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I , t t 1 - i UNITED STATES - i DEPARTMENT OF THE INTERIOR c "fs , Geological Survey J V ,g, cd
.' Washington, D. C.
t l i 1 ENGINEERING GEOLOGY OF THE PROPOSED NUCLEAR POWER PLANT ON BODEGA HEAD, SONOMA COUNTY, CALIFORNIA b7
'}\ Julius Schlocker and M. G. Bonilla - g lI U. S. Geological Survey I
i ' October 1964
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i I N. Prepared on behalf of the U. S. Ato=le Energy Com:nission and approved for public release by the Director, Geological Survey 8707240000 051217 PDR FOIA r FIRESTOB5-665 PDR --
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- UNITED STATES i DEPARTMENT OF 'lHE IRTERIOR
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} Geological Survey -
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l , . ENGINEERING GE0IMY OF TH.E PROPOSED NUCLEAR POWER PLANT
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, ON BODEGA HEAD, SONOMA COUNTY, CALIFORNIA by l
I 'i L Julius Schlocker and M. G. Bonilla U. S. Geological Survey
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, October 1964 I I l -i ifl l'
l'j i r p. t ,I} ( l1 Prepared on behalf of the U. S. Atomic Energy Cnmmission I and approved for public release by the Director, Geological Survey I i
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. CONTENTS., .
Page - Summary statement . . . .:. . . . . . . . . .-. . . . . .. . . 1-Introduction .......................... :6
,- ' Geologic setting . . . 1 .' . _. .1 . . ~ . .. . . ...... 8
. Granitic rocks'of Bodega Head . .- . . ..' . . . . . . . . . . 8 a '.
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Joints and faults . . . . . . . . . . . . . . . . .. 9 ,
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Weathering . . . . . . .. . . . .. . . . .' .. . .. .' .. 11 - k..~..~..
; Pleistocene and Recent deposits . . . .-. '. . .
12' Fossils and age . . . . . . . . .'. .-. . . .l. . .. 13 - s g.
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. Shaft fault /
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Type and angnitude' of novament on Shaft . fault .q,. . 15. ,
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Age of' movement of Shaft fault'in sediments'... .'. 16L i Origin of Shaft fault . . . ... . . . . . ... . . . . 17 Shaft fault and' San Andreas fault zone . ..... 19 Surface ruptures adjacent to San Andreas fault zone . . . . . 21 Future faulting on Bodega Head . ' . . . . . . .' . . . . . . . . . 27 References ......................... 31 e 4
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, ILLUSTRATION-(Atendofreport) .; .
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Figure 1.--Bottom'of shaft showing faults and' probable offset on
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fj , 3 J* ENGINEERING GEOLOGY.0F THE PROPOSED NUCLEAR JOWER PLANT J ~' ~ ON BODEG HEAD,SONOMACOUldY, CALIFORNIA. 4_ . by ,
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Julius Schlocker and M.'G. Bonilla ,
},) U. S. Geological Survey i
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SUMMARY
,STA2EMENT
, This report summarizes ' and . interprets,.the, geologic data presented ' .
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in previous reports by the Geological Survey. These data' soar on' the
; possible effect of large magnitude ' earthquakes on the. foundation of the ^l ' .
proposed nuclear power plant on Bodega gend, California.
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The crucial geologic problem at the proposed plant' site is to predict ,
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the probability of a sudden permanent displacement; by rupt'uring, of thei I foundation rock of the reactor during an earthquake. . Any such prediction must be based to a great extent on experience in earthquake-affected regions particularly near the San Andreas and related fault' zones. .The degree of confidence assigned to such predictions 'is necessarily low . . because geologic knowledge of the phenomena being evaluated is incomplete.- An upper limit on the , probability of faulting at the site in set by l -% I the probability of occurrence of severe earthquakes (Richter magnitude 1 8.0 and above) along the San Andreas fault nahr the Bodega Head site. Several highly qualified seismologists have estimated.that the Bodega . li Head site will experience a severe earthquake in the next 50 years, the assumed lifetime of the plant. ,
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!' The principal hazards to the proposed plant from such a seismic .t event are twofold: (1) shaking of the ground due to seismic.vave '
s propagation, and (2) possible permanent displacement'of the foundation . rock due to faulting. The hazard due to shaking is being . investigated ^ by others,' including the Seismology Division, U. S. Coast and Geodetic ] e Survey. Prediction,of possible permanent displacement must be based i largely on the distribution and characteristics of.the surface faulting produced by the 1906 earthquake and to a lesser extent on. the distribu-a , tion of faults in the excavation for the" reactor and in thet entire' Bodega Head area. The evidence is not adequate ,to suggess more than j a general statement of probabilities. I= , The site is approximately 1,000 feet vest of the west edge of'the "
, activeSanAndreas'faultzone,whichisapproximately1-1/2mileswide
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I here. The main surface rupture'in'this vicinity during the 1906 earth-i quake took place.on the east side of the zone and'had a horizontal dis-l placement of 10 to 20 feet. Throughout Bodega Head, tectonic faults and joints are common in the granitic rocks; the most prominent ones trend northwest, northeast, and east. At the site, a principal structure is the Shaft fault, named from its exposures in the shaft excavated for
; the reactor. This fault, one of many tectonic faults in the granitic rocks, is the only one that has been traced downward from the surface
. through Pleistocene sediments into the underlying granitic rocks. It
, strikes N. 400 E. and has been traced on the surface a total'of about
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j'. The Shaft fault in the bedrock is a zone that ranges from 2 to 10 i i feet in width and has a measured displacement in-the granitic rocks of 24 feet horizontally. and in the sediments of'14 inches vertically. The '
'l fault zone consists of many. intersecting faults;~ this suggests that l~'
i 1 acvement on the fault occurred several times, though'thefamount.of
- I i vertical or horizontal movement during anyLone period of movement can- -
1 I N not be determined. The fault' displaces Pleistocene sediments dated I i from geologic evidence as younger than 400,000 years and from radio-- 3 l3 . m; e 9 3 e ~. +n..w. c ) active carbon as older than 42,000;ye.carsM.s,Itmayplso'havecaffected -j
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plastic deformation rather than' by, rupture. - I j Surface ruptures created during'the.1906 earthquake have been
'l' < 'i described at many localities outside of the San Andreas fault zone-1 (Lawson and others, 1908). The record of these events provides important i clues for predicting future earthquake phenomena on Bodega' Head. . Some of the observed faults parallel the San Andreas, others lie. at acute _ ,
angles to it, and still others are nearly' normal'to'it. ' i' . . The principal observations of ruptures outside'the San Andreas fault ~ zone after the 1906 earthquake were made at the Point Reyes Peninsula, i]J the San Francisco Peninsula, and the Santa Cruz Mountains; fat 11 ting , {, maylhave occurred in large areas elsewhere which were not studied. No 3 investigation was made at Bodega Head. Nevertheless, the data, particularly q
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3'O that from the Point Reyes Peninsula, can be used as a very general guide' I l
, to the expectancy of fault displacements 'at.various distances from the '
main fault zone during some future earthquake. 3 l
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. The 1906 bedrock ruptures on Point Reyes Peninsula vere reported by' l
G. K. Gilbert in general to increase in abundance and amount of displace-I ment toward the San Andreas fault zone. They occurred as far as 10 miles i
.! vest of the zone, but the ones farthest away were barely discernible. At -
j distances of a mile, horizontal displacement of 2 to 6 inches was observed. 1 At Inverness, about 2,000 feet from the zone,the reported horizontal dis-placementwas2-1/2 feet. The geologic setting of Bodega Head is similar to that of Point a l l a l Reyes Peninsula. The granitic rocks of both areas;b'ound the western edge of the San Andreas zone and both bedrock masses are pervasively fractured and faulted. The two areas would be expected to react simi-
; larly to the stresses cubninating in major earthquakes. -
I . 1 In the Santa Cruz Mountains, 100 miles southeast of Bodega Head, a
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surface rupture approximately 1,900 feet from the' main rupture in'the - San Andreas fault zone showed a lateral displacement of four feeti The probabilities of displacements on Bodega Head estimated in the ; following tabulation are qualitative and perhaps somewhat subjective but ' available knowledge does not permit greater refinement. It is assumed that a severe earthquake, say of Richter magnitude 8 5, has its epicenter
- in the San Andreas fault zone in Bodega Harbor.
- l
. Displacement Probability '
l lj, 2 inches or less Moderate to high Approximately 1 foot Lov
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l-) . Approximately 5 feet Remote - ' I, li il l \ ~ l ' ~
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,a j accompanied the 1906 earthquake,' the probability of displacements of -.}
T1 as much as one foot appear to be remote at distances of more than
.t l about 3 miles from the San Andreas fault zone.
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*I IFIROIRJCTION '
-l ' .gn j This report summarizes and ' interprets the geologic data presented-y 8 t
-] intheGeological^ Survey'sreports'TEI-837(Schlocker,Bonilla,and Clebsch,1963, Part I; Eaton,1963j Part II) and TEI-84 (Schiocker and
';h ~ Bonilla,1963), prepared for the .U. . S. Atomic Energy Commission in con-e nection with an application by.the Pacific' Gas and Electric Company for. '
4. j- a license to. build and operate a nuclear power plant on Bodega Head, .'s l peninsular promontory on the Pacific coastline about 45 miles northwest of San Francisco, California. The;1ocation and' geology of the shaft for I the reactor foundation is shown on plates 1 to 4 of TEI-BM. ' 4 - Geologic data obtained during field investigation in May and June,. I .
]
r 1963 are given in TEI.837 Results'of field investigations mostly in. l l connection with the excavation of the shaft for the reactor foundation, ' l between July and November,1963 are given in TEI-84. The' shaft is approximately 140 feet in diameter and 75 feet deep (TEI-84, pls. 3 and' h)' . During September and October when the excavation of the shaft proceeded
, on a 24-hour a day basis the writers were able to study and map exposures of critical areas. Additional geologie data vere obtained in the shaft on March 18, 1964. !
i The crucial geologic problem at the proposed site, which is approxi-1 . j mately 1,000 feet west of the west edge of the active San Andreas fault
- i j" zone, involves an estimate of the probability of sudden permanent dis- -
j j, placement by rupturing of the foundation rock of .the reactor during 'an L Q.i earthquake. Additional hazards that must be considered in evaluating the
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.i suitability of the. site are related to shaking during the earthquake. - !
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7-y:gS wmpyMW9K*iyJWff6@shjI54i%%%M,y%N"."Ykfk %#T4%y . y s .., . The brief historical record and theoretical consideration (Benioff, 1964, p.1,400) indicate at least one strong earthquake may occur on the e ; l San Andreas fault zone near the Bodega Eead site during the assumed 50-year lifetime of the plant. The plant should, therefore, be designed j
)l . I -l* to withstand de. mage from faulting and seismic vibrations.
The accelerations, amplitudes, and frequencies of seismic waves likely to be encountered at the site are being estimated by experts in
' engineering seismology, including the Seismology Division of the Coast 1
and Geodetic Survey. 'N' ' i The granitic rock foundation on the floor of the present reactor i foundation shaft is generally capable of supporting heavy loads. l
- Laboratory tests show that the ultimate, unconfined compressive strength 4 l
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- i of granitic rocksin the shaft rangestrem 1,037 to 16,800 pounds per
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square inch. Most of the rock on the present floor of the shaft probably has strength properties in the upper two-thirds of this range. Nevertheless, it is recommended that the foundation preparation include removal of the soft and plastic rock along faults to depth prescribed by standard ' engineering practices. ! i l-
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GEOLOGIC SETTING-The proposed reactor site is on Bodega Head, a peninsula consisting
! of a body. of granitic rocks partly covered by Recent and Pleistocene .
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sediments. Bodega Head lies along the vest border of the San Andreas i 4
't fault zone which is about 1-1/2 miles vide here. At the site the shaft l 4
for the reactor is approximately 1,000 feet vest of the vest edge of the ct zone and 6,800 feet vest of the main surface rupture which formed during 1 , l the 1906 earthquake by right lateral displacement of 10 to 20 feet.' The y Recent and Pleistocene unconsolidated sediments consist of interbedded nearshore marine, beach, dune, marsh, stream, and slope debris deposits and are as much as 180 feet thick.
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The site is on a buried valley system that was eroded in the granitic ,
.. rocks by a surface stream and its tributaries when ' sea level was relatively '
lower (TEI-BM , pl. 2). The valleys were subsequently filled with marine , and continental deposits. The main buried valley crosses Bodega Head in a more or less east-west direction. At its deepest point, on the east shore of. the Head at Campbell Cove, it is more than 80 feet below sea level (TEI-8 4 , pl. 2). i: Granitic rocks of Bodega Head j 1 ! li
.l The granitic rocks,of Bodega Head are mostly a foliated coarse-grained .I ll .
1 - biotite-hornblende quartz diorite and minor coarse-grained hornblende-1 biotite quartz menzonite. Pegmatite and splite dikes as much as 7 feet in L. j thickness and dark granitic rock inclusions are common. A leucodiorite dike ; i - with a maximum vidth of a foot cuts the granitic rocksof the shaft. E 8
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'i The granitic rocks of Bodega Head are generally severely jointed b.
and faulted. Most of the rocks are cut'into 3- to 5-inch blocks, i.
.l but some rock is cut into 1 to 2-foot blocks, and rarely a mass *'
[. of rock has a joint spacing as great as four feet. The blocks are l comanonly bounded by prominent sets of parallel joints and also by i . l, irregular, curving, and branching joints. ' Prominent joint trends are ' l l northwest, northeast, and east. FaultsareabundantinkhegraniticrocksofBodegaHead. The l widest and best developed faults are zones of sheared rock about three to ten feet wide and are found about 100 to 300 feet apart. Between the vide shear zones' the rock contains a great abimaance of narrow
, fault zones, which generally are spaced only one to two feet apart..
Fault characteristics in the granitic rocks vary. The numerous, i closely-spaced, narrow faults are sharp, clean breaks, or consist of 1/4-to2-inchwidezonesofplasticgouge(clayeymaterialofpulverized and chemically altered rock), breccia (coarsely broken rock), .or nyl'o nite (pulverized, but firm rock). The vider stronger faults consist of complexly interrelated zones of gouge, breccia, and sylonite. Each zone
; is as much as two feet wide. Much of the brecciated and nylonitized rock is also altered chemically to a clayey rock that can be easily i
j- broken by hand. The granitic rocks from widely separated localities on i ea 1. Bodega Head are highly sheared on a microscopic scale as seen in thin I sections of rock specimens., i 1 9 N
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Direction of movement on the faults is revealed by offset of dikes ' j and dark inclusions and by slickensides on fault surfaces. A detailed study of direction of fault movement and its relationship with fault ' orientation was not made. The writers, however, have the general impres- g, i sion from a preliminary study of the offset of markers that reverse j faulting is more enmme than normal faulting. The trend of slickensides '
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on fault surfaces ranges from vertical to horizontal. On different t fault surfaces within a single fault zone, they are diversely oriented.
~l The great abundance of faults in the granitic. rocks of the reactor l l shaft is believed to be typical of the gran1 tic rocks of all of Bodega i
Head. The distribution and orientation of faults seen between elevations i~ -66 to -73 feet on the perimeter of the shaft (fig.1) are also believed to be typical of those in the granitic rocks exposed at higher el' evations I
; during excavation of the shaft. After rock debris was removed from the final floor of the shaft, many of the faults, exposed previously only on the valls, were traced along the strike for many feet.
To judge from reconnaissance field observations the granitic rocks of Point Reyes Penins'ula appear to be as pervasively fractured and sheared as those of Bode 6a Head. Weathering l .
, The granitic rock generally is mantled by 5 to 30 feet of weathered rock and soil. The soil is a mixture of sand, clay, silt, and gravel.
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. Fossils and age l
., The topmost 5 to 10 feet of sediments, mostly soil and vindblown l sand, are believed to have accumulated within the last 10,000 years and
,~ are designated as Recent. Fossil vood is abundant in the underlying i
f,: sediments. Radiocarbon content of wood collected from these sediments j near the shaft at elevations 77, 55, and 49 feet above sea level indicates i . the sediments are older than 42,000 years. The fossil flora in the sedi-ments, 5 to 25 feet above sea level, is similar to the fossil flora found in the sediments exposed in the headlands on the northeast shore of Tomales Bay, 8 to 20 miles southeast of Bodega Head. This similarity l indicates the sediments 5 to 25 feet above sea level on Bodega Head and o g. i those at Tomales Bay were deposited at about the same time (written l communication, Jack Wolfe, Paleobotanist, U. S. Geological Survey). Based on invertebrate fossil data, the beds at Tomales Bay are considered to be younger than the folding of the rocks of the Merced Formation in the San Francisco Peninsula area; Louderback (1951, p. 86) estimates the folding occurred from 240,000 to 400,000 years ago. The radiocarbon and fossil data indicate the sediments between elevation of 77 feet and 5 feet are older than h2,000 years and probably younger than 400,000 years. i
'! . Shaft fault
- The Shaft fault is one of many faults that cut the granitic rocks of l
the shaft area (fig. 1). It is one of the large faults in the shaft and is the only one that has been traced from the Pleistocene sediments down-vard into the underlying granitic rocks. The Shaft fault is a strongly-developed 2 to 10 foot vide zone made up of broken and chemically altered 13 l
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Ej; . . , j 3 l' rock (TEI-84,pl.3,4). Witbin the zone there are many faults and j 1 i l shear zones of varying vidth. Some of the shear zones are as much as ! ia i
! one foot vide, and are made up of soft, slickensided, clay gouge whereas .
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.i l' - others are tight and paper-thin. Other faults in the granitic rocks of f
.l l the shaft have similar characteristics, though most but not all are not . ,
j as wide as the Shaft fault. ' 1
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The direct connection between the Shaft fault in granitic rocks ! J and in Pleistocene sediments is well established on the south vall and i ... .- ,
, .. 1 was observed on the temporary floors, formed during excavation of,the
. shaft. The average trend of the Shaft fault is N./.40 E.(for' location 1
see ple. 2, 3, 4, TII-8 4 ). In the granitic rocks it dips 65 to 80 v. ) and in the overlying sediments between 500 to 850 E. and TO W.(TEI-8M, ,
. pl.4).
'i An en echelon branch of the Shaft fault in the sediments was followed from a point 2 feet southwest of the shaft collar at approximate elevation l
-5 feet to a point 170 feet southwest of the shaft at approximate eleva-tion 21 feet. The fault was not seen in trench 3 (TEI-84 , pl. 2) at elevation 51 to 52 feet, and 250 feet southwest of the shaft. The Shaft fault was not seen in the sediments exposed on the embankments, about
' ..;i 130 feet northeast of the shaft. In the shaft, the fault is not exposed i j locally in the gray, massive, gravelly sandy clay on the south wall i (TEI-BM , pl. 4) nor on'the northeast vall.in these sediments. j . o
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o l l , Type and ma..;nitude of movement on Shaft fault i l
- The field evidence indicates that right lateral movement of as ;
}.. '
l much as 24 feet in the granitic rocks and possibly as much as 3 feet I a- in the Pleistocene sediments occurred on the Shaft fault. In the j granitic rocks the broken segments of a leucodiorite dike, exposed ] on the bottom of the shaft, suggest that the cumulative horizontal ) 1 3 l
] component of movement was at least 2k feet in a right litteral sense, l l' , l
! that is, the ' dike southeast of the fault is found 24 feet southwest j i
ofitslocationontheothersideofthefault(*1g.1). Two faults, l about 2 to 6 feet vest and possibly part of the Shaft fault, offset j i{ 1 a the leucodiorite dike more than 3 feet in a right lateral sense (fig.1). I m: , l
.] Though no evidence was found of vertical movement on the Shaft fault in j l a the granitic rocks such movement may have occurred. ,
I
\
In the shaft the measured vertical separation of beds in the f sediments across the Shaft fault was 14 inches. An apparent vertical l separation of 19 inches was found between elevations -32 and -29 feet, but this amount is uncertain because the correlation of beds across 1 the fault between these elevations is not clearly established. i The movement of the Shaft fault in the sediments also had a hori- , zontal component as suggested by: (1)differenceinthickneseandsuc-cession of beds across the Shaft fault as seen on the south vall of the ! - + shaft, in trenches Nos.1 and 2, and in the 20-foot embankment between the trenches; (2) random variation in magnitude of apparent vertical I ( offset of beds across the Shaft fault; and (3) changes in dip of the
- 1 . .
,j fault, particularly the r16ht-angled bends of the easternecat and highest t l l 1 15-1 l
- m. a. m. .w. v:gn= ~ v ,r -cwyvmivra.m. _ _ _ _ _ . . _ _ _ _ _ _ -
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j , branch shown on ylith 4, TEI-844, near vertical control line 39 at '
)
p". elevation -14. Dip-clip (vertical) movement on faults probably would o j 1
-l not result in th)n sharp edges of the angular blocks along the fault, ttj
- p. .c e
~
f._,.j but would tend to break them forming a'amooth surface with no keyed-in - 'j= ' j interlocking projections and hollows'as an, found alotvi the dip direc-- r v.n: ,. tion on the fault surfdce. The total horizontal disp 1Acement on the '
.M> + . '
6 .
,i, .. . ,
y 3,i
'Sbatt fault in the sediments cannot be measured. However,.from known -1
- w 1,u _ w L. 1 vertical displacement of 14 inches together with the geologic. evidence 1
for a horizontal ccaponent of movement, it is not unreasonable to. 1 i postulate a total horizontal displacement of between one and three I feetinthesedimentI.
-i s
~1 i
Age of acvesunt cf Shaft fault in ' sediments c:
'mn .
2 Geologic field data are used to determine the approximate age of-i the Shaft fault in the sediments. The age of faulting is less then ) l that of the sediments which are fats.ted and greater than that of sedi-l ments deposited over faulted sedi:nents. Thus, if age data d .he faulted
- <. .. i and overlying sediments are known it can be used to set limitt, on the l
' j i time of the faulting. The geologic evidence indicates the Shaft fault /
,y displaced sediments that are more than 42,000, but less than 400,000 i
\ !G . years old. Thus the last movement on the fault which can he definitely. l q j dated took place some time during the last 400,000jaars. Because the M
,, In an earlier calculation 13 feet (TBre8)hp.). 24) was obtained by .
l assuming that the right lateral movemen't on beds dipping $0 south. . i l'd westward was horisontal. a f
,. }
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3 ,. , 3 aa - possibilityexiststhatthefaultingM[currddduringthe'pastfewhundred. e i 7-l ~, &.' ...s J
- i. years,.it is prudent to predict thatJfaulting i's y possibility at the.' site. '
i, j. . . p E, g.' during the r. ext. 50 to 200 years. *
. . g. e y}.
The displacement.across the Shaft fault' in the sediamants may represent . g [- only one and perhaps the last episode of faul;cing. . If this is true' we'any ? ,
- t. .
i...# W
. conclude that there has been only one episode of faulting on'the Shaft '
yj1 - 4 J ) m# fault in more thanik2,000" years, or possibly in as 1cas as 400,000 years. 7 ; , , 1;) Note that this conclusion is also valid if the faulting took place only a ;J L, , few years ago.. j f . ,
?, 1 y ..
Origin of Shaft fault i , s 'r. Because of its bearing on recency of fault movement, the origia of
,j the Shaft fault in the W,*ments is fundamentally important in thte eva$.
'l.d .
untion of its significance to site acceptability and plant design. , c 'r
) Tectonic faulting is considered to be the most' probable origin.3) 'Other 4 l
.., g,*
- mechanisms contfdered, but re,jected, are landsliding, subsidence free compaction of sediments, or lurching caused by seismic waves.
".A 3 y, j l / 1 l M Tectonic faulting is used here for rupturing sad movement related I l ,
to crustal stresses such as those thst produce 1 the San Andreas ' fault zone and the 1906 earthquake, in contrast to local stresses c i
]+ . that produce landslideau , ,
y *~ N t .. / L) '- N Faults other than the Shalt fault were found in sediments near the , shaft and may be caused by in.ndsliding. . One fault,'about 35' feet 'J south of the south rim of the shaft, trends N. 70 0 W. Several interrelated faults, about 100 feet ne.,rtheast of the northeast rim .
- ( on the south cut of the ramp leading the shaft, trend"about N. 550 W. .
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p" g t. j p, . 4 g 1 x 1 l, The continuity of the Shaft fault from the Pleistocene sediments into [. I. the underlying granitic rocks and the close coincidence in trend of this '
=[' fault iri both b types indicate they were formed by the same geologic '!
,. process. In the granitic rocks features such as great vidth, internal
, textural complexity, and straightness are evidence for a tectonic origin
+
l. c;i of the Shaft fault. l The absence of the Shaft fault in trench No. 3 (TEI-84, pl. ' 2), l lN about 250 feet southwest of the shaft, can be attributed to dying out 1 , i of the fault upward and/or laterally, or to deposition of younger sedi-l 4 ments after fr.ulting. j l Tectonic faults of interal mo,*ement such es those pzeduced by the l , .)
!'- 1906 earthquake are characteristically discontinuous, en echelon, or
',j la branching (Richter,1958, p.178-181). The surface ruptures associated
- s. ,!
)
~
L vith the 1906 e ethquake have been studied extensively and excellent ' I l' descriptionsaregivenbyLawsonandothers(1908,p.70-72forthe s Woodville area 27 miles southeast of the reactor sit,e, and on p. 63-65 _j and map 3 in the atlas for the Fort Ross area 18 miles north of the rer.ctor site). In some of these areas the main surface rupture zone is expressed as en echelon cracks whereas in others the surface ruptures g are discontinuous, subpr.rallel, and several hundred feet in length.
- ) .
J They have a few feet of horizontal offset in their central portion but J.)
,.} no offset at their ends. Ivavson (1908, p. 53) describes the en , 'l echelan and branching nature of the 1906 ruptures as follows:
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' q "The width of the zone of surface rupturing varied usually from - , !
a few feet up to 30 feet or more. Not uncommonly there trere i auxiliary cracks either b' ranching.from the main fault-trace ' l obliquely for e few hundred feet or yards, or lying subparallel j
~
to it and not, so far as disturbance of the soil indicated, J directly connected with it. Where these auxiliary cracks were
~ '
i- features of the fault-trace, the zone of surface disturbance j which included them frequently had a width of several hundred.., 't i feat. . The displacements appear thus not always to have been N confined to a single line of rupture, but to have been distrdb-4
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, uted over a zone of varying vidth." '
') i
, The discontinuous, en echelon nature of the 1906 ruptures by 1
l analogy, therefore, may explain the absence of the Shaft fault in the I i sediments in trench No. 3, at elevation 51 to 55 feet, 250 feet south-west of the shaft; and in the sedimants in the embanfcnents, at approxi- f 1 mate elevations 2 to 25 feet 50 to 100 feet northeast of the shaft. In
]* these localities the Shaft fault may die out upward and laterally rather l l 1 ! than by being covered'by sediments. * ' .i .
Locally the apparent absence of the Shaft fault in the sediments on the shaft's south vall between elevations -23 end .26 feet, and above the granitic rocks on the northeast vall is attribt3ted to rehealing of the massive and structureless, gravelly, sandy clay. The clay may have l been shee. red but the visible evidence for shesring was concealed by ' subsequent plastic defomation. i 1 1 Shaft fault and San Andreas fault zone . t
. s The San Andreas fault zone is a more or less vertical complex of faults that trend northwestward and whose major movement has been right '4 lateral. The Shaft fault strikes N. 40* E. and dips steeply vestward.
s 1 - Field evidence indicates that it probably also has right-lateral movement.
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v %)f4 %' & A Q N NINP$1Y"l$5 SENN 1b f $ fE W WNUff.0$$$hsff,hyhf.5$k?f4l= Y y gm f.i . . i l t Activity of the Shaft fault at least once during the last 400,000 years
,-) strongly suggests its contemporaneity and close genetic relation with O q 4
1 the active San Andreas fault zone which is only~1,000 feet to the east l j t*. j and along which movements have occurred for millions of years. L. ;i. .
;i! . The presence of the Shaft fault indicates that local strain release .,
n. 4]l .. in the vicinity of the shaft in the past favored a northeast direction m though the presence of numerous other faults across the bottom of the C
;'l shaft indicates that some strain release episodes also favored northwest
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SUEFACE RUPTURES A WACENT TO SAN ANDREAS FAULT ZONE l< The characteristics of faults adjacent to the San Andreas fault
- zone were investigated to obtain empirical data that would aid in eval.
'i unting the probability of future surface rupturing at the proposed site
- 4 on Bodega Head. Some of the most meaningful evidence was that obtained
.a ', from the work of geologists who studied the fractures resulting from the 1906 earthquake. Surface rupturing of tectonic ori6in, formed during the 'l 1906 earthquake, was described at many places vest of the San Andreas i fault zone, but principally at the(Point'Reyes Peninsula, the San Francisco Peninsula, and the Santa Cruz Mountains.' Faulting may have occurred else-where in many areas adjacent to the San Andreas fault zone, such as Bodega Bead, that were not studied. Because the record of events of the 1906 l earthquake is the only e=pirical information that affords clues to future t .
earthquake phenomena in the Bodega Head area, the writers examined some of the localities described on nearby Point Reyes Peninsula and in the Santa Cruz Mountains southwest of Los Gatos. The Point Beyes area is { especially pertinent because the granitic rocks are pervasively fractured and faulted similar to those found on nearby Bodega Head. The ruptures at some of these localities, which are farther from the San Andreas fault l zone than is the proposed reactor shaft, appear to be of undoubted tectonic 1 1 - origin whereas for others the evidence was insufficient to determine i their origin.
;s The State Earthquake Investigation Com=ission report (Lawson and others, ,1908) contains numerous descriptions of rupturing outside of 4
- the San Andreas fault zone. Ruptures were abundant at some of the localities. For example, Gilbert (p. 75) reports: " Bedrock cracks I
1 . 4
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( g .- . . c; 1 . j - occurred at many' points within the Rift, usually appearing as branches I
-1 from the faults. . They were seen also at a number of. points vest of the l .
J* Rift, their distribution' reaching to the ocean in the vicinity of Point
~
.j 1
1
,t d, '
Reyes,10 miles _from the fault-trace. At the most remote points they ,
-N vere quite amm11, often barely discernible, and no system of arrangement '
w;% Y was discovered. They are peculiarly prominent along the summit of the
- 3 .
(, ridge constituting the southwestern rim of the main Bolinas-Tomales ;
;!f j trough. This summit was visited on four lines of road, and at each l3 ,
Id locality, conspicuous cracks were found. On the road from Inverness to
- 1. 6 l Point Reyes Post Office, about a mile in a direct line from Tomales Bay i
, (that is, a mile vest of the San Andreas fault zone) a crack was traced l l
for more than 800 feet. Its general trend is east and west, but its r . course is not straight and it has a branch diverging at 450 Along
.. 3: .
- +
this crack there is a horizontal' throw of from 2'to 6 inches . . . ." l (Underlining and parenthetical note by authors.) -
' l In the town of Inverness, Point Reyes Peninsula, the writers ,
l l examined part- of a surface rupture fomed in 1906 and described by . i Gilbert (Lawson and others,1908, p. 69) as "an outlying or branch i t fault-trace about half a mile long." This rupture is on Inverness Ridge J' at.a point about 2,000 feet vest of the west edge of the San Andreas l i fault zone (T U-0 % U l.01). At the top of a ridge the writers observeded ,.-{ p
, the rupture to consist of two scarps, 3 to 4 feet high that trend about N.18 W. and bound a amall ridge.10 to 25 feet vide. The scarps expose af 4
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;. 1 i surficiel slope debris estimated to be 10 to 15 feet thick. Gilbert j
. J
; stated (Lawsonandothers,1908,p.69)thatthehorizontaldisplace-
,~
ment here was 2-1/2, feet.~ The ruptures in the surficial material are < j i , { g
!* believed to be tectonic in origin and vere caused by a rupture in the j
,. - 1
..a underlying granitic bedrock, possibly an old fault.
U l
.; Bedrock ruptures formed during the 1906 earthquake on the north-east spur and on the vest slope of Mount Wittenberg, Point Reyes Peninsula, 4-1/2 miles sectheast of the Inverness locality previously described.
The ruptures on the northeast spur trend northwest, and are one mile i vest of the vest edge of the San Andreas fault zone. According to i Gilbert (Lawson and others,1908, p. 75) a ridge 3 to lo feet vide and ,
, up to 1-1/2 feet high was produced by horizontal faulting. Though the i j
j .j- < writers were unable to locate the ruptures at the locality given by 1 i J l Gilbert, it was observed that the spur is broad and its crest is an unlikely place for landslide movement. The bedrock rupture on the "
)
( 1 vest slope of Mount Wittenberg,1.3 miles vest of the San Andreas . fault i
! zone also trends northwest, and according to Gilbert it was traced on '
i.he uurface Ior about 1,000 feet. The writers observed that the surface .
\
, of the main divide and the northeast spur consist of bedrock or bedrock
.i
] covered by 1 to 5 feet of slope debris. .
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7. yn ,%.~..,. 7 - , ]Qi Gilbert's (Lawson and others,1908, p. 76) opinion on the extent of bed. rock fracturing in connection with videspread modification of fiov of springs on Point Reyes Peninsula is expressed thusly: ,
"The spring phenomena and the visible cracks may be grouped together
' + as indications of bedrock fracturing, .and their distribution in-i dicates the regions in which the rocky foundation of the land was
'M more or less shattered. That, region includes the Rift and extends
^) from it to the ocean. The phenomena diminish somewhat with distance
. from the Rift, but the fracturing appears to have been important
,I and genersi through a belt 4 or 5 miles broad."
J In 1906 a 6,200-foot long railroad tunnel, 5 miles south of Los 0,atos, California, about 100. miles . southeast of Bodega Head, was offset five feet horizontally along a.rupt'ure that strikes N. 52 0 W., and dips 75 0 W.(LawsonandotheEa,1908,'p.111-113). This rupture , is probably the main one formed during the 1906 earthquake in the 1 San Andreas fault zone. Thett[nnelvanexcavatedalmostentirelyin j- bedrock. For 5,150 feet in the tunnel so'uthwest of the fault, rails I were bent, timbers crushed, ties broken and heaved, and the tunnel in places caved. The original tunnel line was also Senerally shifted so
! that at a point 4,000 feet southwest of the 5-foot offset the shift .
van 14 inches. Zones of severely crushed timber supports in the
, tunnel between 1,000 and 1,800 feet southwest of the 5-foot offset ] are believed to conceal bedrock ruptures along which the rocks moved i
i horizontally. l , About 0.6 mile south of the 5-foot break in the tunnel, a fence was offset approximately 4 feet in a left lateral sense along a surface e rupture formed during the 1906 earthquake. This rupture is approximately
. 1,900 feet southwest of the probable location of the main 1906 surface
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3 rupture. The San Andreas fault zone in this area is probably only a few hundred feet vide. The left lateral rupture is believed to be the one i 3 shown on a map, figure 75 of the Earthquake Commission report (Lawson andothers,1908,seealsopis.6hB,65A,p.276-277), that trends a ,
..?,
;j N. 30 to 60 W. for b feet and for an additional 225 feet as northwest-N trending en echelon cracks 10 to 40 feet long and 10 feet apart. These l',
ruptures are in surficial materials. The relationship of surface ruptures , to topography, as indicated by a field examination by the writers, led l to the belief that the ruptures formed mostly by fault movement in the underlying bedrock rather than by landsliding. 1 l l At Black Mountain near Palo Alto, 82 to 86 miles southeast of Bodega , Head, cracks were exceedingly abundant for as far as 1.8 miles east of the San Andreas fault zone in a wedge-shaped area between the zone and i 1 two vest-dipping thrust faults that trend about N. 70o W. The cracks ' 4 trended "in every direction"; some of the ' larger ones were several I hundredfeetlong(Lawsonandothers,1908,p.107-108). l In addition to ruptures created by the 1906 faulting, undrained topographic depressions were seen by the writers at many places along the San Andreas fault zone extending about 200 miles south of Bodega , 4 Head, and as far as a mile vest of the zone. These features appear to 51 i t have been created by tectonic menrement such as faulting possibl'y in the , last 20,000 years. i Such a feature is Mud Lake on Point Reyes Peninsula 1j (TEI-8%ppl.21) about 3,000 feet vest of the San Andreas fault zone. 3 &. l l , 1 . ' ,? . l1 - t 1 25
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7_ a It lies lin an elongate steep-walled depression probablyl formed by. {' subsicience,l estimated to be more than 40 feet,- of a block bounded by- i'- t-
. v faults. Similar opinions on such features have been. expressed by
~
ather geologists (Richter,19$8, p. 482). ." Eiggins'(1961, _ p. 57) '
- described the area between Point Arena' and Fort Ross, .18 to 59 miles .
l, northwest.of Bodega Bead,'as follower "
'". . . numerous saml1 ponds and depressions 'on the. rolling summit j? , of the ridge west of, the 1906 fault trace could only have been '
caused by small dislocations along faults.that are either part of, or closely related to, the San Andreas. Yet most of these depressions are at least half a mile southwest of the 1906 ' fault trace." ' The' surface ruptures formed'during the 1906 earthquake in' the B. rocks adjacent to the San Andreas fault zone vary widely in extent, - r W
,- amount of displacement and orientation. In general the' abundance and 1
.. j magnitude of displacement on these sympathetic ruptures' increases. J* .. s .. .. . . . toward the fault zone. Ruptures ha far as 10 miles west of the' zone in the Point Reyes area were " barely discernibl.a.". On Point'Reyes , about one mile west of the zone the horizontal displacement was 2 to 6 i inches. At Inverness only' about 2,000 feet from the zone; the horizontal' ' 1
! displacements were 2-1/2 feet an'd near Los Gatos a 4 foot bor'i zontal
~
j displacement was measured on a rupture about 1,900 feet southwest of j i j. a S the main'1906 rupture. Surface ruptures were generally along surface l expressions of old faults, such as scarps and sags. Lawson(1908, , ,
- p. 53-54), however, reported new scarps on slopes where no trace of a J'
previous scarp was detected. Data on the orientation of the ruptures
- 4 's formed in 1906 in the rocks adjacent to the San Andreas zone' indicate h"'
2
' that'the dominant trend 'is northwest.'
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e . k-r , - .,, . FITI'URE FAULTING ON BODEGA HEAD
\
I The great abundance of faults, many of which are strong er.d persistent, is evidence of a long history of repeated fault displacement in the granitic bedrock of Bodega Head. The marked development and preferred orientation of many of the faults suB8est recurrence of movement along certain favored
' t, ]
1 planes of weakness. Mcnrement on some of the faults took place millions of ! i l j years ago, but measured displacement in the Shaft fault indicates movement ;
-i continued at 3 east into mid-Pleistocene time. Whether the last movement I i
took place on the Shaft fault more than 42,000 years ago or more recently 1 i is not known, but there is no evidence to suggest a decrease in frequency l l of faulting in more recent time. Toth the time span and the magnitude of displacement on the Shaft fault (
; are of en61neering si6nificance. The cumulative horizontal component of move-ao -
] ment on the Shaft fault durin6 the past, presumably millions of years, is a l
minimum of 24 feet. Based on field evidence, the last large movement, prob-ably of 1 to 3 feet mostly of horizontal displacement, took place in the last 400,000 years. Probability calculations of future ground displacements at the site during the lifetime of the plant should not be made on the above inference
.h of one movement in the last 400,000 years, however, because ve know nothfng 9 .
of the past and present rate, magnitude, and pattern of strain accumniation in the crust below Bodega Head that ultimately leads to fault movement and 11, earthquakes. Indeed our lack of knowledge of the re61onal strain buildup t 0 e
.)
anywhere along the San Andreas fault zone is reflected in our inability i j a .
- to predict the time, ma6nitude, or epicenter of future earthquakes along i
I 1 e 9 - = I i ' *
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. p the zone. What is known about strain buildup and the earthquake record permits seismologists to estimate that approximately one severe earth-quake per century will occur on a segment of the San Andreas fault zone (Benioff, 1964, p. 1,400). Thus the Bodega Head site is almost certain ;
l to experience one severe earthquake in the next 50 years, the assumed lifetime of the plant. . In connection with earthquake prediction, Tocher (1959, p. 48) writes:
"The four yee.rs 1954-57 have see.n four strong shocks in the San Francisco Bay region. Whether or not these are the first of a series which vill continue for a number of years and culminate l in e. major shock remains to be seen. The historic record is too
, short and too uncertain in its details for such a definite state-ment; a major earthquake could hit San Francisco before these lines reach the printer or there might not be such a shock in the next hundred years.g' In lieu of information on, local strain accumulation, prediction
. u r ; r w e,., ,, .. e .e c!:tJt u,"f o ,,a .. :ced ~ 3 fyjy va t,h e t.i of possible displacement at the shaft must be based largely on the dis-l tribution and characteristics of the surface faulting produced by the 1906 earthquake and to a lesser extent on the distribution of faults in the excavation for the reactor foundation shaft and on the entire Bodega ,
Head area. Because some sections of the San Andreas fault zone were not examined after the 1906 earthquake and because .other sections were exam-ined hastily and only on the main roads, the evidence is not adequate to e a e g
# e 28 i
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8- . -- . ' ' - 7l r t . { sugEest more than a general statement of probabilities and it is used.
' 4 only as a very general guide to the expectancy of fault displacements l at various distances from the main San Andreas fault zone during a 3
future strong earthquake. Thus the probabilities based on it are ~ believed to apply to all of BodeEa Head and to a zone a similar dis-tance east of the San Andreas fault zone. i The probabilities of displacements on BodeEa Head are estimated in the following tabulation. It is assumed that a severe earthquake,' say'of Richter magnitude 8 5 has its epicenter in the San Andreas fault zone in Bodega Harbor. Displacementcomponentscanbehorizontaland/or vertical. The conclusions are only qualitative and perhaps somewhat sub-jective but cannot be refined from available . knowledge. .
- u. .y. % m.:,w i, m Ama 4
, n , Displacement 0.: _t probability w
2 inches or less *^* Moderate to high l l Approximately 1 foot Lov l
, Apprcximat'ely 3 feet Lov, lower than above, 1 but still a possibility Approximately 5 feet Remote The low probability assigned to displacements of approximately 1 to 3 feet is based partly on the inference that only one movement over this magnitude range took place on a fault in the shaft over a Lperiod
~
of time when several, perhaps thousands, of 1906 magnitude episodes of i fault-displacements and earthquakes took place on the San Andreas fault tone, some of them presumably close to Bodega Head. The site, however, i l e> ? e 29 I
, . - . . .; -- ~m 2&S~ ~~ A~ i; - * *
.~.-
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9 , . . - ', ;; . s.<.. q.~ , , , , . . .; . . t..; . i ' is approximately the same distance from the San Andreas fault zone as ' are localities for which displacements of 2-1/2 to Ifeet are reported for the 1906 earthquake; ,therefore, the possibility of a 2 to'3 foot . ,
- offset at the site abould not be ruled out.
t '
'{l From general obsenations, it is clear that the likelihood of. T i
'.l-' .
occurrence and the magnitude 'of sympathetic faulting outside of. a , 4
- major earthquake fault zone decreases with distance from the fault zone.
Freet observations of sympathetic faulting in bedrock which accompanied the 1906 earthquake, the probability of displacement of as much as one. foot appears to be remote at distances of more than about 3 miles from the San Andreas fault zone. e g i 4
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. REFERENCES .
j_ , , Benioff, Hugo,1964, Earthquaim source' mechanisms: Science, v.143, .
]
no. 3613, P. 1,399-1,406. I' '
.j[
Higgins, C. G.,1961, San Andreas fault north ~of San Francisco, ,
.1 --
Geol. Soc. Ameries Bull., v. 72,' no.1, p. 51-68.
~
, ,. j .,
.. California 3 -
'i - Lawson, A. C., and others,1908, The California earthquake of April 18,-
i 1906. Report of the State Earthquake Investigations Commission: ' , j l : Carnegie Inst. Washington Pdb. &T, v.1, Pt.1, 254 p.) Pt. 2,. l ..i . i P. 255-451)' atlas 25 maps and' seismograms. : Loudefback, G. D.,1951, Geologie history of San Francisco Bay: Calif. Div. Mines and Geology Bull.154, p. 75-94.
4 I
Richter, C. F.,1958, Elementary ' seismology:.- W. H. Freeman and' ' s y Compnay, San Franciseo, 768 p. ,
, m , .. . .- -
; Schlocker, J., M. G. Bam411a, and A. Clebech, Jr.,1963, Geologie and seismic investigations of a proposed nuclear power plant site on Bodega. Head, Sonoma County, California, Part I - Geologie investi-gations: Part II - Eaton, J. P., Seismic hazards evaluations: -
j
, U. B. Geol. Survey report TEI-837, 51 p.
l, Schlocker, J., and M. G. Bonilla,1963, Engineering geology of the
'I ' Proposed nuclear power plant site on Bodega Head, Soncma County L
California: U. S. Geol. Surrey report TEI-844, 37 p. ' 1 . e Tocher, Don,1959, Seisnie history of the San Francisco regica: Calif. Div.; tines 'and Geology Special Report 57, P. 39-48.' -: I , '
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, a . - ( 'S , e U. & Df 7A272Dff 0F THE INTERIOlt *. 4 e % " *
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b "j SEIS'.!!C ITY AND TSUNAMI REPCRT - BODE 0A HEAD, CAL!FORNIA
. c s ., m , ' ..' .i /.%]_ .' 2 0 3 3 i 5, The Division.of Licensing and Regulation of the Atomic Energy Commis-I sion, Washington, D. C., requested the U. S. Coast and Geodetic Survey to e
! report on the seismicity and tsunami ccnditien of Bodega Head, California.
t 3 This report contains an evaluation of the seismic condition of Bodega Head - l i as defined by the Pacific Gas and Electric Company in numerous documents* 1 l l f submitted to the AEC. In addition, the Survey presents an independent I l 1
. evaluation of the earthquake frequency pattern along the San Andreas fault, the most probable ground motions measured in acceleration and displacement for a magnitude 8.3 on this. fault near Soaega Head .nd the tsunami hazard j
at the same location. The Survey is in a unique position to perform this service because it has either the original documents or, a c:mplete file of f
! historical data for earthquake seismelegy, engineering seiscelc3j, and l
i l tsunamis and has mace stucies in these f ales for approximately 33-40 years. 1 In this repar. and all other geologic and seismic reports submitted to 1 the AEC relative to the proposed reactor at Bodega Head, frequent reference 1 is made to geological faults and in particular, the San Andreas fau'.t. This is to be expected in describing earthquakes because a fault is the only sur- -j i face manifestation of earthquake occurrence. Geologists refer to faults or earth fractures as active or inactive, depending upon the recency of move-ments. Active faults are associated with recent earthquake activity, such as the earthquake belt around the perimeter of the Pacific Ocean and across 8 9
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l Asia and along the Mediterranean Sea to the Atlantic Ocean. An example of an inactive or relatively inactive belt it. the Appalachian system in eastern s North America where there are extensive fault systems but only minor and in- 3 1 frequent tremors. l
, .e
~
The San Andreas fault,. which is of principal interest due to ,its prox- h i imity (within 1000 feet of the limits of the western zone) to the proposed ,
, . )
i > site of Bodega Head reactor, is considered by geologists and seismologists 1
,' to be an active earthquake source. This fault extends southeasterly from a 1
! point under the ocean about 300 miles frem the Oreron coast (approximately
! 450 north latitude,1300 west longitude) across California and under the i Gulf of California to a point across from the southern tip of Lower Califor- )
l
- i
) . nia. It is a right hand strike slip fault (i.e., motion is predominately 1
, horizontal) and it has been 'the source of two great earthquakes in historic 1
i I times (1857 and 1906). Other faults trending nearly parallel to this mas - l ter fault show evidence of right hand slip or horizontal displacement. 1 i Faults with trends roughly at right angles to the San Andreas are predomi-nately left hand type. l.
.i In Southern California there is evidence for accumulated shift of about~~
l 25 miles along the San Andreas fault since mid-Tertiary time. Some investi-gators believe much more horizontal motion has taken place. Movements along a
, the fault have been measured by the Survey in the vicinity of Point Reyes to Petaluma, San Francisco to San Jose, Hayward, Hollister, San Luis
[, . Obispo to Avenal, and others. At each location several geodetic lines were I measured across the fault and where movements were noted, they approximated 4 9
. N% , .'
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3 I . 3 an average of 2 cer.L. meters per year. The relative motion indicates ,he t
; west side of the fault is moving northward and the east side southward. .
l j The maximum horizontal shift observed after the 1906 earthquake was 21 3 j 1
.: feet at Tomales Bay and the vertical motion was no greater than 3 feet.
1 - 4 This is typical of the earth. motions resulting from California ear,thquakes d j i.e., greater horizontal than vertical displacements. Even though California and specifically the San Andreas fault are
, considered earthquake prone areas, they experience a surprisingly low
' number of magnitude 6 and greater earthquakes. There have been two " great" b
earthquakes (1857 and 1906) on the fault and approximately 20 earthquakes )
' 1 from 1800 to 1950 with magnitudes 6 to 7+ within 75 miles of this fault. 4
{ 1 i . Many of the largest California earthquakes have been followed by swarms j of strong aftershocks. Usually'the strongest aftershocks have magnitudes i' at least one unit lower than the main shock. The frequency of ine af ter-y shocks increases with a decrease in magnitude. ' As noted in the report submitted by Tocher for earthquakes felt at or near Bodega Head, 1838-1960, none of them were centered at Bodega. 'Of the 58 listed earthquakes,14 were reported felt at Bodega or alona Sodega Bay; 2 caused little or no damage; and 1 in 1906 caused appreciable damage and i some surface fissuring. Studies of the seismicity along the San Andreas fault for the past ,57 j years show that a magnitude 6 to 6.9 earthquake occurred every 7 years on i
- 1 .
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4 j .
-l the average and a magnitude 7. to 7.9 earthquake occurred once during this a'
period. These occurrences agree with the pattern determined by Gutenberg i in " Seismicity of th'e Earth" which precicts'a magnitude 8 earthquake about once every 100 years. . In discussing the frequency of high magnitude earth-quakes, it should be noted that many reoccur in the.same epicentrat areas. l Richter mentions 3 earthquakes occurred in Honshu, Japan at 390 north, . l l 1430 east, in 1897,1898 and 1905 with magnitudes from 7.9 to 8.3. .A num-1 i ber of areas in Italy experienced repeated damaging earthquakes, for example, G i r ifaleo (1626, 1659,1783,1905); Monteleone (1659,1783,1905); Gerace (1720, 1784, 1791, 1907). Accc,rding to Davis, Valparaiso, Chile was de-
, stroyed in 1822 and again in 1906. Skopje, Yugoslavia, of recent memory,
$ was totally or partially destroyed in 1963,1921,1555 and 518.~
l [ To design and construct earthquake resistant structures it is necessary
, to know not o..ly the above-mentioned seismicity infor a ion etc : destruc-
'ive eart? akes, but also the displacement, velocity' and acceleration of ground motions and the response characteristics of structures to these motions. Since 1933, the Coast and Geodetic Survey has made such measure-ments of strong earthquakes in the Western United States and in Latin I! America. All interested parties in this investigation are aware of this h.
f,
- work and the data collected have been used extensively by all. There is I
general uniformity in the interpretation of the direct recorded strong motion data at El Centro, San Francisco and Seattle. However,.there is some
*I .
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i, . . l 5 j 4 dispersion in the computed results when attempts are made 't'o extrapolate i from magnitude 6-7 earthquakes.to magnitude 8.3 (San Francisco,1906). l i l Extrapolations are necessary since no magnitude 8 or greater earth' quakes 1 4 have ever been recorded by strong motion seismographs.
; The strong motion seismographs operated at a number of locations in
{ California since 1933 have recorded several intermediate magnitude earth-i . i i quakes. Among those recorded were the Long Beach of 1933, El Centro of i j 1940, Kern County of 1952, San Francisco of 1957, and Olympia-Tacoma, i Washington of 1949. The greatest recorded accelerations were of the order of 1/3 g in Seattle, Washington, and the wave periods for the greatest j o accelerations were 0.2 - 0.4 second. Most of the recordi_ngs have been made
, by instruments on unconsolidated material with the exceptions 'of -the Golden Gate recording of the March 1957 earthquake yihich was on granite. To date, no recordings have been made within 7-8 miles of the epicenter so there are no experimental data for accelerations or displacements of ground motions
]
l within a mile or two of an earthquake epicenter. l I Through the use of these strong motion data in ecmputing response 1 . spectrum and in correlating intensities of strong earthquakes, the Survey
~
[ t i has estimated acceleration factors for Bedega Head. in addition to these l i - 4 earthquake computations, the Survey has performed similar experiments in
.I connection with nuclear explosions. Even though the source mechanism for i
j the release of energy by earthquakes and explosions differ, it is inter-esting to note that the accelerations of both are of the same order of 1 c 4 ..
-4
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magnitude. Results of the Survey's studies show that on rock, a maximum probable ground acceleration of 2/3 g at periods from 0.2 - O'.6 second
'E 'should be expected at Bodega Head and that ground accelerations as high
,. as.1.0 g in the same period range could occur and should be taken into t- .
t i account in the design of the facility.
'q i
Relative to the possible dislocations in the site or adjacent parts I . of Bodega Head during a major earthquake, there is a scarcity of data upon which to estimate a probable displacement. The geology of the site
, was studied and reported by the Geological Survey based on excavations of the shaft. It must be emphasized that this study covered an infini-tesimal segment of the geological structure associated with the San i
- i. ,. Andreas fault. Therefore, if such concentrated surveys were extended to l-i other areas even very close to the proposed site, it is our opinion that evidence of recent faulting would be found. Moreover, the occurrence 1 during large earthquakes of offsets on minor faults in sympathy with a large displacement on the causative fault cannot be disregarded. Certainly l in the case of the Point Reyes Peninsula during the 1906 earthquakes the i
displacements in bedrock indicate that faulting does occur outside the San a Andreas fault zone in sympathy with large displacemer.ts within the zone. 1 t Such an occurrence of offs &ts on Bodega Head during future earthquakes is a definite possibility. The Survey believes that the permanent displace- . [ ments at Bodega Head could be as large as 2t feet, as experienced at Point j Reyes du' ring the 1906 earthquake. l I 4 i i F
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1 7 l Another factor for: consideration in evaluating Bodega Head as a pos-t j sible site for a reactor is the probability of a damaging tounami, it is :a t i well established that the San Andreas fault extends into the ocean off ' q the northern coast of California. Since 1898 there have been two submarine i earthquakes along this fault that could have generated a tsunami. More- l
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over, there is a possibility that the next earthquake of magnitude greater i i than 8.along this fault could occur off-shore and generate a tsunami. l i Now it might be in order to digress briefly and say something about j tsunami causes and propagation. For the most part. tsunamis are generated by submarine earthquakes or earthquakes located c!ose to coastal areas. f y However, it is well established that on.'y a small percentage of submarine earthquakes generate measurable water waves. The most common explanation is the vertical displacement of submarine blocks of the earth's crust. j Since it has been observed on land that great earthquakes have caused uplifts of 30' - 50 feet and affected e ustal blocks hundreds of miles long j and up to a hundred miles wide, it is easy to conceive of such a crustal 1 movement under the ocean generating a huge water wave. Slides rlong the t e oasts are also thought to oe sources of tsunamis. in the case of great 1 I j earthquakes originating on the sides of deep oceanic troughs, huge masses ]
; of unconsolidated material may slide into the depths, displacing a great j j amount of water. It has been suggested that there is a possible coupling Jl mechanism between tsunamis and great seismic surface waves with periods J
j c-) l over a minute.
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, it is not surprising that the tsunami peril was not mentioned in con-nection with the applicant's report because, with the exception of a wave reportedly generated by a local earthquake on December 21, 1812, there is-
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no record of a destructive' tsunami beleg generated along the California j
-l coast. The 1812 wave reportedly reached land elevations of 50 feet at 1
Gaviota, 30-35 feet at Santa Barbara, and 15 or more feet at Ventu'ra. l Inasmdch as historical recerds for locally generated isunamis are so f j i sparse, the dimensions of tsunamis that have been established through rela-n tively frequent occurrences in Japan should be considered. lida has done . I considerable work in establishing statistical relationships from available ,
! Japanese data using both earthquake magnitude and focal depth. His for-mulas show a small tsunami will be generated for a shrillow earthquake of .
. I magnitude 6t-7 and disastrous tsunamis for s. allow earthquakes with magni- l
{ tudes of 7 3/4 or greater. Based on lida's formulas, a tsunami classified as destructive will have
! a height of about 33 feet or greater. The earthquake of March 3,1933, of f i.
the Saneiku coast of Japan, had a magnitude of 8.3 and was of shallow focal I 1
; depth; the wave rose to heights of 77 feet on the coast. The recent Alaskan j i )
l earthquake had a magnitude of 8.4; maximum waves of 30-35 fact were reported 1
-i l j at Kodiak and may have been exceeded elsewhere. Local waves were reported !
1
! cf 50-60 feet for the Chile tsunami of May 1960 (earthquake magnituds of
$-S}). These support lida's formula as being reasonable (even though far from being rigorous evidence) and suggest that his conclusions for the Japa-nese area may apply approxirdately in other areas.
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y .__ n .,.y ;i.% ;, 4...\ 9 it is not possible at this time to cite values for specific areas such i as the Bodega Head site. However, for the California coast as a v.hol'e, ) I d
'i there is one justification for differentiating'between runup due to locally I{
' generated tsunamis (epicenter within several hundred m!!es) and that associ- f
.l t ated with distant earthquakes. For tsunami runup from nearby severe marine earthquakes,asiteshouldbeprotectedtoaverticalheightof5dfeet b above mean lower low water. There appears to be sufficient experience along the coast of Californja to limit the required protection against tsunamis J
; from distant generating areas to 30 feet above the same datum. <
The seismological risk of a local tsunami-generating seismic event may q
. vary from place to place, but the numerical height that must be assigned 'f I
for the subsequent runup must be the same. Future refraction studies may
! Indicate that it is possible' to lower scmewhat the 30 foot limit at this site for tsunamis from distant generation areas. The Coast and Cechetic l
Survey expec'ts to complete an electronic computer progrsm describing tsu-I nami propagatfor, in the Pacific in about one year. This program will cal- )
! i
- culate the relative convergence of rays of energy for different coastal
; :enes from various source areas and thereby permit some degree of differ-o entiation in the required elevation for protection of special sites. How- ,
I ever, this differentiation will be significant only if it is decided that i I the danger of a locally generated destructive wave is so small that it may j be disregarded.
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; C0fCLUS 10N3 1 (1) The seismicity study of the San Andreas fault area near the pro- 1 I
i - ,.,. posed Bodega Head reactor shows the possibility of a magnitude 8.3 earth- _j
.4 o quake about every 100 years. For this reason, the Coast and Geodetic Sur-t 9
vey believes that tan earthquake of magnitude 8.3 should be expectqd during .
; the lifetime of .the reactoe' plant.
f , l4 (2) The Skrvey believes.that a maximum probable ground acceleration'.
' s
! of 2/3 E at periods from 0.2 - 0.6' ancond should &g expected at Bodega 1
l Head and that grou id 4. accelerations as ni;F es 1.0 ; in the same period range could cecur and should be taken into account i:- t' a fesign of the l i j facility. I (3) The Survey celleves that relative ground displa:.:ements of 2f feet f sh:uld be censidered in the design of tha facility. l
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4 (4) For tavnami runup'fm.r. nearby severe marine w6g.::U:es, a site l should ba pecte.cted to a vertical hel2Ni of 50 feet above mean . wer low
)
water. Bare appears to be sufficient experience along the coas.' of Cal- l L i ifornia to limit the required protection against tsunscr.' from c:stant gen- .j i erating areas to 30 feet above the same datum. ' l; .
.i r U. S. Coast and Geodetic Survey I
., '?ashington., D. C.t
'. October 1964 i :
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