ML20234A844
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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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