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. 1 BEFORE THE UNITED STATES ATOMIC ENERGY COMMISSION In the Matter of PACIFIC GAS Docket No. 50-205 +
AND ELECTRIC COMPANY Amendment No. 5 Now comes PACIFIC GAS AND ELECTRIC COMPANY (the Company) and submits herewith Amendment No. 5 to its above-numbered application. This amendment consists of the follow-ing reports:
'l 1. " Geologic and Seismic Investigation of the Site for a Nuclear Electric Power Plant on Bodega Head, California" by Don Tocher and E. C. Mar 11 ave, dated January, 1964.
- 2. " Movements and Seismic Destructiveness Associated with the San Andreas Fault" by Hugo Benioff, dated January 21, 1964.
- 3. " Report on the Geology of the Pacific Gas and Electric Company Bodega Head Power Plant Site Excavation During I the Period May 26, 1963 to July 17, 1963" by william Quaide.
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- 4. "A Re-Examination of the Geology of the Southwest l
l Part of Bodega Head" by Don Tocher and William Quaide, dated l
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July 7, 1962.
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Subscribed in San Francisco, California, this 22nd !
day of January, 1964. '
- Respectfully submitted, PACIFIC AND ELECTRIC COMPANY By + ..
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Rottert/ II.%erdes l President RICHARD H. PETERSON -
PHILIP A. CRANE, JR. . f l
Attorneys for Pacific Gas and.. ectric Company g B . PhiliyyA. Grane,'Jr. *j s
1 Subscribed and sworn to before me this 22nd day of January, 1964.
i (SEAL)
Rita J. Green',/ Notary Public in l
. and for the City and County of j San Francisco, State of California s 1
My Comr.A saion Expires July 16, 1967 I 1
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P. GEOLOGIC-AND SEISMIC INVESTIGATION, i=
' 0F DIE SITE FOR; A NUCLEAR ELECTRIC POWER PIANT '
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\ ~ 'i ON BOIEGA IIEAD, CALIFORNIA B7 l
Don Tocher Consulting Seismologist 2740 Derby Street -
Berkeley, California and E. C. Marliave Consulting Geologist h466 North Park Drive Sacramento, California ms l h i
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Prepared for
' Pacific Gas and Electric Corgpany 245 Market Street San Francisco, California
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3 94106 January 1964 ,
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CONTENTS Pages '
Introduction 1-2 l
Acknowledgement 3 1
Previous Investigations 4
.{.
Geologic Setting 56 Western Limit of the San Andreas .
Fault Zone T-9
. Postulated Faults Cuttin8 Bodega Head 10-13 .
l Criteria for Evaluation of l Seisnic Hazards 1k-15 ,
i Geology of the Plant Site 16 1
Quartz Diorite 17-18 !I l
Sedimentary Rocks l 19-21 )
1 1
Structures in the Sediments 22-33 i i
Likelihood of Secondary Faulting Throu6h the Reactor Site 34-36 l Conclusions 38-39 References h0 41 List of Illustrations 42
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l INTRODUCTION l This report presents information on geologic features observed during preliminary phases of preparation of a site on the southerly portion of Bodega Head, Sonoma County, California, for the construction of a large nuclear-fueled electric power generating plant by the Pacific Gas and Electric Company, together I
with the authors' interpretation of these data with regard to the suitability and safety of the site from seismic activity which mi*ght originate in the neighborhood of the site. This report supplements an earlier report submitted 1
to the Company on September 14, 1960 by Don Tocher and William Quaide entitled j
" Report on Earthquake Hazards at the Bodega Bay Power Plant Site" (Tocher and i
. . . . Quaide1960). This earlier report (Tocher and Quaide 1960) was included as
/Gpendix IV of the " Preliminary Hazards Summary Report" filed as Exhibit C I
to the Company's application for a license authorizing the Company to construct ]
and operate a nuclear reactor as a part of the plant at Bodega Head. This application is now pending before the Atomic Energy Commission.
In mid-October 1963, the Company completed excavation of a pit, 142 feet in diameter, to a depth of -73' (datum for all elevations given in this report is mean lower low water) at the site of the proposed nuclear plant.
This work, and excavation of the p1' ant yard or working area, was carried out in part to permit a detailed examination of geologic features at and in the immediate vicinity of the site for the nuclear reactor in order to develop full information on the suitability of the site. This report presents the results of approximately 50 days of fie.i work at the site and in nearby areas in the months of May through November 1963, plus a review of pertinent sections of several published and unpublished geologic and seismic reports on the site.
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Figure 1 is a map of the Bodega Harbor and Bodega Head region, showing 1 . . i the relation of the plant site adjoining Campbell Cove to the surrounding area. l. i Figures 2 and 3 are general views of the excavation for the plant yard area, ,
1 1 l and show the reactor pit on October 12, 1963 An interior view of the pit,. ,
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l also photographed on October 12, 1963,.is seen in Figure k.
An additional purpose of this report is to answer from geologic and ~$
seismic points of view the questions that have been raised'in the public press and elsewhere with respect to the suitability of the Bodega Head site for -
construction of a nuclear electric power generating plant. ~!
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. 4 Acknowledgements Detailed examination of the geology of the excavated areas was greatly facilitated by the complete cooperation of officials and employees j of the Pacific Gas and Electric Conpany and of the Cosqpany's contractor for j
the reactor pit excavation work, Peter Kiewit Sons, Inc. Geologic conditions
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at the ~ site were evnmined continuously during excavation of the plant yard down i 1
to an elevation of +25', and near the pit to elevation +5' during the period
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May 26, 1963 to July 17, 1963 by Dr. William Quaide, who vus then a geologic consultant to the Ccrapany, and who kindly made his observations available to the authors. l
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Previous Investigations Discussions of the geology of the general region of Bodega Bay are.
given in reports by Johnson (1934,1943), Travis (1952), and Higgins (1961).
Mort detailed discussions of the geology of Bodega Head are given in reports of Tocher and Quside (1960,1962) and Schlocker, Bonilla, and Clebsch (1963). -
Ibe latter report in addition describes observations made in the course of a detailed examination of the excavation for the plant insofar as it had been coqpleted on June 6,1963 Detailed observations of plant site geology as exposed through July 17, 1963, havebeenreportedbyQuaide(1963).
Certain data on subsurface geologic conditions, particularly on depths to bedrock at and near the site, have been interpreted from logs of boring 8 3
appended to reports to the Pacific. Gas and Electric Company by the foundations engineering firm of Dames and Moore (1960s,1960b, '1962). Additional borehole ,
infor=ation was available for use in the present report from logs of borings drilled by Peter Kievit Sons, Inc.
The seismic history of Bodega Head -- historic and instrumental --
is given in Tocher and Quaide (1960). Observed effects of the California 1 Earthquake of 1906 on the region around Bodega Bay arc included in the classic report on that shock edited by Lawson (1908).
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- .$. r-GEOLOGIC SETTING s-Inasmuch as the present report is intended to supplement existing
\
reports by describing details of geology revealed in the course of site '
preparation and preliminary excavation work, a detailed discussion of the general
~
geologic setting of Bodega Head vill not be included herein. Instead, the reader l
is referred to the reports of Tocher and Quaide (1960) and of Schlocker, Bonilla, '
and Clebsch (1963), or to the review article by Koenig (1963) for the more general aspects of the geologic setting of Bodega Head.
The areal geology of Bodega Head itself is shown in Figure 5, adapted fromTocherandQuaide(1960). The bedrock of Bodega Head is granitic in general character; although the bedrock is predominantly quartz diorite in co= position, samles of rock thought to be more nearly quartz monzonite in composition have 3 been noted both at Windmill Beach (on the Pacific Ocean side of the Head) and .in +
)
the reactor pit (Julius G. Schlocker, oral communic'ation, November 26, 1963). The ,
bed. rock is cimilar in character to rock exposed on Point Reyes peninsula, and doubtless is a part of the same intrusive series. ' According to Curtis, Evernden, and Lipson (1958), the potassium-argon method of age determination indicated an age of 83 9 million years for a sample of similar rock collected from the Point Deyes peninsula.
l Prior to excavation work at the site, bedroc3; exposures in the immediate vicinity of the plant site were limited to those which could be seen below a horizontal wave-cut bench northwest and southeast of Caz:pbell Cove. The bench lies a few feet above sea level on the bay side of the headland, but was not .
observable in Campbell Cove. The nearly flat contact between bedrock and the overlying sediments is marked by a rather firmly cemented conglomerate horizon l , a few feet thick both northwest and southeast of Campbell Cove. The bedrock l exposures and overlying conglomerate northwest of Campbell Cove are no longer 4
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. 4 observable, having been covered by spoil from the plant excavation placed there
- in the cource of construction of an access road to the site along the vesterly l
edge of Bodega Harbor.
A generalized bedrock surface contour map based on borehole information -
l (Dames and Moore,1962, plate 2) shows the reactor site to be located on the northerly edge of a buried valley cut into the bedrock surface. The axis of the buried valley slopes downward from the vestern side of the headland in the vicinity of Windmill Beach in a Benerally easterly direction. The deepest point of the valley encountered by any of the boreholes was not far inland from the .
beach at Cer:pbell Cove, near the southeast corner of the yard excavation.
Here the bedrock surface lies approximately 75 feet below sea level (Boring
, No. 2, Dames and Moore, 1960b). Se buried valley is filled with sedimentary .
deposits of Pleistocene and Recent geolo61 cal aSe. The hiatus in the horizontal I
vave-cut bench in the bedrock observed on both sides of Campbell Cove thus '
reflects the existence of the buried valley on the bedrock surface. f 1
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Western Margin of the San Andreas Fault Zone e
As indicated by Tocher ar.d Quaide (1960) and as discussed in greater detail 17 Schlocker, Bonilla, and Clebsch (1963), the vestern edge or margin of the. San Andreas fault zone cannot be precisely delineated in the immediate vicinity of the power plant site on the southern part of Bode 5a Head because the usual geolo61c criteria for defining the fault zone are absent or concealed by dune and beach sand and by water. The line.shown by Tocher and Quaide (1960) as the " Western Limit of the San Andreas Fault Zone" and in fig. 5 of the present report as the line A - A vas and is intended to represent the most extreme possible vesterly position of the vestem edge of the zone which may be drawn 1
consistent with curface rock exposures in the vicinity of the site, and which has ,
a direction consistent with the general direction of the San Andreas fault zone where it is better exposed both northwest and southeast of the Bodega region.
Available geologic evidence does not demand that the line be drawn so far to '
the vest, but could indeed allow the line to be drawn some distance to the east of A - A.
The remarkably straight physiographic feature indicated by line B - B on fig. 5, where recent beach.and dune sands have drifted up against the higher ground of the Head itself, is almost certainly controlled by faulting. If the l
direction of this feature is adopted for that of the vestern edge of the San Andreas fault zone, at least locally, the line B - B is then the vesternmost position of the vestern edge of the San Andreas fault zone which is consistent with both _ ;
geolo61c and topographic evidence. The direction thus suggested by the topographic evidence is about 8' to 10' counterclockwise from the Eeneral direction of the i
San Andreas fault zone where it is exposed in Marin County to the southeast and !
l in northern Sonoma County to the northwest of Bodesc Bay, and about the same cmount counterclockwise frora the direction of the 1906 surface fault break acrcss the j sand dunes northeast of Bodega Head as shown by Lawson (1908). Variations in the l 1
direction of the edges of the San Andreas fault zone of this or Greater maEnitude are observable in other areas.
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The results of gravity observations made along two profiles approxi.
mately normal to the San Andreas fault in the vicinity of Bode 6a Bay by personnel 7 of the U.S. Geological Survey have been presented by Don R. Habey and Donald L.
pcterson of the U.S.O.S. as a section entitled " Geophysical Observations" in the report by Schlocker, Bonilla, and Clebsch (1963, pp. 11-13). These data can be regarded as strongly suggesting that the steep gravity Eradient observed approximately one-half mile east of line A - A (fig. 5) on a gravity profile following Doran Beach represents the westerly edSe of the fault zone. B e steep Cravity gradient must result frem a steeply dipping density interface at approximatelythatposition;thisdensityinterfacecouldbeeither(1)anincrease in the dip of the base of the lower density near-surface sediments toward the east or (2) the vestern margin of low density sheared rocks of the zone of most m'
recent faulting. Either interpretation muld suSCest that the vestern limit of recent fault activity lies considerably to the east of either of the lines A - A or B - B of fi 6. 5 In discussing the San Andreas fault zone, Taliaferro (l$hSIP.159 61) concluc'ed that the vide zone (up to one mile or more) of faulting commonly referred to as the San And.reas fault zone is largely an old zone of Eocene faulting which originally determined the boundary between crystalline and sedimentary basement rocks, but that pleistocene and Recent fault activity has been confined to a much narrower zone superimposed on the older and broader zone of Eocene activity.
Se only surface fault ruptures known to occur in historic times in the San Andreas fault zone in the vicinity of Bode 6a Head vere those formed during the California earthquake of 1906; as shown by Lawson (1908), the surface fault break in that earthquake lay approximately 1 mile northeast of the reactor site.
]
The authors of this report have both examined a number of localities in the ,
San Andreas fault zone and other fault zones of similar character, and have seen
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ample physiographic and geologic evidence to support Taliaferro's conclusion that recent fault movements have been confined to narrow zones, even where the '
zone of older activity is much vider. Succeesive fault movements tend to occur ' '
in the same limited, weakest part of the broader zone; in the Bodega region, '
this narrov zone lies near the eastern margin of 'the San Andreas fault zone.
From this,, we conclude that any activity that may oc6ur in this segment '
of the San Andreas fault zone during the lifetime of the plant and for a long time thereafter is very likely to occur more than a mile east 'of the reactor site.
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<10 POS'IUIATED FAUIES CUTTING BOIEGA HEAD Without exception, all previous geologic reports describing the guartz diorite of Bodega Head have touched. upon thie jointed and fractured
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nature of this rock, which is evident to the most casual observer. Some observers have attengted to attach more inportance to the jointed and fractund nature of the bedrock than is warranted.
. At the recent end of the time scale, the remarkedly undisturbed ,
character of the sedimentary deposits (including, in the buried valley in which the plant site lies, a thickness of more than one hundred feet of
' sediments lower in e3evation than wood samples with ages of more than 40,000 years as indicated by Carbon-14 age determinations) e:qphasizes the lack of recent movement on these bedrock faults.
N .
In his unpublished Ph.D. thesis, F. A. Johnson (1934)' presented
- n i.
a geolo61c map on which was plotted a fault on the southwestern portion of Bodega Head. From the text in the thesis, it is clear that Johnson regarded this area as the likely location of a zone of faulting in the bedrock, not the only such zone, but one of the more prominent ones. Tocher and Quaide (1962) examined the area in which Johnson shoved this fault, and concluded: "All lines of evidence lead us to conclude that there is little probability that a fault exists in the position shown by' Johnson." Schlocker, Bonilla, and Clebsch (1963) adaress the same question as follows: "
...ve found no compelling geologic evidence for mapping a fault in.the location and with ,
the attitude as mapped by Johnson in prefer =~e to numerous other locations and attitudes."
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1 In contrast, Saint-Amand (1963) takes a more alarmed view of the ,
possible Johnson fault. However, it should be noted that Saint-Amand ,
1 also views with alarm other fault zones in the bedrock, some of which have ,
allowed more rapid erosion of the rocks in them than at adjacent points g
l near present sea level where active erosion by the surf is going on at present. The jointed and sheared character of the bedrock on Bodega Head is . adequately described by Tocher and Quaide (1960) and by Schlocker, Bonilla, and Clebsch (1963); differential erosion rates by the surf in rock of such character is rather to be expected than viewed with alarm. Also, displace-ments of the mch younger sediments overlying the bedrock above a bedrock fault zone do not necessarily indicate that recent tectonic slippage occurred in the bedrock. In particular, a conclusion of very recent tectonic movements in the bedrock should not be drawn from disturbances of I
T the young sediments which were in fact caused by landsliding down a bedrock ,
j slope oversteepened by recent wave action. ,
California is and has been throughout its known geologic history both i a tectonically and seismically active area. There are almost no areas where i some earthquakes cannot be felt and much of the area is undergoing tectonic activity. As a result, large areas of deformed, fractured and faulted rocks may be seen in almost any locality. Indeed, it is rare that one can find an area free of fracturing and faulting. Faulting does not always produce earth-quakes. Many California faults, particularly the small local features, result from folding and deformation of the earth's crust. When the elastic limits .
l of the rock are exceeded, offsets occur. These movements may be so small and 1 >
slow that no earthquake is produced, but nevertheless shearing and faulting result.
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It has been publicly stated and written that'there are faulte, or a fault at the Bodega Head site. A fault by definition is a surface in tlie earth where 1
movement has taken place. The term does not indicate whether movement is infini- )
- 4 1 tesimal or large nor whether it is up or down, sideways or pulling apart or a combination of these. It is a common practice to refer to small faults as slips, l I'
offsets, shears or sheared zones. This generally indicates something other than a major or important or extensive fault. In general the'public thinks about faults only as the generator of earthquakes and lience the term fault usually brings to ,
the layman's mind an earthquake-producing feature. Thus it is frequently desirable to refer to minor faults as small displacements or offsets. Shearing and brecciation do not usually produce earthquakes. Bedding shearing and brecciation may occur very slowly yet do some dama6e to the rock mass. Offsets due to differential ;
3 ,
, settlement, consolidation or slumping may and usually do occur without producing shocks. However, major shocks by shaking may produce, induce or assist with consolidation or slumping. These suocks may act as a triggering force for minor displacements. However, many of the faults found even in seismically active areas are not earthquake producers but are due to slow crustal disturbances. These move- -
ments have severely fractured, crushed, sheared and faulted much of the rock over most of' California. Their existence is common knowledge.
In investigation 6 and reporting upon foundation conditions for over 300 dams, tunnels, power plant and penstock locations in various parts of california, these fractured and faulted conditions were found by the authors to exist. During the investigation for over 200 other locations as possible. sites similar conditions of varying degree vere noted. By recognizing the geologic and seismologic criteria it is possible to create a suitable and safe desi6n. Many cities have Brown and major en61neering structures been built to service them near seismically active fault zones. A partial list of such cities and of structures with which the authors are acquainted c.nd which they have worked on is shown to indicate the
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reliance placed on study and design for present day structures in seismically
- active areas
- -
Along San Andreas & Hayward Faults .
l San Francisco Hollister San Juan Bautista San Bernardino Berkeley Oakland Hayward Humboldt Bay Atomic Power Plant Ruth Dam j Bay Bridge Golden Gate Bridge Crystal Springs Dam Anderson Dam Lexington Dam Austrian Dam Coyote Dam Cuyama Dam 011 and gas lines to various parts of California Little Rock Dam Palmaale Dam Metropolitan Water District Aqueduct
-) Lawrence Radiation Laboratory Gabd Du '
San Pablo Dam Vallecitos Ecactors -
Fairmont Reservoir and Tunnel - City of Los Angeles Dept of Water & Power I=perial Irrigation District Distribution System Along San Jacinto Fault l
San Bernardino Colton Hemet
- Metropolitan Water District Aqueduct Lake Nathews Along Newport Inglewood Uplift Inglewood Long Beach Newport Balboa So, Cal. Edison & City of L.A. Power Plants l ?,
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r CRIn:RIA FOR EVALUA1 ION OF SEISMIC HAZARDS An evaluation of the seismic bazards affecting any particular site for a large structure must consider two questions:
- l. How hard vill the ground under the structure be shaken by i the strongest earthquake that conceivably might occur in ';
the vicinity of the site?
- 2. What is the likelihood that the strongest earthquake that ,
1 conceivably might occur in the vicinity of the site vill cause dislocation of the ground under the structure?
In brief, earthquakes may damage structures by shaking them or by dislocating the ground on which they stand. .
The Modified Mercalli Intensity scale, like the older intensity scales which it has superseded in this country, attempts to answer both of the foregoing question's' with one number. This is particularly misleading im .
When one is concerned with very strong earthquakes, which are usually , ,
accompanied by both videspread dama6e to buildings and Bround dislocations of various types and ma6nitudes. For example, in several major earthquakes in the vestern United States in this century, one aid easily assign i
intensity XI on the basis of dislocation of the Ground by primary faulting (Modified Mercalli scale: " Broad fissures in ground") at a given locality, whereas the intensity based on the criterion of damage to structures a few feet or tens of feet from the fault was only VI (Ibdified Mercalli scale: "A fev instances of fallen plaster or cracked chimneys. Damageslight.")
Another drawback in the use of the }bdified Mercalli scale lies in the fact that so many of the criteria in it, and especially in its higher grades, attempt to relate the intensity of an earthquake to the degree of damage to buildin6s. The response of buildings and other structures to
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strong-motion carthquak:s is of particular inttrest in tha field of Engineering
-{~,; Seismology. And yet, for most purposes of Engineering Seismology the Modified Hercalli scale is neither very satisfactory nor very useful. This seeming paradox 9
stems in a large part from the vide differences in the abilities of buildings to o resist earthquake-imposed forces, . and in part from the general lack of precision of such an empirical scale which depends so heavily in its application on i
subjective observations and judgments by many people.
]
I The use of the 2dified Mercalli scale by Tocher and Quaide (1960) has been videly misinterpreted. In considering the first of the criteria mentioned above, we nov believe that the best guideline available is to be found not in j 1
the }bdihed Mercelli scale, but in the response spectrum techniques developed l over a period of years at the California Institute of Technology by Dr. George
, Housner and others (for discussion of response spectrum techniques, and for l
, additional references on the subject, see Hudson 1956). Damage to a proposed l T i
' 1 structure from shaking can be minimized or eliminated by proper design analysis i and construction; the information contained in response spectrum curves is far more meaningful and useful to the designer than that offered by an estimate of the maximum intensity or by a statement of maximum expectable ground acceleration with no statement of the corresponding period of the ground motion. The method followed by Housner (1961) in scaling the El Centro 1940 strong-motion accelerograms to apply to the Bodega Head site is the best approach to the problem. We believe that* be has thereby established a conservative seismic design criterion.
With regard to the second of the criteria mentioned above, evaluation of the likelihmi that a strong, nearby earthquake would cause disruption of the ground under the site must'necessarily be based on geologic evidence. Although the quartz diorite of Bodega Head is extensively jointed and faulted on old minor faults, these bedrock faults do not offset the overlying sedimentary deposits, and this indicates that there has been no displacement for many tens of thousands of years.
This question is considered at greater length in subsequent sections of this report.
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GEOLOGY OF THE PLANT SITE The plant site has been excavated to an elevation (nominal) of +25' .
That portion of the plant yard excavated to this approximate elevation is boun:bd 'by' cut banks on the north, vest, and south sides, and by an irregular ; ,
l boundary between cut and fill on the eart, or Campbell Cove side. Approximate lengths of the toes of the three cut banks are as follows: north side, 375'; . 4 vest side 480'; and south side, 250t. Elevations of the' or16 nal 1 ground surface prior to excavation of the yard were hi6h est near the northwest corner of the yard (+120'), and around a substantial portion of the top perimeter of the yard, I original ground elevations exceed +100' .
I j To facilitate excavation of a circule.r pit belov,the general yard level, '
i a further excavation was made near the northwest corner of the yard down to an s ' .
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elevation nomine).ly +5', but actually variable between +5' and O'. The approxir. ate outline of the excavation to elev. +5' is shown in figure 6. Within the confines of this excavation, a circular pit approximately 142' in diameter and with vertical ,
sides was excavated to a final elevation of -73'.
Rocks exposed in the various excavations can be grouped into two general series, the quartz diorite bedrock and the overlying sediments.
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- 9 17.. .k Quartz Diorite Quartz diorite is exposed in a portion of the north bank of the yard excavation in a broad spur or nose whose axis plunges south 'to southeast.
Highest point of the exposure in the north bank is at elev. +68.0' . The exposure is approximately 250' wide at the toe of the bank (elev. +25'). i An additional area of quartz diorite was exposed tecrporarily near the northwest corner of the yard in the bottom and sides of a twelve-foot i
deep trench cut along the base of the banks for installation of yard sub-I drains; the surface of the quartz diorite here does not intersect the final ;
yard level, and this exposure was covered with backfill after installation of drainpipe in the trench.
The north-wal3 exposure of quartz diorite extends across the i
1 adjacent yard surface (elev. +25') to the top of the bank cut down to elevation +5', at which elevation quartz diorite is exposed at the surface around the top perimeter of the reactor pit for a distance (measured on the pit circumference) of 65 to 70 feet. The entire floor of the pit, at elevation -73', is in quartz diorite.
Figure 6 shows contours on the bedrock surface in the vicinity '
of the reactor pit interpreted from the exposures described above, from logs of boreholes drilled for exploratory purposes by the firms Dames and Moore and Peter Kiewit Sons, Inc.,ad from elevations measured in the vertical walls of the reactor pit.
'Ihe general character of the quartz diorite exposed by excavation is similar to that of quartz diorite seen elsewhere on Bode 6a Head and described in this and earlier reports. The bedrock ic generally weathered strongly immediately beneath its contact with the overlying sediments, but the degree 4
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It was not possible to trace any sin 6 e1bed continuously throughout-the exacavation at any elevation, but at most elevations numerous' beds and laminae could be traced with sufficient overlap of different ones to demonstrate, with the exceptions discussed below, the absence of faults cutting the sediments on a scale observable by' $his method.
The age of the sediments has not been precisely determined, but a
- minimum aBe has been established by radiocarbon dating techniques applied to wood samples collected at several elevations. Schlocker, Bonilla, and Clebsch (1963, pp. 31 and 32) reported that a wood sa=ple collected at elevation +49 2' yielded a Carbon-14 age in excess of 38,000 years. This determination was made at the U.S. Geological Survey's Washington, D.C.
laboratories. Age determinations by the same method made at the Los Angeles
,y .
Laboratories of the Institute of Geophysics and Planetary Physics (University
- 3 of California) indicate that two sarqples collected at elevation +18' vere more than 40,000 years old. (LetterdatedSeptember 16,1963, from I
G. J. Fergusson, Associate Professor, to F. F. Mautz, Chief Civil En61neer, Pacific Gas and Electric Company.) To the knouledge of the authors, no vood sample collected from the site hr.s yielded a Carbon-14 age younger than the raximum age determinable by the particular laboratory making the determina-tion. The specific ages mentioned above should therefore be re6arded merely as minimum ages; the actual ages could be tens of thousands or more years older.
Since the age of the upper half of the sediments is over 40,000 years, the lower half would probably represent at least another +k0,000 years for a total age of at lemt 80,000 years. '
Belo i elevation +25' (approx.), the sediment immediately overlying the quartz diorite is most commonly a dense, massive blue-grey clay containing
] numerous white angular fragments or crystals of pla61oclase feldspar and quartz.
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- p. . In a manner similar to the more sandy sediments'imediately overlying the bedrock at higher elevations, these clays comonly grade laterally and ,
vertically away from the bedrock surface into sediments exhibiting better bedding. At these lower elevations, the clay groundmass near the bedrock l contains little sand, but the proportion of sand increases with distance 1 .
l from bedrock. Occasional, fragments of fine roots and small twigs or l
l limbs are found in the blue-grey clays, but most comonly such fragments
~
are lacking. In places a slightly brownish cast to the clay, seemingly l
associated with wood fragments,. suggests the presente of more finely' I divided organic residue disseminated in the clay, but this has not been confirmed by laboratory analysis.
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I O Structures in the Sediments The sedimentary rocks in the yard excavation are nearly flat or .
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dip gently to the south or southeast. From the geologic evidence available j l
at the excavation, it is not possible to tell .vhether these southerly dips have resulted from a general tilting of the entire head (or a large ;
portion of it), or from greater compaction of the thicker sedimentary column fillin6 the buried valley in the bedrock south of the reactor pit, or perhaps in part from initial non-horizontal deposition. As the sediments appear to j i
be largely water-laid marine, estuarine, or possibly, in part, fresh lake deposits, appreciable initial dips do not seem likely. If the nearly horizontal contacts between bedrock and overlying sediments observed a few feet above .
l l ,
sea level both southeast and northwest of Campbell Cove on the east side ,
1 l
' of Bodega Head have a common origin, their present stand at approximately .
the same elevation suggests that appreciable tilting of this entire portion of the headland has not occurred since this bedrock surface was cut.
4 Dips observed in layered sediments in the walls of the reactor ,
pit were if anythin6 Sentler than in sediments at higher elevations, except l l l locally. During the excavation of the pit, evidence for faults cutting the l
sediments was sought by the same technique employed higher up in the column.
Exposures in the vertical valls, of the pit could in general be examined in !
I more detail than in the sloping walls of the yard excavation. Although it j l
vas not possible to record accurately the positions and elevations of each j I
! and every traceable layer or lamina, a considerable body of data of this j nature was recorded. These data have been plotted in figures Ta, Tb, and Tc, vhich together represent a vertical section of the geology exposed in the !
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A. valls of the reactor pit. The amount of detail' sitown varies considerab1'y' from '
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place to place, being most detailed in regions of greatest interest. i One minor offset was found'to cut the sedimentary layers within ,
the limits of the reactor pit, and was the subject of intensi.ve. study. .
from the time it was first noticed on September 11, 1963 The special effort devoted to this feature is reflected in the' greater . detail shown in Tc; the feature under discussion is situated in the ' vicinity of figure station 39 between elevations -13' and 36' . . In the valls of the reactor pit, the offset was followed during the course 'of excavation from the highest near observable point, elevation approximately -13', downward to/the. contact between
- the sediments and the underlying bedrock, elevation approximately -35' .
The zone of disturbbd sediments at elevation -13 is shown in figure 8, -
where three minor offsets cut a prominent, nearly horizontal boundary ,
'l between underlying orange-brown sands and overlying blue-grey clay containing c-l
, *1 l abundant crystals of plagioclase and quartz. These lines of offset cut the >
color boundary near'the tup of the pick; all three are dipping steeply to the left (roughly south) in this section and view. Taken together, the offsets detine a miniature horst behind the pick handle. Approximate amounts of offset and apparent sense, from left to right (fig. 8) are 1/2 inch normal,2inchesnormal,and1-1/2inchesreverse. Near the left side of fig. 8, the' color boundary rises to a higher level, but the prolongation ofthesameboundaryfromtherightsideoftheviewcoincideswitha1/2 inch
, thich seam of orange-brown (organic?) clay, noticeably different in color.
The downward continuation of the zone of offset at elevation .
. approximately -16' is shown in figure 9 Again three lines offsetting the bedding surfaces and color boundaries can be seen, one in the middle of the
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view, the other two close together in the lower right corner. This photograph was taken at the bottom of the pit vall as it existed at that time, and loose material masks the cut surface in the bottom quarter of the view. The right hand offset in fi6ure 8 is continuous with the center offset in figure 9 1
Vnere bedding surfaces in the sediments are as well marked as in .
I
(
fig. 9, the offset or offsets are found near the steepest part of a mono-l clinal fold in the otherwise flat-lying sediments. The elevation of a I sedimentary Icyer boundary is typically l' to 2' hi6her on the southeast i side of the fold than on the northwest; the actual offset of such boundaries ,
was typically about 5".
The zone of minor offsets was followed out onto the floor of the pit !
I l at several levels as excavation proceeded. Despite many hours of dilicent l l
l searching, the zone of offsets could not be followed more than about 55' into iT the pit from the southwest pit vall at any elevation in the sediments. No ,"'
l.
disruption or offset of the sediments could be found in the east vall of the -
1 pit on the prolon6ation of the zone outvard from staticn 39 !
As excavation progressed and the offsets in the vall were followed l to lover elevations, it was not possible to observe any line of offcet continuously,throu6h all elevations in the pit vall; the largest hiatus was about 3' in length near elevation -25', but the sediment here was unusually devoid of marker surfaces which hight help to pinpoint offsets.
At the pit vall near station 39, a bedrock shear zone was found in the ,
general area of the offset in the overlying sediments; however, the top surface of the bedrock was not found to be sharply offset as vould be
, expected had the offset in the sediments resulted from sudden faulting in the bedrock after 'the sedimetrts had been deposited. The underlying shear zone d
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/ in the bedrock was traced across the pit floor and back up the east vall of the
]
pit to a point approximately 3 feet below the top' surface of the bedrock. The l 1
top 3 feet of bedrock was not offset by this shear zone, and no distortion of i
)
the overlying sediments was seen. The approximate position of the bedrock fault is shown on fi6ure 6. ,
A small reversal of slope on the bedrock surface is associated with the fault in the bedrock in the southwest part of the pit, but was not evident l near the east side of the pit (see fig. 6). It may be due, to differential erosion.
The general slope of the bedrock surface in the pit area is rou6hly south and l
southwest, but in the same region in which offsets were observed in the overlying l l
j sediments, the bedrock surface northwest of the fault was noticeably lover ,
l l I l
(perhaps one foot lover) than southeast. Within the pit area, positions of the -
rock surface contours are only approximate, as it was not possible to record )
m j surface elevations as precisely in the floor of the excavation as in the valls. -l I'
The general features of the -35' and ho' contours are believed to be reasonably ,,
correct, however. l After excavation in the pit had reached a depth such that the entire pit floor was composed of quartz diorite, an exploratory trench was dus Just outside the southwest rim of the pit opposite the offset which had been observed I first in the sedimants' at elevation -13' in order to determine the upward course '
1 of the offset. The Ground level in this region was then approximately at elevation O' the location of this trench is shown in figures 6 and 10, near the southwest edge l of the pit, and the trench can be seen in the lover right' corner of fig. 3 The bottom of the trench was cut down to elevation approximately -148 Near the bo~ttom of the northeast vall of this trench, faint color bands in the orange-brown ,
sands vere seen to be offset approximately one inch at a point nearly alon6, the
)
prolongation of the zone of offsets in sediments first observed in the pit vs11 at elevation -13'. Figure 11 is a photo 6raph of this feature in the exploratory trench; the faint color bands can be seen across the center of the view, which is
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looking in the direction of the pit van. The surface in fig. H is approximately nine feet from the pit vall section shown in fig. 8, and at approximately the -
came elevation. No other indications of offsets of this or Breater ma6nitude were found at this elevation in the trench. The line of offset in the trench is rather ,
indistinct, but dips to the right in this section'and view.
The irregular zone of parting which crosses fig.11 just above the -
measu' ring tape is marked by a well-defined seam of brown (orcsnic?) clay about
}" to 1" thick (indistinct in this photograph). The clay seam was quite persistent, and was traced along the vall of the exploratory trench in both direc-tions from the point shown in the photograph. Although the line of offset clearly l
extended upward to the clay seam, the seam itself was not offset, either directly above the offset color bands or anywhere else along the vall of the trench.
Neither was any offset observable above the clay seam, although distinct horizontal or nearly horizontal layering surfaces were well exposed higher up in the northeast
)
1 vall of the trench. The clay seem thus truncates the line of offset and marks l l a minor unconformity in the sediments, at least locally; the line of offset was l
not observed earlier in the vans of the trench excavated for the concrete sleeve which later formed the rim of the reactor pit for the simple reason that it did not extend as high in elevation as the bottom of that trench.
The salient ' features of the zone of offset sediments first observed near station 39 in the van of the reactor pit may therefore be summarized as fonovs:
(1) Horizontally, the zone extended into the area of the pit about 55' northeastward from the southwest edge. It did not extend all the vay to the opposite vall of the pit. Southwestward from the vall of the pit (i.e., outsid9 of the area of the pit), the zone had '
diminished to a single line about nine feet away from the pit vall; 1
l -
it was not observed at all about three feet further from the pit 1
- vall in the southwest van of the exploratory trench.
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.(2) Vertically, the zone extended from close to the contact between the sediments and underlying bedrock (at elevations in the pit between ,
-35' and 40' approximately)~ upward to approximately elevation -13', l 4
at which level the offset was truncated by a nearly horizontal clay seam.
J (3) Although the zone of offsets in the sediments was approximately )
i l coincident in position with a shear zone in the underlying . bedrock, 'l' l
I the boundary surface between bedrock and sediment was not sharply l 1 offset by a fault cutting both units.
l (4) As observed in the vall of the reactor pit, the zone in its upper i l l part dipped southeastward 65' 80'. The dip steepened downward, and .
l reversed to a steep northwest dip just above the bedrock surface.
l l
l (seefi6.Tc) ,
(5) The offsets in the sediments between elevation -23' and -13' lie near the steepest part of a monoclinal fold in otherwise flat-lying sediments. Differences in elevation of well-defined markers across the fold are one to two feet. Like the offsets themselves, the monoclinal fold is evident up to but no higher than elevation -12' or
-13'.
With the completion of excavation of the reactor pit and exploratory trenches, sufficient observational data are available to pemit the piecing together l
l of a reasonably comprehensive picture of the sequence and timing of the various l
l' events and processes in the geologic history of the reactor site which culminated in present-day conditions. The quartz diorite bedrock was emplaced as molten material more than eighty n1111on years ago; complete solidification doubtless was a slow process, and in the course of the cooling and solidifying, zones of
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-28
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the rock which solidified early were later injected alon6 fractures by residual liquid fractions of the melt. In this way, aplitic and peg =atitic dikes were formed, l and some fracturing and faulting accompanied their formation. There is a very lon6 gap in the 6eologic record at the site following the final cessation of solidification . No rocks thought to be older in age than Pleistocene are found on Bodega Head. This indicates that immediately prior to the deposition of the oldest sediments on the Head, the Head stood higher relative to sea level than 1
at present, and the quartz diorite surface was undergoing subaerial erosion. ]
l This is supported by the present configuration of the bedrock surface, with a i
buried valley cut into the bedrock to elevaticn -75' in the vicinity of Ca=pbell '
Cove.
Following this yeriod in which any and all older sediments were removed and subaerial erosion shaped the present bedrock surface, sea level began to rise and the deposition of the sediments begtn in the lover parts of the since-buried vaney. During most if not all of the time required for the deposition of the sediments in the vicinity of the reactor uit, the area was protected rather well from the open sea, and deposition proceeded in quiet vater. Some of the sand Icyers i
seem to have been derived entirely from the erosion products of quart- diorite and.
some contain abundant well-rounded grains of red ehert and greenstone doubtless derived from an area of Franciscan rocks on the east side of the San Andreas fault zone.
From this we may infer that the degree of accessibility to open water varied from time to time.
The rise in sea level which resulted in the accumulation of nearshore pleistocene deposits at the present reactor site probably did not continue monotonicany and without reversal. In the area immediately south of the reactor i
pit,-minor offsets of sediment boundaries were observed which were stron61y
, suggestive of slumping or sliding of material in a southerly direc?, ion; these
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. 29-I O features vere seen in the vans of the excavation for the proposed turbine generator, at elevations near +108 Slumping in this direction could most easily have occurred if deposition of sediments had been interrupted long enough for l
sor.e crosion and downcutting of the sediments to occur roughly along the axis - <
q t
of the buried vaney.
At the time of deposition of the hi6 hest Pleistocene sediments sea level must have stood some 75' to 90' higher relative to Bodega Head than it does at the present time. Inastauch as sediments as high as 49' above prese,nt sea level contain fossil vood dated by the Carbon-14 radioactivity method at more than 38,000 years, and as sea level is not thought to have stood so high since the last period of continental 61 aciation, we conclude chat the high stand of sea level which permitted deposition of these sediments occurred prior to the most recent glacial l period. Followin8 the high stand of sea level during which the highest Pleistocene I .
l l ', sediments were deposited on Bodega Head, sea level again began to fell, presumably l 1- .
at the time of a later increase in continental glaciation. Deposits of Recent age are thus represented here only by a rather thin (perhaps 5' or at most 10') of vindblown sands overlying the considerably older water-laid deposits.
In assessing the approximate a6e and the significance of the zone of offsets 1
observed in the sediments in the southwest sector of the reactor pit, it is useful to distinguish clearly three successive periods of time: (1) an intt.M approximately 1
80 million years long beginning with the emplacement and slow solidification and cooling of the quartz diorite bedrock and ending with the start of deposition of sediments in the deepest part of the valley eroded into the bedrock surface;
! (2) an interval of uncertain.len6th, probably better measured in thousands rather than millions of years, during which sea level gradually rose and sediments filled the valley as high as present elevation -13'; and (3) a later interval, considerably ,
longer than 40,000 years in duration, during which additional sediments were deposited as sea level rose to or above present elevation +75', then sea level retreatet f
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30- e to below and later again rose to its present position.
At a time possibly coincident with, but certainly no earlier than, the close of the second of these interyn18, the sediments lying below present elevation
-13' vere warped into a moncelinal fold. The varping progressed to a point where breakage or slippace of the sediments took place along the zone of offsets nov observable in the valls of the reactor pit near Station 39 Younger sediments
. . 1 now overlying this zone are neither va: ped nor offset; the warping and offsets I are cut off at the top at present elevation -13'. Therefore, whatever ucy have been the cause of the warping and offset of the sediments below present elevation -13', it ic clear that this varping and offset took place before the sediments which now overlie the zone were in that position.
l Although the sediments in the reactor pit area are relatively undistorted, a=ple evidence was seen that minor deformations of non-tectonic origin occurred at several different elevations. Loadcasts of sand in underlying finer Grained j
. I material vere observed at two or three boundaries of the type favorable to tha formation of this feature. A number of the loadcasts were deformed by mover.ent of sand over mud at a time when the mud was still oversaturated with water, prob::.bly soon after depocition of the overlying sand. One such deformed loadcast is shown i in fig. TB, two feet left of Station 37 at elevation -23'. According to the l
description of this type of feature by DeSitter (1956, pp. 301-303), the direction i of the horn vould indicate that movement of the overlying sand had been to the right or northwest relative to the underlying mad. Movement at this elevation and in the indicated direction, had it occurred at the same time as or later than i
I formation of the zone of offsets near Station 39, would account for the reversal of apparent dip and even for the hiatus in the zone of offsets which was obsched
,) at Station 39 l
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l Hear Station 3 (fig. TA), several sandballs completely severed from l r,, _
their source beds were observed near elevations -10' and -18', suggesting that the gliding movements here were more severe than at Station 37 Another feature suggestive of gliding movements on nearly horizontal surfaces in the sediments vas observed at several elevations and locations in the valls of the pit, to vit, ,
thin seams of plastic clay continuous for tens of feet horizontally but 3/4" or '
less in thickness. Slickensided surfaces could sometimes be found in such seams l
if they were pulled apart carefully. A zone less than one foot thick containing i several thin clay seams of the nature just described lay directly on the bedrock surface immediately to the left (southeast) of the zone of offsets near Station 39 The lowest seam in the zone was in contact with relatively unweathered bedrock for a distance of h} feet southeast of the offsets, but departed from the bedrock surface 1
x: ,
l ,
to follow a straighter and more nearly horizontal course than the bedrock surface itself for a distance of about 8 feet centered at Station 38 (fig. Tc).
i .
Numerous exposures of features thus indicative of extensive gliding movements along nearly horizontal surfaces in the sediments not long after deposition l were seen, and have been described here at some length because the zone of offsets l
near Station 39 appears to be intimately related to these movements. Slumpin6 on l
I a Grand scale with attendant gliding of the newly deposited, water saturated sediments over the pre-existing irregular bedrock surface would easily account for the offsets having the observed sense, yet it is difficult to work out details of such a process l
in terms of direction and limits of the movements from the limited available exposures and from what is known of the topography of the bedrock surface.
Although the exact details of origin cannot be given, it is clear that
.the hypothesis that the sediment offsets were generated directly by a sudden movement 3 on one of the fault planes in the underlying bedrock runs counter to observations. ,
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Offsets of sedimentary coundary surfaces are clear and. distinct where the layering
- e. ,
f.' is observable (see fig. 9). If these relatively soft sediments can preserve the l' I offset with such clarity and sharpness, it is difficult to understand why the boundary between bedrock and sediment should not exhibit at least as sharp and clear an offset. Instead, the offset surfeces, both in bedrock and sediments, seem to bend over and tend to flatten out as they approach the surface separating I i
the two types of material. Also, it is difficult to envision an offset in bedrock sufficient to offset overlying sediments five inches or more at one point, but which >
was incapable of producing any observable offset in the sediments 55 feet along the strike of the fault. Also, the rather ti6ht bend in the offset surface at elevation
-26' is mechanically incompatible with either simple removal of support from beneath the downthrown side or upward thrusting of the upthrown side; its configuration
, slmost certainly resulted from movements in a nearly horizontal direction after the offset itself had formed, but before the sediments had devatered to such an extent that they could no longer readily deform elastically. ,
Certain additional remarks may now be made with regard to the previously mentioned three successive periods of time distinguished by observations of features associated with the offset zone in the sediments. Because the zone of offsets is truncated by a distinct clay seam at elevation -13', the sediments now overlying the zone either had not yet been deposited when the offsets were formed, i
or, if they had already been deposited, they were transported to their present location i l
at some later time by Sliding along a surface in the nearly horizontal clay seam.
If the former alternative be correct, it is safe to say that the offsets observed belov elevation -13' were formed tuch longer ago than 40,000 years, and that no subsequent movements have occurred in the zone. If the latter alternative be correct, . ,
a the zone of offsets at higher elevation than -13' has been transported out of the
)>
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3 present confines of the pit in a generally southerly direction; this movement !
/3 -
J along a nearly horizontal surface mst have taken place before the waterlaid 1
q sediments had& watered very much and hence not very long after original deposition. j
.i The topography of the site prior to excavation was not favorable to slumping in a ]
l southerly direction, and if movement in this direction did occur, it must have taken place at . time prior to the final development of the topography. )
1 With completion of excavation of the reactor pit, the edge of Bodega Harbor is only about 300 feet east of the pit. The bottom of the pit is 73' below l
l Mean Lower Low Water, yet no seepage of salt water into the pit has occurred. .l This testifies to the impermeability of the foundation material and to the absence j of open or continuous fissures, fractures or seams, either in the bedrock or in the 1
sediments below sea level.
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i Ten 00D OF SECONDARY FAUL'ITNG THROUGH THE REACTOR SITE n.
The San'Andreas fault, like many similar major faults in other areas, is not a single line or fracture bounded by completely unbroken rock extending j many rdles in either direction, but rather is a zone of fractured rock of ,
varying vidth. Without doubt, every part of such a zone has ruptured at some '
time in its long geologic history. As has been discussed earlier, however, in regions where the fault zone is vide (say on the order of a mile), major dislocations in the zone are far more likely to occur near the immediately previous dislocations.
In other words, the migration of the limited zone of recent dislocations is a geologic process that proceeds at a slow rate. The San Andreas fault zone is a very old l
1 feature in the geologic sense, doubtless grew to its present vidth very slowly, l
1 and vill either become dead at some time in the distant future, or, if it does grow in vidth, the process vill be slow enough in the geologic sense that it is e
of no concern with regard to sites well outside of the present zone of fracturing.
Although large dislocations associated with a major fault will doubtless '
I recur in the limited zone of the more recent previous ma;or dislocations, major earthquakes in such fault zones are frequently accompanied by minor auxiliary fractures outside of the main fault zone. The great tajority of such auxiliary ,
fractures are quite superficial in nature, even though appearing spectacular to the layman, and involve movements of unstable masses of surficial materials under the influence of the vibratory motions associated bth the earthquake. The condition:
necessary for such surficial movements to take place are well known, and have been considered in the engineering planning for the proposed construction on Bodega Head.
We are firmly of the opinion that movements of this nature vill in no way ,
constitute a hazard to the plant. '.
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1 In addition to the foregoing class of movements, auxiliary fracturing i of a more profound nature must be considered. The volume of rock in which is stored the elastic strain energy which is, through the mechanisu 'of a great earthquake, i
converted into clastic vave energy, is considerable, and extends for come distance f 1
l away from the principal fault zone. In the process of conversion of strain energy I i
into wave energy, some minor displacements may take place on auxiliary minor faults j in rock outside the principal fault zone. Some of the fractu?es which can always l
be found in rocks near major fault zones doubticas represent fractures of this nature.
It has taken many millions of years for all the fractures of this class to be I developed in the neighborhood of any particular fault zone like the San Andreas. !
1 \
Such auxiliary fractures are generated by locally high stresses set up in the country rock by the gross movements in the principal fault zone. Movements of the principal j
, fault blocks tche place at a very slow rate, and, consequently, so do variations in I s
the locations of the local stress concentrations that give rise to movements 'on j auxiliary bedrock fractures. For this reason, some auxiliary fractures in bedrock l '
that have suffered dislocations in recent major earthquakes are most likely to suffer such minor displacements in major earthquakes in the near geologic future.
The identification of such bedrock fractures that have undergone movements in the recent i
geologic past is clearly the best guide to determining where such minnr dislocations vill occur in the near geologic future. On the other hand, where bedrock fractures l have been inactive for a 1cng period of time, these fractures are not likely to suddenly again become active in the next earthquake on the associated ' principal fault.
Evidence of . lack of activity on auxiliary bedrock fractures in the recent geologic h
past is therefore evidence that the local stress concentrations which originany '
i l
produced the fractues was long ago relieved in that region.
~
In the California Earthquake of 1906, fracturing and fissuring of the l ground was noted at many places outside of the San Andreas fault zone; all but a l i
i handful of these fractures and fissures were clearly superficial in nature, and were
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. , 1 so described and identified by the investigators of that earthquake. A few breaks )
's ;
in the bedrock which vere ncrt identified as incipient landslides or other super- j.
l ficial disturbances were described in the report on the 1906 earthquake by l Lawson(1908). one described by Roderic Cramlall, then a student at Stanford University I i
(in Lawson 1908, p. 253) was on Sawyer's Ridge, San Mateo County, in a region of l Franciscan bedrock. It exhibited a maw =m downthrow to the southwest of 2 to 3 inches.
Several other such cracks were described by Grove Karl Gilbert (in Lawson 1508, p. 69 )
i and p. 75); those described by Gilbert vere on the Point Reyes Peninsula some miles )
southeast of Bodega Head. As is perhaps natural, those which exhibited the larger f movemento vera described more ccc:pletely than' those which vere merely open fissures with little or no evident displacement, and in fact, it would not be possible even to find the locations in the field of some from the brief references to them. It
, is noteworthy, however; that of those which exhibited measurable movements, most are described as touching a pre-existing fault sag or beirs associated with a pre-existing bluff or scarp, indicating that earlier movements had occurred at the same place in the recent enough past that normal e:rosion had not yet had time to efface the obvious effects of those earlier movements.
At the Pacific Gas and Electric Company's reactor site on Bodega Head, careful, detailed examination prior to any excavation failed to reveal any suggestion that movements of this nature had occurred in the recent geologic past, either at j the site itself or anywhere else on Bodega Head. During all stages of excavation I
for the plant yard and of the reactor pit, particular care has been taken to examine the boundary between the quartz diorite bedrock and the overlying rediments, wherever it has been exposed. In addition, recognizable markers in the sedimentary rocks have been examined carefully for evidence of dislocation, both in regions where ,
bedrock was already or was subsequently exposed, and elsewhere. /s described elsewhere in this report, the sediments overlying the bedrock are in general remarkably e
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undisturbed, and the few breaks which have been found in them have arisen from causes other than faulting. Specifically in the immediate reactor pit area, no dislocations of the sediments from any cause whatever have occurred in a period of time longer than, probably naich longer than, forty thousand years. .
l l' '\
We therefore conclude that movements on subsidiary faults in the bedrock under the reactor site are not to be expected. Any possible movements that might occur would be of such minor amount as to be negligible.
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t 38 CONCLUSIONS m
- l. The foundation of the reactor and suppression chamber vill rest on quart: dicrite which, althou6h sheared and jointed, is relatively unweathered in the vicinity of the base of the reactor pit, and is agly strong to support the anticipated load. *
- 2. Cutslopes in the sediments overlying the bedrock can be adequately stabilized by proper attention to slope steepness, herming and draining.
3 The sediments can serve as a suitable foundation for accessory bu11 din 6s by adequately draining and proper desi6n. ,
L. No evidence has been found at any stage of the investigation of any active fault cutting the ground surface in the vicinity of the site.
5 No evidence was found to indicate that bedrock fault movements have ..
occurred since the overlying sediments were deposited on top of the quartz diorite.
- 6. A zone of minor offsets of sediment beds. cuts the southwest vall of '
the reactor pit. These offsets do not extend to the east vall of the pit and were '
found to die out a short distance southwest of the pit.
7 The relatively undisturbed character of the sediments is surprising in view of the proximity of the San Andreas fault zone and by comparison with other young sedimentary formations near the San Andreas fault zone in other areas. j This indicates that whatever movement or shakin6 has occurred on the nearby j
\
San Andreas fault zone has not affected the sediments in the pit area.
- 8. Several faults postulated in the sediments by other investi6ators have been reviewed and field. checked. These faults vere not found. In addition, there has been a tendency on the part of some to overeghasize the importance of particular bedrock faults (which have not disturbed overlying sediments), thereby conveying a distorted impression of the overall jointed and faulted character of the quartz diorite.
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I 9 At the very minimum, the reactor site lies 1,000 feet from the nearest j point of the San Andreas fault zone; it is probable that the concealed western limit of the zone is one half mile or more from the' site.
- 10. As in several other locations along the San Andreas Fault the recently active zone is here confined to a very narrow belt. This belt is over one mile cast of the reactor pit.
- 11. No si6 nit:, cant offects were observed in sediments at elevations higher than +25' anywhere in the yard e,xcavation. F2cm Cxrbon-14 age de. terminations on vood samples, sediments at elevations n's high as +49' are probably older, perhaps ;
much older, than 38,000 to 40,000 years old. Ergo, no moverants have occurred on ;
any faults cutting the reactor site in at least the past 40,000 years, and probably not for 100,000 or unre years. The few minor ' offsets in the sediments can vell be attributed to slu: ping or differential consolidation.
}
I
- 12. Movementtr en subsidiary faults in the bedrock of the reactor area tee not
.) l expected to occur for a period vell beyond the lifetime of the plant. Any mcsvement .
I that might possibly occur wuld ba of such minor amounts as t'f be ne61gible. 1 )
l l
13 From the ceismic hazard standpoint, the quart: diori*e bedrock of Bodega l Ut {
l Head is as good as the rock that might be fcnnd at ary';1te up et /down the coast !
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for many miles, and is superior to most. The site ) is suitable 1d. the proposed plant. ]
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, kO-References l ,
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