ML20234A844

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: IA-85-665, Discusses Seismic Design of Nuclear Power Plants,Including Const of Nuclear Plants in Seismically Active Zones ML20234A761 ML20234A769 ML20234A792 ML20234A798 ML20234A803 ML20234A812 ML20234A816 ML20234A827 ML20234A830 ML20234A838 ML20234A841 ML20234A844 ML20234A847 ML20234A851 ML20234A855 ML20234A858 ML20234A861 ML20234A864 ML20234A868 ML20234A871 ML20234A874 ML20234A893 ML20234A896 ML20234A900 ML20234A911 ML20234A922 ML20234A929 ML20234A932 ML20234A937 ML20234A944 ML20234A956 ML20234A965 ML20234A968 ML20234A976 ML20234A984 ML20234A993 ML20234A996 ML20234A999 ML20234B005 ML20234B008 ML20234B023 ML20234B032 ML20234B035 ML20234B040 ML20234B062 ML20234B066 ML20234B076 ML20234B084 ML20234B098 ... further results
References
FOIA-85-665 NUDOCS 8709180152
Download: ML20234A844 (5)

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[g UNITED STATES 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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Abstract c

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,l This report summarizes and interprets the geologie data presented i.

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

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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 because geologic knowledge of the phannmana being evaluated is incomplete, e

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and in some instances rudimentary.

I' An upper limit on the probability of faulting is set by the probabil-P f-b ity of occurrence of severe earthquakes (Richter magnitude 8.0 and above)

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

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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

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of possible displacement must be based largely on the distribution and asI characteristics of the surface faulting produced by the 1906 earthquake

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

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

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adequate to suggest more than a general statement of probabilities.

s4 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.

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

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

g-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

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been traced downward from the surface through Pleistocene sediments

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into the underlying granitic rocks. It strikes N. 400 E. and has been f

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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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~ The Shaft fault in the bedrock is m' sons that ranges from 2 to lo

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feet in width and consists of several intersecting faults.. This-4

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suggests that movement on the fault occurred several times,.though the 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+=

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,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 =:

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dated sediments, and any be younger than sediments that are not displaced, I

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for in the soft sediments displacement-may be taken up by plastic K+

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deformation rather than rupture.

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Surface ruptures created during the 1906 earthquake have been -

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

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

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

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

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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l 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

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

t They occurred in the Point Reyes area as far as q

I Andreas fault zone.

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10 miles west of the San Andreas sone, but the ones farthest out were 2

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

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

Ed. i sone a horizontal displacement of 2d feet was measuzwd.

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b The geologic setting of Bodega Head is similar to that of Point I

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Reyes Peninsula. The granitic rocks of both areas bound the western m

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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

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'* :h are pronounced. Point Reyes is higher, thereby intercepting more R@{

rainfall; it is larger and contains a better-defined system of stream hc drainage. The topographic differences are considered to be of secondary g-

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importance insofar as predicting future faults is concerned. The two I.

areas would be expected to react similarly to.the stresses culminating.

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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 k

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P fault zone in Bodega Barbor. The conclusions are only qualitative j

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and perhaps somewhat subjective but cannot be refined from available 1

knowledge.

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Displacement on a fault Probability L

in granitic rock of shaft U.,,I i

LZ 2 inches or less Moderate to high i

N Approximately 1 foot Lov i

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pf l Approximately 2 feet Lov, lower than above, e

but still a possibility E

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Approximately 5 feet Remote I

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

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major earthquake fault none decreases with distance from the fault

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scoe. From observations of sympathetic faulting in bedrock which s

h accompanied the 1906 earthquake, the probability of displacements f:

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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