ML20234A844

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Engineering Geology of Proposed Nuclear Power Plant on Bodega Head,Sonoma County,Ca
ML20234A844
Person / Time
Site: 05000000, Bodega Bay
Issue date: 05/07/1964
From: Bonilla M, Schlocker J
INTERIOR, DEPT. OF, GEOLOGICAL SURVEY
To:
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ML20234A767 List: ... further results
References
FOIA-85-665 NUDOCS 8709180152
Download: ML20234A844 (5)
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'gupp/d g [g UNITED STATES  : 5/7/N DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY WASHINGTON 25.D.C.

ENGINEERING b M OF THE PROPOSED NUCIEAR POWER PIART

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ON BOIEGA BEAD, 80 NOMA COUNTY, ChLIFORNIA i bY Julius Schlocker and M. G. hnilla

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,l This report summarizes and interprets the geologie data presented

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in previous reports by the Geological Surrey. These data bear on the F..

5i] effect of large magnitude earthquakes on the foundation of the, proposed L.1 '

nuclear power plant on Bodega Head, California.

Q The crucial geologic problem at the site involves an estimate of the probability of a suddan permanent displacement, by rupturing, of the foun-dation rock of the reactor during an earthquake. Any such prediction

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must be based to a great extent on experience in earthquake-affected B?r regions; it can be made only with a relatively low degree of confidence LZ e

because geologic knowledge of the phannmana being evaluated is incomplete,

{ and in some instances rudimentary.

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An upper limit on the probability of faulting is set by the probabil-f-

b ity of occurrence of severe earthquakes (Richter magnitude 8.0 and above)

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g on the San Andreas fault. This has been ertimated by several highly r

j qualified seismologists to be on the order of one or two per century.

The Bodega Head site is almost certain to experience one severe earthquake in the next W years, the assumed lifetime ofet.be plant.

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The principal hazards to the plant from such a seismic event' are twofolds (1)shakingofthegroundduetosciamicwavepropagation,

and (2) possible displacement of the foundation rocks due to faulting. - l

, The hazard due to ahairing is being investigated by others, including the Seismology Division, U. 8. Coast and Geodetic Survey. prediction -l l' b ; as of possible displacement must be based largely on the distribution and I characteristics of the surface faulting produced by the 1906 earthquake

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and to a lesser extent on the distribution of faults in the excavation I 3/

Efh, for the reactor and on Bodega Head as a whole. The evidence is not ps

, adequate to suggest more than a general statement of probabilities.

4  ! The site is approximately 1,000 feet west of tbs vest edgs of the r-g SanAndreasfaultzone,whichisapproximatelyl}milesvidehere. The b main surface ruptum during the 1906 earthquake 7,ock place on the east j p.3 side of the scoe and had a horizontal displacement of 10-20 feet. '

p Throughout Bodega Head, faults and joints are cosmon in the granitic r;<

h rocks with the largest ones trending northwest, northeast, and east.

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95 l At the site, a principal structure is the Shaft fault, named from its W ,

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erposures in the shaft excavated for the reactor. This fault, one of r' ,

many tectonic faults in the granitic rock, is the only one that has e i

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e D been traced downward from the surface through Pleistocene sediments into the underlying granitic rocks. It strikes N. 400 E. and has been f e j t aced on the surface a. total of about 230 feet. ma mart == measured i displacement in the sediments is 14 inches vertica5.ly and in the granitic rocks is at least 24 feet horizontally.

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l' 1 i' i ~ The Shaft fault in the bedrock is m' sons that ranges from 2 to lo .j

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feet in width and consists of several intersecting faults.. This-l suggests that movement on the fault occurred several times, .though the ,

-l amount of vertical or horisontal movement during' any one period 'of l j

apresent cannot be detenmined. It is not unra-ale, however, to - I

! expect that displacements to+= M several feet have occurred at one j f , ,1 f::- [i tias. The fault displaces Pleistocene sediments dated from geologic )

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F4 evidence as younger than W,000 years and from radioactive carbon as ]

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$r older-than M!,000 years. Faulting is, therefore, younger than the- i r =: 1 g dated sediments, and any be younger than sediments that are not displaced, I EV i:i_ for in the soft sediments displacement-may be taken up by plastic K+ 1 5 deformation rather than rupture. "

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$, Surface ruptures created during the 1906 earthquake have been - l c.--

[5 er described at many localities outside of the San Andreas fault zone i

'(Lawsonetal,1908). The record of these events provides important k(

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clues for predicting future earthquake phencuena on Bodega Head. The G .

C'r character of the measured ruptures at the'se localities indicates that '

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y sans of the faults parallal the San Andreas, others intersect it at w , '

f acute angles, and still others are nearly normal to it.

.! l The principal observations of ruptures outside the main fault zone yi i after the 1906 earthquake were made at the Point Reyes Peninsula,.the

. San Francisco Paninsula, and the Santa Crus Mountains; undoubtedly

l. faulting occurred in large areas alsewhere which were not studied.
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No' investigation was made at Bodega Head. Nevertheless, the. data,  ;

particularly that from the Point Reyes Peninsula, can be used as a very general guide to the expectancy of fault displacements' at j I

various distances 1 rom the main fault zone during some future earthquake.

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In general, the 1906 bedrock ruptures were reported by G. K. Gilbert

' r to increase in abundance and amount of displacement towards the San -

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I Andreas fault zone. They occurred in the Point Reyes area as far as q

.' f p,2 10 miles west of the San Andreas sone, but the ones farthest out were

~_ i barely discernible. At distances of a mile, horizontal displacement j m l

{ of 2 to 6 inches was observed. At Inverness, about 2,000 feet from the  :

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sone a horizontal displacement of 2d feet was measuzwd. l b The geologic setting of Bodega Head is similar to that of Point I W l m Reyes Peninsula. The granitic rocks of both areas bound the western a

b.f.' edge of the San Andreas sone and both bedrock masses are pervasively

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fractured and faulted. Topographic differences between the two areas -j h R@

are pronounced. Point Reyes is higher, thereby intercepting more rainfall; it is larger and contains a better-defined system of stream

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hc drainage. The topographic differences are considered to be of secondary g-

[.. importance insofar as predicting future faults is concerned. The two I. areas would be expected to react similarly to.the stresses culminating.

1 in w or earth unkes.

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,- The probabilities of' displacements on Bodega Bead are. estimated j

, i in the fol. lawing tabulation. ' It is assumed that a severe earthquake, ')

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say of Richter magnitude. 8 5, has 'its epicenter in the San Andreas l 4

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' ! fault zone in Bodega Barbor. The conclusions are only qualitative j s . '

! and perhaps somewhat subjective but cannot be refined from available 1 knowledge.

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.gj. Displacement on a fault Probability L in granitic rock of shaft U., ,I i

LZ 2 inches or less Moderate to high l i N

%  : Approximately 1 foot Lov i k . \

pf l Approximately 2 feet Lov, lower than above, e but still a possibility E  ;

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1 Approximately 5 feet Remote I

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g-From general observations, it is clear that the 14k=14haad of li ME occurrence and the magnitude of sympathetic faulting outside of a  ;

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[". . - major earthquake fault none decreases with distance from the fault M

scoe. From observations of sympathetic faulting in bedrock which s ,

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accompanied the 1906 earthquake, the probability of displacements F- of as much as one foot appear to be remote at distances of more than a.

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3 or so miles from the aan Andreas fault zone.

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