,. I;.by.s;%

j.

. -a.

i

-2 Piedras R' $

@' E- %. ' $ qA.%)> Q

',m'., d.a.s 4

o Blancas A -...

e#c,*':..M@.

o

~=--

l Antiform %

e

~_

c

~;, g..

.....: n, -

ns.

-;e m

o y, x % \\.

%3 c..

~

.-).i; _4.. tjg.p?',

__:)~o Rt, t,:

'n

. $',, ..W %; +,i. ge. i

. 2, w..., 4,.

o

'. m.

q

g.w.., a, ;. y\\ s.

r...

-s-

~. -y y

,=

g p'<

3

..' a -

c

. '., h;y

'~

O

1. +4 s

v

.i,.A.

...s \\

,f.

s v4 %..

p o

0;,gl, n

e f-

- (ft 7~

Canyon

0..,.

o EXPLANATION m

~@

W 4

o.

u g

T.

I Deformation Rates W.

G i

?-

G-4~

C :, -

g-4 g'

g, Santa cu

< 1 mm/yr 9., k h.

t4 Q Maria

-d -

'w.,

,o Ms-N, %gg#

e N

^

1 10 mm/yr

'.S.-. i Y.

\\

6 \\/ alley o,

3 F

\\

c r

( \\

h,N'},\\l 10+ mm/yr m

F'

).. '

\\

Fault; dashed where approximately located,

\\

arrows indicate appar.

\\

l*

-g'['da ' JF.

f

.,. M;-

ent relative direction of /

of displacement.

,W[es, tern;

• iTransversen S "f gyy *g,

^ - - Reverse / thrust fault O

20 mi

• J

,. jg?

s C

^ ^== Buried reverse / thrust c.

i I

[

s I

Geologic Units in the 0

20 km "y h e%

Coastal and Offshore Region 1

1 Quaternary U Mid to late Tertiary

-@ Mesozoic to early Tertiary g

g

As more information is accumulated during the field studies and data analyses that constitute the Geology / Seismology / Geophysics tasks, the data compilations shown in Figures 59 through 61 will be updated.

Of key significance will be the incorporation in this kinematic model of the crustal structural information to be derived from the field studies of early November.

The synthesis of a tectonic framework continues to be a significant and evolutionary product of the Long Term Seismic Program.

I I

l

-101-

b;

[-

5 IMPLICATIONS FOR SEISMIC SOURCE CHARACTERIZATION

[

Analyses done in support of the Phase II study (PGandE, 1986) provided a full characterization of the potential seismic sources in the Diablo Canyon region.

The parameters and associated h

-uncer a nt i ties were based on PGandE's best assessments of the current ranges of opinion of the scientific community.

Uncertainties were formally incorporated through the use of logic trees.

These Phase II studies, associated results, and extensive

[-

sensitivity analyses served to focus the Phase III scope of work on the considerations significant to evaluating ground motions at the site.

Because of the substantial effort that went into the

(

Phase II source characterization, it provides an appropriate

" baseline" for work in Phase III.

{

A major goal of the current geology / seismology / geophysics studies is to provide specific information to update the characterization of seismic sources in the Diablo Canyon region for deterministic r

and probabilistic ground motion assessments.

The integration of L

data and analyses will occur as part of Task 9, Seismic Source Characterization (Appendix A).

The-geologic data-gathering aspects of the Long Term Seismic Program will continue through about mid-1987, to allow for the incorporation of these data into the later parts of the Program (ground motions, soil / structure interaction, seismic hazard analysis, fragility analysis, and probabilistic risk assessment).

However, rather than wait for the completion of the geology /

seismology / geophysics tasks, these later analyses are proceeding with preliminary inputs that are periodically updated as data are developed.

The present schedule calls for appropriate updates to the characterization in November 1986 and July 1987, based on

{

the ongoing field program.

The sequence and priority of the tasks are based on the relative p

importance of technical considerations determined during Phase II.

L For example, the Phase II study clearly showed that the Hosgri fault zone is likely to be the dominant contributor to both deterministic and probabilistic ground motion estimates, and that

[

particular aspects of its behavior are most important (for example, downdip geometry, sense of slip, and slip rate).

As a result, these characteristics have been the focus of the program thus far.

Because of this focused approach, the recently

{

acquired data can be considered to update the Phase II source characterization the Hosgri fault zone.

The philosophy adopted in this updating will be to not narrow the range of uncertainty

[.

given in the Phase II study unless the new data provide a reasonable basis to do so.

For these interim updates, if no new data have been gathered or come to light regarding a particular

{-

seismic source, then the Phase II source characterization will be used.

-102-1

[

The onshore and near-shore geologic structures in the San Simeon

[

area have proved to be valuable in constraining critical aspects of the behavior of the Hosgri fault zone, including its sense of slip, near-surface geometry, slip rate, earthquake recurrence r

intervals, and amount of displacement per event.

Initial results L

of the offshore seismic reflection data have served to clarify the lateral continuity, segmentation, structural relationships, and history of development of the Hosgri zone. Further analysis

{

of seismic records ranging from high resolution data to reprocessed 2-to 5-second records promises to further define the downdip geometry of the fault.

At present, no " surprises" or significant findings have come to light that were not effectively included for the source characterization in the Phase II study.

For example, the

(

emerging characteristics of the Hosgri fault zone all fall within the ranges considered in the Phase II study, as illustrated in Table 5.

Shown are the ranges of Hosgri fault zone parameters

{

given in the Phase II report (PGandE, 1986, p. 2-29) and the current results of the geologic investigations in the San Simeon area.

Uncertainty still exists as to the exact structural p

relationship of the San Simeon fault zone to the Hosgri fault L

zone, and the nature of the fault zone at crustal levels deeper than those observed in the geologic studies.

(Both are the subject of ongoing studies.)

However, it is clear that the

{

studies to date are providing data on particular, critical aspects of the Hosgri fault zone.

In turn, the studies are reducing the uncertainty in these aspects, resulting in higher levels of confidence in the evaluations.

TABLE 5 E

COMPARISON OF PHASE II RANGES IN VALUES FOR THE HOSGRI FAULT ZONE

{

AND PRELIMINARY PHASE III FINDINGS Maximum Maxinrn

~

Prob. of Fault Downdip Total Rupture Displac.

Historical Capability Dip Width Length Length Per Event Earthquake Slip Rate Phase Sense of $110

(%)

M (km)

(km)

(km)

(m)

(Maanitude)

(m/vr)

Il Right/ Thrust 0.99 15 90 9 58 150 400 10 200 0.5-2 5.5 7.3 0.4 23 III Right 1.0 70 90 12-15 1.2 1.9 5.8 2.4 The results of these studies and the other studies discussed previously are providing data for subsequent updates of the source characterizations for ground motion estimates.

In sum, the Geology / Seismology / Geophysics program to date has proved to I

be well-equipped to focus on technical issues significant to the Diablo Canyon site, and it is flexible enough to accommodate any changes that may be needed in the event there are unexpected findings.

-103-

6 REFERENCES

Bull, W.

B, (in press), Correlation of flights of global marine terraces; in Morisawa, M.,

and Hack, J.,

editors, Tectonic Geomorphology:

Proceedings of the 15th Annual Geomorphology Symposium, State University of New York at Binghamton, George Allen and Unwin, Boston.

Crouch, J.

K.,

Bachman, S.

B.,

and Shay, J.

T.,

1984, Post-miocene compressional tectonics along the central California margin; in Crouch, J.

K.,

and Bachman, S.

B.,

editors, Tectonics and Sedimentation along the California Margin:

Society of Economic Paleontologists and Mineralogists, Pacific Section, v. 38, p.

37-54.

Dehlinger, P.,

and Bolt, B.

A.,

(in press), Earthquakes and associated tectonics in a part of coastal central California, Bulletin of the Seismological Society of America.

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.

Eaton, J.

P., 1985, Regional seismic background of the May 2, 1983 Coalinga earthquake:

U.

S. Geological Survey Open-File I

Report 85-44.

Envicom Corp., 1977, Fault investigation section, Hearst Ranch environmental data base:

unpublished consulting report prepared for the Hearst Corporation, Sherman Oaks, California, 30 p.,

1 plate.

Hall, C.

A., 1973, Geologic map of the Morro Bay South and Port San Luis quadrangles, San Luis Obispo Caunty, California:

U. S. Geological Survey Miscellaneous Field Studies Map MF-511, scale 1:24,000.

Hall, C.

A., 1976, Geologic map of the San Simeon-Piedras Blancas region, San Luis Obispo County, California:

U. S.

Geological Survey Miscellaneous Field Studies Map MF-784, scale 1:24,000.

Hall, C.

A., 1978, Origin and development of the Lompoc-Santa Maria pull-apart basin and its relation to the San Simeon-Hosgri strike-slip fault, western California:

California Division of Mines and Geology, Special Report I

137

p. 25-31.

Hall, C.

A.,

1984, 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, scale 1: 62,500.

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[

Hamilton, D.

H.,

1984, The tectonic boundary of coastal central California, unpublished Ph.D. Thesis, Stanford University,

[

290 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, no. B14, p.

12,503-12,522.

{

Manson, M.

W.,

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.

Minster, J.

B.,

and Jordan, T.

H.,

1978, Present-day plate

[

motions:

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p. 5331-5354.

Pacific Gas and Electric Company, 1975, Appendix 2.5E to FSAR

[-

for Diablo Canyon Nuclear Power Plant:

AEC Docket nos.

50-275 and 50-323.

{

Pacific Gas and Electric Company, 1986, Long term seismic program, development of scope of work for Phase III (results of Phase II scoping study):

Diablo Canyon Power Plant, Docket nos. 50-275 and 50-323.

Shackleton, N.

J.,

and Opdyke, N.

D.,

1973, Oxygen isotope and paleomagnetic stratigraphy of equatorial Pacific core,

(

V28-238; oxygen isotope temperatures and ice volumes on a 6

105 year and 10 year scale: Quaternary Research, v.

3.,

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Sieh, K.,

and Jahns, R.

H.,

1984, Holocene activity of the San Andreas fault at Wallace Creek, California; Geological Society of America Bulletin, v. 95, p. 883-896.

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M.,

and Wheeler, W.

H.,

(in press), Possible evidence for subducted sedimentary materials beneath central

{

California:

Geology.

Weber, G.

E.,

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[

terraces of the San Simeon region and Pleistocene activity L

on the San Simeon fault zone, San Luis Obispo County, California:

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S. Geological Survey, Final Technical Report, Contract 14-08-0001-18230, 66 p.,

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E

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APPENDIX A WORK PLAN FOR GEOLOGY / SEISMOLOGY / GEOPHYSICS The work plan for the geology / seismology / geophysics element of the Long Term Seismic Program scope of work was described in

{

detail in the Phase II Scoping Study report (PGandE, 1986).

The work plan was developed from a detailed identification and analysis of the technical considerations that are relevant and f

significant with respect to the Diablo Canyon operating license L

condition.

A comprehensive specification of tasks was developed to form an integrated work plan that addresses the significant considerations.

For each of the~nine tasks comprising the

(

geology / seismology / geophysics work plan, the following outline identifies the significant conditions and lists the subtasks that organize the planned investigations and work activities.

TASK 1 -- CHARACTERIZATION OF THE GEOMETRY AND BEHAVIOR OF THE HOSGRI FAULT Sianificant considerations

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Fault behavioral characteristics sense of slip dip and downdip width I

total length L

segmentation slip rate Neogene structural evolution

(

Relat3onship to other structures San Gregorio fault San Simeon fault Santa Maria Basin folds and faults

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Western Transverse Ranges folds and faults regional decollement Scone of Work Subtask 1.1 Review of geology and geophysics I

Subtask 1.2 Review of geodetic data Subtask 1.3 Geologic studies of onshore portion of Hosgri trend l

Subtask 1.4 Analysis of geophysical data Subtask 1.5 Development of fault map and areawide structural contour / isopach maps l

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TASK 2 -- NEOTECTONIC AND OUATERNARY STUDIES

[.-

Sionificant Considerations Distribution of Quaternary geomorphic surfaces and (J

associated deposits Dating'of surfaces and deposits Chronosequences for regional and local correlation Characteristics of neotectonic deformation

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Timing and rates of Quaternary deformation Scone of Work Subtask 2.1 Mapping of geomorphic surfaces Subtask 2.2 Analysis of seismogenic potential of h-active folds Subtask 2.3 Interpretation of neotectonic deformation TASK 3 -- SEISMOLOGY STUDIES Sionificant Considerations Update catalog of regional seismicity and source parameters

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Recent crustal velocity models for earthquake relocations Association of earthquakes with faults and regional tectonics rL Location and source characteristics of the 1927 Lompoc earthquake

(

Scone of Work Subtask 3.1 Seismicity analysis

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Subtask 3.2 Refine crustal velocity models Subtask 3.3 1927 earthquake analysis TASK 4 -- STUDIES OF EDNA AND SAN MIGUELITO FAULTS AND SAN LUIS/PISMO FOLD TREND

{

Sianificant Considerations Edna and San Miguelito faults r

capability L

sense of slip total length slip rate

(

San Luis/Pismo fold trend Neogene structural evolution Quaternary deformation Unknown relevant nearby faults and folds E-existence and capability physical characteristics F

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Scone of Work Subtask 4.1 Review and analysis of available geologic I

data l

Subtask 4.2 Geologic field studies Subtask 4.3 Offshore geophysics review TASK 5 -- STUDIES OF LITTLE PINE /FOXEN CANYON FAULT TREND AND ONSHORE SANTA MARIA BASIN Sionificant Considerations Little Pine /Foxen Canyon fault trend capability total length sense of s1_ip Santa Maria Valley / Santa Ynez Valley neotectonic evolution relation to Western Transverse Ranges I

relation to offshore Santa Maria Basin Scope of Work Subtask 5.1 Review and analysis of available geologic and geophysical data Subtask 5.2 Geologic field studies TASK 6 -- STUDIES OF WEST HUASNA, RINCONADA, AND NACIMIENTO FAULTS Sionificant Considerations West Huasna, Rinconada, and Nacimiento faults sense of slip late Quaternary slip rate Scone of Work Subtask 6.1 Review of available geologic data Subtask 6.2 Geologic field studies Subtask 6.3 Analysis of seismic reflection and refraction data I

TASK 7 -- DEEP CRUSTAL STUDIES Sionificant Considerations Neogene plate tectonic evolution Comparison with analogous tectonic settings Construction of crustal model Existence and capability of regional decollement

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I Scope of Work Subtask 7.1 Review deep crustal models for continental transform boundaries Subtask 7.2 Review deep crustal data for Santa Maria Basin Subtask 7.3 Synthesize deep crustal model for Santa Maria Basin region TASK 8 -- DEVELOPMENT OF REGIONAL TECTONIC MODEL Sionificant Considerations Interplate strain distribution Alternate tectonic hypotheses Implications for seismic sources Scone of Work Subtask 8.1 Synthesis of data Subtask 8.2 Development and implications of model TASK 9 -- SEISMIC SOURCE CHARACTERIZATION Sianificant Considerations Source characteristics for each source three-dimensional source geometry maximum magnitude earthquake recurrence interval Evaluation of parameter uncertainties Scope of Work Subtask 9.1 Specification of seismic sources Subtask 9.2 Maximum earthquake magnitude assessment Subtask 9.3 Earthquake recurrence assessment I

I I

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