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. ;'*JCC SITE 0:* 203I0A EEAD, 20%07." 00ll::TY, . CALIFORNIA PART I--CICLOGIC IF.J,TIGATICHS hy Mius Sch:.ocker, hnuel G. Son 111a, and Alfred Clebsch, Jr.
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PART II--SEIS:CC HAZARDS EVALUATIONS i
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Page ntroducti;n--------------------------------------------------------- 1
. 1 A: % . le d; _a nt s - - - - - ---- - - - -- - -- - - - --- - -- - - - - - - ------ ---- -- --- -- .3 Pre vicu s ~nve st igat ions ----------------------~ ~----------------- 3
, Part I--Cecisgic Investigations by Julius Schlocker, Manuel G.
Bonilin, and Alfred Clebsch, ~r . --------- ----------------------- k Geologic setting------------------------------------------------ k Ocpocraphy------------------------------------------------- k Coology---------------------------------------------------- 5 The San Ardreas fault zone and seis.icity of. the Bodega
- c. ar co r area - - - - - - - - - - - - - --- - - , --- -- -- -- ---- ---- -- -- - - - 6 ,
o l *'ectern a
limit of the San A:vireas fe. ult zone -------------------------- 9 l , Geophysical Investigations, by Don R. Mabey and Donald L. Peterson--- 11 l Faults vest of the S.n Andreas fault zone---------------------------- 14 l The fault rapped by F. A. Johnson near the southwest shore of 1 l
Bode ga H e ad - -- - - --- - - - - --- - - --- - --- - --- -- - --- - -- - -- - - - - - - - 18 Q. art z dio rit e a-4 related roc ks ------------------------ ------------ 21 .
Ge oloc/ of the s it e and v ic inity------------------------------------- 25 Qaart z diorit e at the s it e -------------- .------------------- --- 26
! Qaartz diorite in the proposed reactor excavation---------- 27
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Pleistocene and Recent deposits at the site--------------------- 28 t
. Age of the deposits----------------------------------------
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Structures in the Pleistocene deposits at the site------ -- 33 li j Possf.ble fault control of the south shore of Campbell Cove---------- 36 1
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Bo' c k -cut t e rrac e ------------- --- -- ---- ---------------------- ----- -- -- 37
-l Faulting through the site during future earthquakes-------------- '--- 38 I
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. Recommendations for future. investigations---------------------------- 41
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ILLUSTRATIONS :
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[" Figure 1. Map of Bodega Head. area. i!
i f.3.:j 2. General view of. northwest corner of site excavation. j u e 3 General view of north face of site excavation.
2 4. Bouguer gravity Profiles, Bodega Head.
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Ej 5 Aeromssnetic profile, Bodegn Head.
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South shore of Bodega Head. '
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- 8. East end of north face of excavation. '
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9 Close view of north face of excavation.
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- 10. Close view of north face of excavation.
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- 12. Closeup of organic layer. '
, 13 Stereographic projectf.on of poles of faults in quarts diorite.
i 14. Observed ground displacements in the Arvin-Bear Mountain-Bealville areas.
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4 Table 1. Fault zones observed in quartz diorite on traverse east of
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Geologic and seisnic investigations of a proposed nuclear power plant site on Bodega Head, Sonoma County, California INTRODUCTION N"
The Pacific Gas and Electric Co=pany (PG&E) has applied to the -
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U. S. Atomic Energy Comission for pemission to construct a nuclear-povered electric generating plant on a site adjacent to Campbell Cove or. Bodega Head, Sonoca County, California. Questions have been raised
} as to the suitability of the site for such a plant because of: 1) its j
J position relative to the San Andreas fault zone; 2) the possibility that minor faults, related to the San Anircas fault, night actually i
pass through the site; and 3) the seismic hazards posed by the nearby 1
San Andreas fault zone.
h*. The Geological Survey is conducting a study of the site in an effort
-q to resolve these points, and this report presents the results of approx-i=ately 8 days of field work at and near the site between May 10 and June 6,1963, plus a review of published geologic and seismic reports on the general area and unpublished reports on the site itself, The unpub.
lished reports are part of the Facility License Application, as amended, I
AEC Docket No. 50-205 and are thus available to the reader. About two-c' thirds of the field work was spent in a detailed examination of the S
excavation for the plant and of exposures in the adjoining Campbell Cove area. bout a third of the effort was devoted to the study of the geol-osy of other parte of Bodega Head, principally around Horseshoe Cove ani
// on the vest-central, southern, and northeastern margins of the Head.
s Results of geologic and geophysical studies of this part of the San Andreas fault zone are im indad in part I. *
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l Qaestions relating to the frequency and intensity of earthquakes 1 that might be expected at the site and the kinds and degree of earth j rotion to be expected are important in assessing site suitability and c ..
setting design criteria for the plant, and are presented in part II.
1
- 1Q l"
- The term " site" as used in this report, unless otherwise specified, c:.
- vill refer to the main plant site adjoining Campbell Cove which includes
' G- .
G.e reactor, turbine-generator, administration building, etc. Figure 1 shows the location of the first of four proposed reactors at the site.
l Figures 2 and 3 show the stage of excavation reached on the north and vest faces on May 16, 1963 l l
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. n-Acknowledgments l
, 1 The cooperation of officists and esployees of Pacific Cas and i Electric Company greatly facilitated tihe work. They made copies of
,]. 1 g reports, maps, and other data available and further facilitated the ,
field work by providing on occasion surveying services and improving exposures in the site escavaties by scraping with' heavy equipment and .
y
- q by washing loose soil and fill so that the faces could be observed in detail. .
! Gravity measurements were made by Donald L. Peterson and. Victor i
W. McAllister of the U.S. Geological Survey under the supervision of I Don Mabey, U.S. Geological survey. The section is this report'on 1
- e. 4 I
') i fi s oybysical investigations was written by Mabey and Peterson.
ne, i Frevious Investigations The goalogy and seismology of Bodega Bead were studied by Don Tocher ar.:. Willian quaide on behalf of PG&E and are reported in PG&E's " Pre-
. i l liminary Basards Summary Report, Bodega Bay Atomic Park - Unit Number 1" l as Appendix IT " Report on earthquake hasards at-the Bodega Bay Power i Plant Site, Pacific Gas and Electric Company." . Details of subsurface geology at the site as interpreted from boreholes,are contained in reports prepared for PG&E by the engineering firm of Dames and Moore .
~
(1960,1962). gems of the information from these reports has been checked A '
independently in the field but because of lack of time , .it has 'not been a.g possible to field-shoek all such information that we, have'utilised. More ,
a pteneral diseassions of the geology of the area are tacinded in reports
. 1 a . ,
by Jehseon (1943), Travis (1952) and Eiggins (1961). The geology 'of
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Point Reyes Peninsula, which is part of the saae' geologic province as ;
,; Bodega Head, has been mapped and described by Weaver (1949) and Ganoway u .
)[ (1961,1962). h age of the quarts diorite on Point Reyes has been e
- estimated by Curtis, Evernden, ~ and Lipson (1958) . . h effects of the {
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. .l '. 1906 earthquake in that area are described in a report casqpiled and
.}
edited by Iewson (1908) . .
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PARf I--GE0I4GIC INVESTIGATIONS I l .
i
]a by Julius Schlocker, M. G. Bonilla and Alfred Clebsch, Jr. '
Geologic setting Topography
~i N site is on the eastern half of a saddle between the two southern-I e
j most hills of Bodega Head (Figure 1). Bodega Head is bounded on the north.
t j by Horseshoe Cove and ashes up the southern two-thirds of a wedge-shaped bedrock peninsula that extends from Mussel Point southeastward for 2.2 mile's.
- Bodega Head consists mostly of three broad,' low-created, gently-rounded hills that, beginning with the northernmost, are.'266, 238, and .
200 feet above mean sea level. N Head is 2,000 to 2,800 feet vide and j is bounded on the western or Pacific Ocean side by cliffs 100 to 200 feet high and on the eastern or Bodega' Bay sieb by cliffs about 75 feet high.
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- Detached low sea stacks and small,~1ow, rocky islands are abundant beyond j
a the vestern and southern shores of the peninsula.
'l N. northern third of the peninsula swaches a height cf.108 feet-
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,2 a above sea level in a hill of gently rounded slopes about 1,250. feet south
'} of Massel Point .h peninsula is about 1,000 feet / wide in this area.
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N N pania=i1= is tied to the =4=1m=A on its northern half Oj .by a strip of dune and beach sands 2,700 to 6,000 feet. vide that makes
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' . 8 up the northern and western border of Bodega Barbor.. g'almon Creek Beach
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is the Pacifte Ocean shore of this strip. The prevailing winds, blowing
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- 8. 50' E., have an unobstructed sween from the beach and have created a j 1 .
i series of sand dunes 120 to 152 feet high that are parallel to the beach 1
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.i and estead 2,300 feet to the southeastward. goutheast of this crest,'and 1 in the lee of the hill at k ssel Point, the dunes are generally aligned with the prevailing wind direction.
'{
, 2 Doran Beach, a low sand bar probably created by currents that moved sand generally northwestward in a counterclockwise direction, forms the southern border of Bodese Barbor. It is about 7,500 feet long and in 1942 came within about 450 feet of Bodega Head at a poi [e about 1,700 feet morth of the straight southern ' shore of Campbell Core. Since 1942,
'l j its shore has been modified by dredging and man-made fill so that it is-
.i
, now 800 feet east of the Head at a point about 1,000 feet north of the southern shore of Campbell Cove.
Geology ,
N bedrock of the hemel Point-Bodega Head peninsula is a granitic rock that is predominantly quarts diorite in composition. Rock of similar i
composition, believed to be of the same origin and approximately the same 1
, , age, forms the bedrock of Point Reyes Peninsula, 5 miles southeastward of i
Bodega Road. A sample of the Point Reyes rock was reported by.C'urtis and others (1958, p. 9), to be 83.9 million years old by the potassium-argon
.t
, anthod of age determination. '
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h granitia weeks of Bodega Eead are now partly covered by flat-I' lying and pantly dippias mesammated to umakly-eemented depeette, probably' i
of Flaisteeses age'and by yeonger Basant deposits.., N Flasatseems
, depeetts are mostly' sand'ad $tity and simysy sands tha.everlying Resent "i , ,.-
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deposits are mostly uncemented to weakly-cemented silty sand soils and .
clay, sand, silt, and pebbly colluvium. The term colluvium is applied ;
l here to weathered slope debris that has moved downslope mostly by gravity.
The Pleistocene deposits appear to be nearshore marine and nonmarine deposits, such as beach and adjoining sea floor deposits, tidal marsh or fresh-water lake deposits, interstratified marine and nonmarine terrace deposits of alluvium and colluvium derived locally from steep shores, and I l
minor windblown sand. .
The general distribution of the Pleistocene deposits is shown on the geologic map by William Quaide that accompanied the report by Tocher and Quaide (1960). I
.* r f
/ This map was field-checked by us in critical areas. In such areas the i contact between quarts diorite and terrace deposits was found to be depicted on the map within reasonable limits of accuracy. We found a few small patches of. terrace deposits lying on quarts diorite, mostly near the western and southern shore of Bodega Head, that are not shown on the mapg soms of the area shown on the map as quarts diorite is covered
. irregularly by organic, silty, vindblown sand soil and by slope debris.
Surface exposures of quarts diorite are spotty in the latter areas.
1 The San Andreas fault zone and seismicity of the
'l ..
Bodega Barbor area Sedrock in the area east of Bodega Barbor is the Franciscan Forma-tion, considered to be of Jurassie and Cretaceous age, and is composed I
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i 1 mostly of pervasively-fractured marine sandsto..c. The Franciscan Forma-ticin is separated from the granitic rocks of Bodega Head by the San
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Andreas fault zone which traverses Doran Beach, Bodega Barbor, and the
.d . neck of dune and beach sand that connects the Head to the mainland. I h
'1 ,,
In other places, where the San Andreas fault zone is better. exposed i and has been studied in detail, it is found to be a belt within which bedrock is generally thoroughly sheared and broken and the older i
l surficial deposits greatly disturbed. Relatively intact, little-sheared i or broken blocks, several hundred feet in size, are present. Such condi- j tions are believed to exist in the fault zone in the Bodega Bay area because they are common elsewhere in the fault zone.
}
Because the Franciscan Formation which lies ea'st of the San Andreas j fault sone is not intruded by the younger granitic rocks which lie west of the fault zone, and also because of inferences on the time-rate of l
offsetting of certain geologic features, many geologists believe that -l
.1 the total horizontal movement on the fault by repeated rupturing during .)
i
,the past 25 million years has been at least 160 miles (Crowell, 1962,
- p. 49; curtis and others, 1958, p. 14).
{ There is some evidence to indicate that an individual rupture tends i
to occur along the same line, or in the same part of the zone as the one I preceding. For example, in the valley 20 to 30 miles southeast of Bodega' Bead, between Tomales Bay and Bolinas Lagoon, and elsewhere along.the "
i fault sone, traces of older ruptures are located close to the 1906 rhyture. l The strongest earthquake of record at Bodega Head was that of April i
1 8, 1906. The intensity of shaking and its effect in the Read area are .
not recorded, though the intensity was probably strong. The trace of- , ,I 4
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- the surface rupture within the San Andreas fault sone is 'shown by Lawson j and others (1908, atlas map 4) as striking N. 35' W. and lying 6,600 feet northeast of the westernmost shore of Caepbell Cove. Observations along ,
j this rupture showed that the movement was mostly horisontal; the west
- side moved northward relative to the east side. The amount of horisontal
. displacement of the 1906 rupture in the Bodega Barber area was not recorded.-
. Horizontal displacement on the rupture 21 miles southeastward in the Point
! Reyes Station area was about 20 feet. An 18-inch vertical displacement, ,
^l l up on the west side, was recorded at the rupture near the town of Bodega l 1
Bay (Lawson and others, 1908, p. 65). It is not known if this was true vertical displacement or horizontal displacement of. a small mound, which
- - i' could have produced an apparent vertical displacement; however, a study l of the distribution of living and dead barnacles and muasels along the l ! ,
east shore of Bodega Head 6 months after the 1906 earthquake indicated that no uplift of Bodega Head had occurred (W. E. Ritter in Lawson and l l
others, 1908, p. 88).
I Other aspects of the seismic history of the Bodega Bay area are discussed in detail in the report by Tocher and Quaide (1960, p. 2-4, 9-10, table 1).
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WESTERN LDtIT OF THE SAN ANDREAS FAITLT ZONE' l Definition of the limits of the San Andreas fault sone near' Bodega 'I
'Barbor is more difficult than elsewhere because most of the geologic and ,
]
j
. ',;;. topographic criteria used to define the fault sone are absent'or con-
[ coaled. In other places the sone is commonly marked by a well-defined -
trench that may contain straight valleys and ridges, disrupted drainage, i
undrained depressions, gouge, breccia, and a mixture'of rock types. If
.j these features are present near Bodega Harbor, they are concealed by water-or by dune and beach sand.
Inasmuch as direct criteria for the width of the fault sone are j i absent near Bodega Barbor, indirect criteria from adjacent portions of ,
the some must be used. The general position of the sone can, of course, -
be established by the 1906 trace and by projecting the attitude of the fault sone from exposures northwest and southeast of Bod'ega Barbor. The 1906 trace was not everywhere in the middle of the fault zone and in relatively short distances the trace crossed from one side of the sono to the other (Higgins, 1961, fig. 2). The fault sone is well defined ;
southeast of Bodega Bay by the trough extending from Tomales Bay to Bolinas Lagoon. This trough is 1 to 1% miles wide, but the most active
'I i
some is narrower than the trough. Gallowey (1962, p. 395) speaking of .
an area about 3% miles north of Bolinas Lagoon says "...the whole faulted .
. some is about half a mile wide." Northwest of Bodega Barbor the fault passes out to sea and re-enters the land near Fort Boss. For about 10 e
miles northwest of this point the fault is not in a well-defined valley but northwest of that reach, it is la a well-defined trough for more than'
, 25 miles. This trough, which is occupied by the Gualala and Garcia Rivers,. '
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f: also 1 to 1% miles' wide but the most active ' sone is probably narrower l l
than the valley. Thus based on the width of this trough, the fault zone 4
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in reaches southeast and northwest of Bodega Harbor is probably no more
. ql ,
than 1% miles wide. The eastern limit of the zone is probably east of
' ~ ' ~
the east shore of Bodega Barbor, based on the intensity of shearing of sandstoneoftheFranciscanFormationandon' topographic'ehidence..The
- distance from the east shore of Bodega Harbor to the western limit.of the ,
l i .
fault zone as shown by Tocher c;d Quaide (1960) is f. bout' 1% miles and'is I I
thus of a reasonable width when compared to adjacent. portions of the sone. l l Over most of its length in northern California the San Andreas fault ~
l sone occurs between granitic basement rocks on the west and Franciscan i basement rocks on the east. This relai; ion is true in. the Foint Reyes '
.g Peninsula, which is inmeediately south of Bodega Harbor; granitic rocks-
, .do not crop out north of Bodega Head. By this general rule then the fault should lie eastward of the granitic rocks of Bodega Head. However, along l some parts of the San Andreas fault zone bodies of granitic. rocic as large as Bodna Head are included within the San Andreas fault zone (Santa Crus l 1
Sheet, 1959, and San Luis Obispo Sheet, 1959, Coolog'ic Map of California). l Nevertheless, the granitic rock of Bodega Head to'be within the San Andreas ,
fault zone would require a considerably greater width of the fault none
- than exists in nearby areas to the north and south, and furthermore there i
is no compelling geologie data for postuisting 'a greater width near Bodega l 3
1 . . . - .
o Read. Tocher and Quaide (1960) drew the western limit of the San Andreas .
I fault sone about 1,000 f.et east of the center of th'e proposed reactor,- j t
going, among other ' criteria, the casternmost 'line 'of .exposuraa '...' the 't y: , ' u. .
2 granitic rock. Itishlievedthatthes'raniticrock'inthesubsurface esteedseastofthesurfaceexposuresandthereforei:hewesternlimitof
.t
'j .the San Andreas some is at er east of the line shown by Tocher' and Quaide. ~
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Gs0FETgICAL INTEg71GATICIls by Don R. Mabey and Donald L. Petersee
- 2. ' 4 Gravity observations have been made at 57 stations sloeg two p're-m :q gg files approximately normal to the gan Andreas fauic in theIric'imity of w4y:
[6 Bodega Esad (fig. 1). . h stations are about 500 feet apart except at Of
. %;; the entrance to Bodega Barbor, where there is a break in the southera j
\
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- - profile. Position control for the gravity stations was ob'tained by PG&E personnel. The relative elevations are known to within one foot i ,
and the horisontal position to within 10 feet. Gravity observations
. .l
.} were made with a Worden gravity meter having a dial constant of 0.4907 l l
.y milligale per division. Reoccupation,of five stations repeated the
.w'.
si original observations withis 0.15 milligals.
iM The grarity data were reduced to the simpin Bouguer enamely asing 3 for the material above sea level.
] t the standard density of 2.67 g per em l Near the center of profils A-A' (fig. 4) minor irregularities are produced l 1
a by deviations of surface material from this assumed density. Profile 1
}-)' would not be substantially changed by corrections for changes in ;
i surface density. i 4
h Bouguer gravity maa-aly relative to an arbitrary datus is shown
- .I 5 '.] -
in figure 4. A, total gravity relief of about 13 milligals is passat on I
both profiles. h w= mat features are a gravity low centered over
. Bodega Barbor superimposed on a moderate westward increasing regional
..,.- tread. The regional trend Ladicated by the difference in the gravity- j
- .. c . .. ..
si values se or amar bedroek at Bodega Eend and on the mataland east of-i 93 .l
,7, Bodo4a Barbor is probably a esabined affect of differias densities of t asar-sarfaes useks on opposite sidea of the San Andreas fanit and deeper
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mass anomaties. Tha local gravity low is probably directly or indirectly related to the fault sone.
i 1
.s The character of the gravity variations along the two profiles ce '
v.h 1 l
similar. The flattening at the ends of both gravity profiles suggests l h.
that the profile extends beyond the range of effect of tha low-density i
mass causing the local gravity low. A line connecting the Lowest anomaly l values on both profiles is parallel to the fault sone. Both profiles !
I show steep gravity gradients of limited extent which must be produced by near-surface mass anomalies. For interpretative purposes, profile 2-l' was selected. It is more nearly a straight line, and there is less
,y~ local surface relief in the vicinity of the local gravity los, l l
- h. Two causes are suggested for 'the gravity low: 1)' low density ,
Q
$; sediments deposited in a depression along the fault sone, and 2) low density rocks within the fault sone. The average density of the undisturbed rocks adjacent to the fault sone-and water saturated sedi-l mestary deposits along the some are estinstad to be, respectively, about l, 3 2.7 and 1.8 3 per em . If a linear regional gradient is removed from profile }-}' the residual gravity low of 9 milligals could be produced i i by a thickness of about 1,000 feet of sediments 0.9 3 per em3 lower in l
- density than the enclosing rocks. If the sediments are assumed to be 400 feet thick under the center of the profile then a 5-mi111 gal gravity l , low remains to be explained by rock in the' fault sone. The density of sheared rock may be as low as 2.2 g per em 3 . A thickness of less than
'}{,' 2,000 feet of this material could produce the 5-milligal anomaly.
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i Although an analysis of the gravity' data alone till not yield a:
unique solution for. the negative mass anomaly producing 'the low along
. ,i
- 0 ..j profile 3-1', tuo inferencas are justified. First, the limited extent q
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of the gravity anomaly require's that. the low density material be within j
g'W a few thousand feet of the surface. Second, the steep gravity gradient 1
k.a . .
N. measured 4,500 feet east of 3 indicates a steeply dipping density inter-
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face. This could be either an increase in the slope of the base of the 1 d; sediment er the western margin of low density rock in the fault sone.
l A similar' break occurs about 3,000 feet west of I'. A preferred inter-l l i pretation of the gravity data, but not the only one, is that the uncon-
> :i ;
. o. solidated sediments thicken eastward from godega Baad to a point about ,
5!.d
- ((' 4,500 feet east of 3 East of this point the sediments either thicken M.
WV abruptly or are underlain by sheared rocks in the fault mone. If the l i
sheared rocks are the cause of part of the anomaly they must increase in density with depth so that most of the density contrast across the-fault some has disappeared at a depth of 5,0'00 feet. East of a point abou: 3,000 feet west of l', the sediments either thin abruptly or the sheared some terminates.
, , The total magnetic intensity was measured along one profile across ,
M
] the fault some near Bodega Read (fig. 5) in conjunction with an aere-1 - ,
radioactivity survey floun in 1959. The flight level was 500 feet above the ground surface. A magnetic high of about 12 gammas was observed across the fault some. With only one profile available it
'y cannot be determined if this small anomaly is elongated parallel to the J} fanit sens. The coetern edge e8 the seagnetic high aorrelates with the y' .
eastern edge of the gravity low suggesting that the tuo asemalies may
- t
,q have a stagle sourse. ~ Be significant change in the magastic profile -
,.ss-> I is apparent en the west side of tho' fanit sene.
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FAULTS WEST OF THE SAN ANDREAS FAULT EONE - The report on the 1906 earthquake (Lawson and others,1908) mentions , ,
.l
. 1 1/ that breaks in the bedrock appeared outside the sone of marked fault
.. o 4.L e3 topography. Most of these were incipient landslides but some apparently TJ - ,
G. K. Gilbert, on page 75 of this report, describe's features gi, were not. 9v . near Mount Wittenberg, which is 1% miles west of the 1906 trace and about
'" 25 miles southeast of Bodega Head: , .
j , 1:
"On Mount Whittenberg there are two bedrock cracks. One of these 4
crosses the northeastern spur of the peak near its junction with the l
]
j main crest. Its trend is approximately northwest and southeast and at 1 one point it margins a fault-sag. As it assumes in one place the ridge j
. -]
phase of the fault-crop, I infer that it has horisontal displacement. On the opposite side of the main ersat is a' crack which'was traced for i about 1,000 feet. Its general course is northwest-southeast, but it is f not straight and exhibits a vertical throw of 1 or 2 feet to the south-1 l -! west. At one point it touches a fault-sas. Between these two long 1 i cracks a group of short cracks occurred, with a similar trend, on a knob constituting a portion of the main divida. A
] "About 6 miles farther south, at the head of Pine Gulch Creek, ca another road crosses the range, and in following this a group of cracks
,m.c,. . .
was seen. A short distance west of the divide, and about a mile in a
- direct line from the fault-trace, is a fault-sag trending northwest-m'
.' southeast. On each side of it a crack was seen, the eastern crack
+
. 'k - . being the wider and showing a small throw to the southwest. This crack i.W
'[y was traced for about 0.75 mile and found to suave through an are of .
'. o nearly 90* from seetheast to southwest.. At'its southwest end, or at ,
,3 . .
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2, c' . 7: ns, least the southwestern limit of tracing, it is on a ridge, and it there I
) expands into, or else is replaced by a group of cracks diverging fan-wise.
On each member of the group faulting took place, the downthrow being. 7p toward the northwest except in the case of two apparently short cracke i ub' Wy? . with downthrow to the southeast. On four of these cracks the throw was ,
.g.
greater then 1 foot, and at one place it was about 5 feet. Each crack L' i . was associated with a pre-existent bluff or scarp, indicating that earlier ) movements have occurred at the same place. The field in which the I 4 principal phenomena occur is cultivated with the exc'eption of the steeper
' i I scarps, whose faces retain a bushy growth."
i l Roderic Crandall (in Lawson and others, 1908, p. 253) describes cracks on Sawyer Ridge, which is on the San Francisco Feminsulas l f "...there were cracks several hundred feet long almost at the top of the ridge. N se were parallel to the line of the main fault, which
; is a mile to the east, and there was a marked downthrow of from 2 to 3
) inches on the southwest side, which'in this case was the uphill side.
If the downthrow were on the downhill side, then it could be possible
; that these were landslide cracks."
The descriptions quoted above suggest that active faulting can occur
.? outside the generally accepted limits of the San Andrena fault sone.
s,g 7' . Although the reactor site is outside the fault sone, examination of the 1: excavations at the site has failed to reveal conti sive' evidence of r., . faulting in the sediment 6ry deposits overlying the granitic rock. As 1 stated eleeubers in this report, the sediments overlying the granite
. are generally intact and the few breaks in them can be .esplained by
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'n j .-
l causes other than faulting. In the vicinity of Horseshoe Cove, some
.j lineer features trending southeastward can be seen on the ground and on aerial photographs taken in 1954. One depression, less' than'1 foot deep u
and about 4 feet wide, extends southeastward from the Cove for about we
$ , 100 feet. Southeast and east of the Cove are very low and poorly defined
?/' linear ridges which also trend southeastward. Terrace deposits are well
.t[ -
. exposed on the east and south sides of the Cove and examination of them I
along projections of the linear featurse failed to show any faults in . ' i tha sediments. Although the exposure is not entirely continuous, there l is little doubt that the beds that crop out on the east continue around i the south and of the Cove, where they overlie the quarts-diorite. The linear depression and ridges trend 86 52' F which is also the trend of the longitudinal sand dunes east and south of the Cove. The linear features near Borseshoe Cove are probably related to wind action, possibly modified by cattle making their way to the spring at the south end of the l { Cove. '
\
i The possibility should be considered that a branch of the San Andreas fault might lie west of Bodega Head and that it either parallels i t the San Andreas or curves southeastward and joins the' San Andreas at the j mouth of Tomales Bay. A parallel fault would cross the Point Reyes Feninsula, and the Fault Map of California published by the Seismological Society of herica in 1922 shows two faults on the peninsula nearly
\
l parallel to the San Andreas. However, the map explanation indicates' i
. theseasprobablefaultsofuncertaincharacterandiocation,andtha existence of the faults has not been substantiated by subsequent geologic ,
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l mapping Oleaver, 1949; calloway, 1962). We find no evidence to support j
- ~
J. i the hypothesis of a fault curving south astward to join the San Andreas
,a
,. r.;.
at the mouth of Tomales Bay.
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1 The Fault Mapped by F. A. Jchnson near the Southwest Shore of Bodega Head 1 A geologic map accompanying a paper by F. A. Johnson (1943, p. 624) l
, ,,M] !
' , ![ on the geology of a portion of Sonoma and Marin Counties shows a fault M ,
having a strike of N. 47' W. (more or less parallel to the San Andreas a fault zone) cutting both the quarts diorite and the terrace deposits on the routhwest part of Bodega Head and emerging along the south and west shore. We examined the shore at and for several hundred feet on either i I e } side of the fault location shown by Johnson. (Traverses are shown on l l figure 1.) No faults were seen in sediments that discontinuously over-I lie quarts diorite along the shore, however, numerous faults were seen in quarts diorite and'related rocks along these stretches of shore. The
'] faults show a great range in orientation and magnitude of movement. To i
- 'l '
l illustrate this, the faults observed along the south shore (traverse D-D', fig. 1) are recorded in table 1. An earlier traverse (C-C', fig. 1) failed to show a fau1r in the location along the western shore as shown by Johnson. Several faults listed in table 1 and one fault seen on traverse i 1 C-C', figure 1, have approximately the same orientation as the fault Mq . mapped by Johnson, and a zone of closely jointed rock was observed on , 1 the south shore near the point where Johnson mapped the fault. Table 1
. and figure 13, however, show numerous other faults having drastically different orientations, and we found no compelling geologic evidence for j3 ,
mapping a fault in the location and with the attitude as mapped by. ud .
.j Johnson in preference to numerous other locations and attitudes.
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]i Table 1.--Fault nones observed in cuaren diorite on traverse eas,t of )
e, , southernmost ooint of Bodena Hegd i i i Approximate distance in
^1 feet from Point D en figure Strike Dip Remarks 4 1 as = ==. ired miaan -h re
,z:-
-,e .
130 N. 80' W. 60* 3. 1-inch clay -gouge.
~
160 W. 10' E . - 80* W, 1-inch clay souge. 1 190 - N. 75' E. 90' 3-inch, 1/2- to 1-inch, and
! 1/4 inch clay souge in 14-inch mylonite sone, i
220 N. 30' W. 70* E. 1- to 1/2-inch clay gouge. 320 N. 65* W. 70* - 85' W. Numerous 1- to 3-inch shears, . joints, and clay souge sones
- . .- stained brownish gray across a total sone width of 8 feet..
Rock between shears, etc., is fresh and unbroken.
, 556 N. 70' W. 70' E. A sone about 10 feet wide i i
consisting of.several 2- to { 3-foot-wide sones of sylonite i and breccia and intact rock. Slickenside striation on main shear surfaces oriented in
! many directions from horison' tal to vertical.
640 - N. 45' W. 90' - 80* E.W. Hard sylonite, minor clay ,
.i
, gouge in 18-inch-wide sone. I
~
i J 655 N. 50' W. 90' Clay souge and sylonite in sone 1 to 3 inches wide. Vertical slickenside stria-tions.-
.. 485 N. - 3. (f) 40' E. (t) Mylonite sone 3 to 5 inches '
,- . . th. i l 4
- i. 705 N. 50' W.- 90' '
Clay souge and sylonite sone I
.;r
~ ~ *
. 1 to 6 taches wide. Dis- l placed 18-inch pegmatite dike j
/P .
about 3 inches.
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Table 1.--Fault zones observed in cuaren diorite on traverse east of southerwe st noint of Bodeen Head--Continus$ 4 l Approximate distance in feet from Point D on figure Strike Dip Remarks
.w _1 as measured alona shore w .
755 N. 50' W. 15' E. Mylonita 1 to 2 inches wide; no clay gouge. Upper plate moved to west. Average fracture spac-ing of upper plate 1 to 3 inches; lower plate 1 to 2 feet. l 605 (due south of N. 50' W. 75' W. (f) Zone of close fracturing and rock i hill 200) slide 30 feet wide. .Contains a few braided 1- to 2-inch-wide zones of clay gouge. Fracturing in rock is almost random and
' pieces are 2 to 3 inches in size -
; compared to 0.6 foot to 4 feet in adjoining rock. Pieces of fresh-appearing rock are easily broken with geologic pick.
810, N.-S. 45' - 60* E. Mylonite sone 1 to 4 inches wide, t 660 N. 45' E. 75* W. 1-inch clay gouge; cuts two pegutite dikes that are 1 to 2 feet thick. Displacement 6 feet either by upper pists moving up or by right lateral movement. 1,030 N. 25' W. 70* W. Clay souge and mylonite, 1/2 inch 1 . to .1 inches wide. Offsets 14-int.bwide pegmatite near beach level. Displaced dike not seen. 1,060 N. 70' E. 85' E. Braided gouge and reylonite zones, 1/4 inch to 3 inches wide in sona
.i 2-1/2 feet wide. Parallel to prominent joint ad, foliation of small fine grained dark inclusions and several 6- to 8-inch pegmatica dikes.
1.160 N. 70* 2. 40' W. Mylonite and clay gouge 1/2 to 1 inch wide. Can be seen for 250 feet en head 3end. 1.175 E.-W. 70* N.. Clay googe and sylonite 2-1/2 foot wide sees near and of headland.
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QUAR 2E DIORITE AND REIATID EDCIS
]
,l The granitic rock of Bodega Baad is mostly a coarse-grained biotite-
~
j hornblende quarts diorite. Part.of it is slightly foliated,'as is shown i by the more or less planar arrangement of the dark minerals and feldspar,
, and part of it is enfelisted. Elongated dark inclusions of fine grained
, , rock, generally 1/4 to 1 foot long and 1 to 4 inches wide, are common in the quarts diorite and are generally alined with the foliation of the l
- matrix rock. Nearly black, coarse-grained inclusions of irregular shape ,
1 and from 2 to 10 feet in diameter were seen in two places in the quarts diorite on the shore west and south of the site. Passatite dikes are common. They range in width from an inch or two to about 3 feet. Osmont (1905, p. 43) saw one 7 feet wide on the southeast shore of the Bead. Pegmatite dikes in fo11sted maarts diorite are not foliated. '
; This indicates that the folistion developed before the pegmatite-material-
? .
filled the joints in which they lie. Speciment of granitic bedrock from five localicies at and near the site were studied microscopically by usana of thin sections and by embedding graiee in refractive index oils, and negascopically by use of l feldspar staining on saved pieces. Most of the granitic rock seen at the Esad is a biotite-herablande quarts diorite consisting mostly of plagioclase feldspar, lesser amounts of quarts, biotite, and bornb13nde
, t and minor amounts (1 to.3 percent) of small orthoclase faldspar cryetal's .
, Speciassa sollected on the west shore about due west of the site contain eboet 25 persent osarse grained orthoelaae crystals 3 their mineral i s .
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. composition indicates that they are quarts monsonite or adame111te but the abundance and distribution of these rocks on the Head are not known.
l Spotts (1962, p.1223) sampled the granitic rock at four places in the j .
, j area between Mussel Point and the south shore of the Head. . He found n- i
;j them all to be. quarts diorite. Textura11y the granitic rocks are coarse 30 .J
', . grained and hypidiomorphie granular in which only part of the minerals 1 are bounded by crystal faces. Quarts shows wavy extinction and also-mosaic br**: cia texture. Feldspara show a slight alteration, but not i j more than that shown in' typical fresh granitic rock.
4 The small dark inclusions contain.the same minerals as the enclos-ing quarts diorite matrix, but they are finer grained and contain more i l biotite and hornblende. No feldspar is present-in the large dark irregular inclusions. They consist of biotite, hornblende, and quarts I
.. and minor apatite, garnet, and aircon. -
The pegmatite dikes are generally soned and consist of margins of graphie granite or quarts and plagioclase and a-core of large pink.
! potassium feldspar crystals and minor biotite.
The granitic rock exposed at high tide on the western and southern shore of Bodaga Head generally shows little or no signs of chemical
,,.I alteration. The rock above, about 20 feet above high tide, shows signs S.P -
.p of alteration due to weathering which is progressively more advanced toward the top of the sea cliff. Near the top, the texture of the quarts
.i diorite has disappeared, and the rock has been weathered to a sand-silt-h 44.
clay soil. On the south shore of Campbell Cove near sea level the quarts-
'y .p. . ,
diorite is covered by Pleistocene gravels.
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Joints and faults are common in the granitic rock of Bodega Road, -l t although they vary greatly in abundance from place to place. Most of i l , the rock is broken by joints into blocks 3 to 5 inches wide; however,
,d rock with joints 1 to 2 feet apart is not uncommon, and rarely a mass O of rock has a joint spacing as much as 4 feet. Closely jointed rock l
- l . .
l is cut by four or more sets of joints and also by irregularly branching l
~. j -
joints. In less jointed rock only o:ne er two prominent sets of joints is the rule. Orientation of prominent joints varies greatly between [ l f closely spaced localities. Along the Pacific' Ocean shore, west of the i l
! l l site, prominent north-south joints' at one place are replaced by,promi- i nent east-west joints 500* feet away. Figure 6 shows strong, steeply
, dippina joints oriented about N. 70' 3. in the headland south of hill i
200. . Specing. type, and orientation of faults is highly variable from j place to place. Most abundant are sharp breaks with offsets of 2 to 12 inches. spacing is shout 1 foot apart where they a,re abundant, but is greater in most rock. Many of the small faults are clean, or have a 1/4 to 2-inch some of breccia (coarsely broken rock), mylonite (pulverised but firm rock), or gouge (clayey material of pulverised
.l and chemically decomposed rock). In some places a sone as much as 10 4
feet in width consists of closely spaced breccia, braided and inter-woven thin mylonite and songe sones, and blocks of relatively unbroken
- rock bounded by stickensided surfaces. such disturbed sones are 30 to -
~ '
more than 100 feet apart. Breccia senes,1 to 3 feet wide, of granitic
.- reek eenented is a granitis rock matriz are found in only one of two places. Mswanent of these-took place before eaglete melidification.ol q 4
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j the granitic body. Table 1 gives spacing and descriptions of prominent fault zones seen on shore near sea level between the southernmost point._ i I ,
] of Bodega Bead and the headland south of hill 200. I l
-J The magnitude of movement of faults can be measured ~directly by
,; noting offsetting of pegmatite dikes or dark inclusions. Although most m:. v
[~I^ ,, faults show'only a few inches of offset, some show 4 to 8 feet.- On a ,
, j
/
few faults the dike or taclusion is offset beyond observation at th'a ' shore; the total movement for some of these was greater than 20 feet. I t Faults bounding foliated and unfoliated rock or closely fractured and - l i j relatively unfractured rock also indicate considerable movement of un-
. I known magnitude, but probably greater than 10 to 20 feet.
- No persistaat orientation of faults was seen in the granitic rock I
of Bodega Esad, though locally, over a distance of 50 to 200 feet, some - _. but met all of the faalts are more or less parallel. Neither is there a systematic pattera of fault displacement as shown by slickenside ' striations even withis a 4- to 4-feet-wide some of faults. i e I e
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; GE014CT OF THE SITE AND VICINITY '
_l h site is located on a buried valley cut in quarts diorite and
, filled with Pleistocene and Recent deposits. Borehole investigations
\ l
-i by Dames and Moore (1962, plate 2) show that through the central part .
l
,- ' of the Eead the valley trends easterly; near the southwest corner of ,
a the excavation boundary of the site, it trends northeasterly. ; h deepest part of the valley penetrated by the borings is near the east
~
end of the south face of the excavation where 178 feet of Pleistocene
.; l.
l and Recent deposits were penetrated. hoe deposits become thinner up ' l
. i the slopes of the bedrock valley and cover quarts diorite to an eleva- l tion of approximately 135 feet on hills 238 and 200.
In the east half of the' north face of the site excavation a spur j
! of quarts diorite has been exposed whose axis (the line connecting high 1 -
p<1nts) plunges eeutheastward (fig. 3, 7, 8). h highest part of the ] i exposure is at an elevation of approximately 65 feet above mean sea 1evel at a point about 110 feet east of the northern target marking the centerline of the reactor; the exposure is 189 feet wide at an elevation approximately 37 feet above mean sea level. N surface of the quarts diorite is generally irregular. Near the top of the spur the local t
;; relief is about 5 feet.
h bottom of the buried valley near Campbell Cove is approximately 4 1 60 feet below sea level. h valley was probably eroded to e. hat depth .
, during a continental glaciation when sea level was several hundred feet l 3.
[ lower than it is at: present. N glaciation was probably older than the last one, the Wisconsia, which reached its =-4== about 20,000 7,' years age, for fossil wood that accumniated above the valley floor is
+
older than 34,000 years. i i IS' - i.,p.vol.d O me , ii. M1pmgeegszegeaggeswwnsece> p.cmmum:swv s 1
p ; . ~ :: 'y %: p ;-a3;py;g.:3g ,
, , p;- ::3 3q ~ .php r g~.z~p.ygpi~.;,g.
- 7. ,
- C'l,, ' . ', .
- 1. .
, Exposure of Pleistocene and Recent deposits in the site excavation -
afford a fine opportunity to investigate faulting later than the deposition of Wee deposits. This is particularly true of the north
, , face of the excavation, at the contact between the quarts diorite and J,:E. the sediments. If' faulting occurred through the site parallel to the-
, ^ .j . .
'l 1
trend of the San Andreas fault sone, displacement of beds'should be observable in the north face; it faulting occurred at large angles to the San Andreas, displacement should be observable in the west face of the cut. Faults dipping at low angles and transacting the seM=ents
, below about 30 feet shave sea level would not have been evident in any I
face of the excavation. Quarts diorite at the site In May and early June 1963 the quarts diorite was exposed only on the north face of the site excavation, where its surface reaches an-elevation of about 63 feet above mean sea level. Though the quarts diorite texture is preserved, rock in the face is decomposed, mostly by weathering, to a weakly to moderately well indurated material. The deeper rock tends to be less altered and more coherent, though pegnatitic rock is relatively little altered even at the highest exposures of the f granitic rock.
. Figures 7 and 8 show the location of the more prominent. faults in i . the quarts diorite exposed in the north face. Details of texture and
...i
'"i structure such as joints and faults were clearly revealed by washing 1
] . the face with water. This was done June 5-6, 1963, after figures 7 and
., 8 were photographed. Most' of the faults are oriented between N. 300 W.
to N. 838 W. and dip 6$8 to 808 eastward, and most have 1/2- to 3-inch-
. vide gouge sones. the some amar the right headedge of figure 7 consists
, ' 26' L q ,,
'-+ f] r ,. .i e , ' : < s a. 2 , - ~,. , . (; 1: j '. <
, ,. ,f waaes-eeummennwme..mee s.ee - mum .*
r, e~- y, p p;m m a, :~~~;m y
.- _ 3:.;: : u >:,;p r ~. y , .
~"*
. ]) .
i
! of several braided and branching 1- to 3-inch souge sones within a sone .
4 to 7 feet wide of hydrothermally sitered quarts diorite, which is somewhat more friable than that adjoining. Except for the faults noted d in figures 7 and 8 the quarts diorite in the north face of the excavation, i
, though decomposed, is not jointed and faulted more than the average rock
- .1 seen'near sea level on the western and southern shore of the Head.
-} ,
j - Directions of fault movement as shown by striations en slickensided fault
, surfaces are mostly parallel to the dip, though some are more or less I
, parallel to the strike.
i iouarts diorite in the nronosed reactor excavation l Flates 3 and 4 of the April 30, 1962 report by Dames and Moore show granitic rock in the botton 26 to 42 feet of the proposed reactor a excavation. Their conclusions are based on a borehole investigation and I l appear to be a reasonable interpretation of their observations. j On May 23, 1963 we examined core obtained by Dames and Moore in borehole 16, located at the center and extending to about a foot above !
, the bottom of the proposed reactor excavation, and from borehole 14, ;
i located at the south edge and extending to about two feet above the ; t
; bottom of the proposed reactor excavation. NK-size core pieces (2-1/8 4
inches in dimenter) obtained 1.5 to 6 feet aberre the bottom of the pro-
. .l posed reactor excavation (borehole 16) were between 1 and 2 feet long.
, The rock is slightly foliated, coarse grained biotite-hornblende quarts diorite. Although sound, the rock is not quite as fresh-appearing as
, .. j
, rock exposed at sea level on the western and southern shore of the Head in that the plagioclass foldepar appears more chalky white in the core.
This was confirmed,by microscopic' examination of thin sections of a core w
, ,' piece from depth 155.5 feet (1.5 feet above bottom of proposed reseter -
,j
, excavation) given to us through the courtesy of J. Dean Worthington, ,
27 ' a . s % e >
' - h-+ s , , e *~bc h
NMQNy@%%$$d%@@EM$VM@MM$h$} MAN [k@3EN$fsk
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.d i.
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)
i i I Chief Civil Engineer, Pacific Gas and Electric Co. Thin sections chow
)
the feldspar, biotite, and horneblende to be slightly more weathered than that of the rock collected from outcr'ps o on the shore. i n; 1
.; The bottom 39 feet of borehole 16 (elevation - 27 to - 66) was
.i
. cored. Rock from core 6 to 9 feet above the bottom of the proposed i
.v p _
reactor excavation is broken into 2- to 3-inch blocks by joints or shear j i zones both parallel to and at 45' from the vertical core axis. Core I
. rock from 9.5 to 22 feet above the bottom of the reactor excavation is '
1 not quite as fresh as rock below. Core lengths broken by joints or ! l l l
- shears, mostly lin~ed with chlorite, have an average length of 4 to 6 inches and a maximum length of 1 foot. Rock from core 22 to 39 fcat 1
above the bottom of the proposed reactor excavation is so=ewhat = ore
. weathered and considerably more fractured than deeper rock. Average length of pieces in the top 10 feet.is about 2 to 3 inchas; 14 feet from 1 .
q the top of the core, it is 5 to 7 inches. The rock is cut by nu=erous clay- and chlorite-lined fractures of various orientations. Their orientation is nearly vertical about 5 feet below the top of the core. 1 A clayey zone of unknown thickness, from which 3 inches was recovered was penetrated about 11 feet from the top of the core. Granitic rock from cores recovered from borehole 14 is fnirly sound, i i but core pieces are broken into 3-inch lengths by clay- and chlorite-
; lined joints. The rock is generally more shearod than rock penetrated by the bottom 22 feet of borehole 16.
i Rocks such as quartz diorite of Bodega Head typically extend'to a d, depths of thousands of feet beneath the earth's surface. The physical y.g integrity of the rock generally improves with depth; that is, effect of 4 I . 27a
- a d, k
+ v kih k MNhdd ih9.f Yj jhkMhh$+h 'dNhf : E f N
i
- .'[FfEbbi.S$Mwr2.u-mx-emi.Ams4112mudaLa i a.b w.;,$4AA ;.41 A ,C....' ~.
W m7.. ,Y, A ( { , j q- - 9 1 1 i weathering is less pronounced, jointing is less intense, and the physical ' i strength increases. No geologic evidence has been found to indicate that
;' the iguarts diorite is not typical in this respect. , ; z. .
Pleistocene and Recent deposits at the site
- )
. Prior to excavation of the plant site, Pleistocene and Recent deposits were reported by Dames and Moore to cover the quartz diorite to a maximum thickness of about 180 feet. In the acrth and west faces of the site about i
60 feet of sediments are revealed, mostly beds of clayey sand, silty sand, sand sui lesser smounts of sandy silt, candy clay, and organic sandy clay. The top layer is 5 to 8 feet thick and consists of dark-gray to grayish- l brown, friable to loose, gravelly, silty sand. The sand grains are well rounded, fairly well sorted, and have frosted surfaces coated with a thin i layer of carbonaceous material. Root holes and scattered pebbles of quarts diorite are comanon. The layer is thought to represent a mixture of windblown sand and soil developed on the underlying material and on j windblown sand. It grades downward to a dark ye1.lovish-orange, fine- to coarse-grained, clayey, friable sand, of variable thickness, up to 40 I 1 feet, and variable clay content, permeability, and bedding. l Tentatively the Pleistocene-Recent boundary is placed at a horizon I e at or near the base of the top layer, mostly of wind-blown sand which U , here overlies seMaants that are mostly water-deposited. Samples of wood from the water-laid material are now being analyzed for carbon-14
.v
., content in order to determine the age af'the deposits. Results may l
,, necessitate modifications of this conclusion. Most of the underlying e
4 s
.se i
- a. . #. -
s.g. w . al n :. e 2m m-;,%bi s s4,f :f:iW~c' w:Q Q . y x y @ T11: W n z. +MC'W WV W- +? c L' n a.m m a. w$ -; ~~ ~ ' ^
m n :n*- '
, ;-yyy;7 w a meerunewtw.s21:aneuner ar,umsens 7: ;,:w-y
- aw_L:3&Ty,7~-:g ;q:;yz yyg7,;,y' ; ;-
l- ,', .1:* , ~_ 1?:,
~. m makeL u e w, m m w
.2 - -- -
zf,+,'. ' q i Pleistocene deposits are believed to be marine terrace deposits or beach
'n deposits. Detailed stratigraphy has not been obtained.
Immediately overlying the quarts diorite in many places is a greenish j
'gg;T.
1
"; $, grey, poorly sorted, gravelly, cisyey sand' of irregular thickness as great ,
- X 1
'{P ,
as 40 feet. It is generally without bedding,.but in some places it grades l "j laterally and vertically into slightly better sorted material with
. ,. i
+. - moderately won developed. beds 1/8 to 2 inches thick. 'The sand and gravel .1 grains are obviously derived from guarts diorite and the deposit undoubtedly ,
i represents soil that developed on quarts diorite and was transported I
, short distances largely by gravity or by water currents. In.some places, j i i
! where the soil fomed by weathering remained in place, the quarts diorite I 1
-c.
grades into this material and it is difficult to find the ack surface. i
. 1
- t
(. 'On the south shore of Campben Cove the quarts diorite is covered, j a few feet a6ove sea level, by a sandy gravel bed about 4 feet thick unde l up of well-munded pieces having an average diameter of 2 inches, anzimus j I
! diameter 8 inches, in a sandy matrix.
In exposures near sea level on the western and southern shore, old
, marine terrace materials are found deposited against former sea stacks i
. of quarts diorite.
1 17 In some places the greenish gray clayey and sandy material has been l 3 eroded from the quarts diorite surface and younger sediment has been
<~
. deposited on it. This condition is shown on the righthand half of g ,
y
.m; figure 7 The large wedge-shaped deposit labeled (1) is a yenowish 3.2.,
'eman, poorly eensated, highly pemeable, wen-bedded, medium-to
.f'
~ .
coarse-grainst sand. It lies partly on quarts diorite and to the left '
- q. . . .
-t (west) on extensively gunied clayey sand. The dark patch labeled (2) is 4 g 6
. 4 tt , . .
i
-' ,, a N- 9 m q , ,I- ~ '
';
- O' MkMM%MDMS3;yN%d@fMMN'tW@55ITM5M9M5MMP6
%.. ,) . ._ "5x %D'~ ,-
%% t_ Wi&' WW"W-. WWG%l9% c:6 W C li M W ._ ; ,
1 *nt *.
. I l
- 1 I
1 I a resmant of the asssive greenish-gray, clayey sand that was protected
, from erosion before the deposition of the ' overlying sand because it
. .i, lies in a depression in the guarts diorite surface. ,
c;n dgg The extensively gunied deposits on figure 7 west of the sand wedge <' Ti labeled (1) are mostly light-brown to yenovish-brown, clayey, graveny, y:i t .t
- coarse-grained sands. The clay content ranges from about 3 to 20 percent.
o , P . Part of the deposits are . wen bedded and part are massive, poorly sorted, .j i i. and contain angular guarts diorite debris as large as 1 inch in diameter. Detailed views of it are abovn in figures 9 and 10; their location on I i l the excavated face is shown on figure 7 As is seen in figures 9 and i i i l j 10, the M m e is irregular and part of the deposit is cross-bedded. ' I i It is thought to have been derived from erosion of debris lying on the i
. quarts diorite slope immediately north of the site. )
Figure 9 and 10 show details of the bedding of Pleistocene deposits i i at the site and suggest the relative ease with which the sediasnts could l be examined for evidence of disruption by faulting. I A31/2-footthickblackorganiclayerconsistingoftreefragments 1 in a matrix of petty clay and sandy clay was encountered in a low tem-porary face about 150 feet south of the north face (fig. n ). Elevation (2}} ofthetopofthelayeratthestadiarodontheleft(east)sideof figure n is h6.1 feet above mean lower low water (MLLW). The deposit evidently did not reach the area of the north face. Stratigraphicany vE ~ .
.h , equivalent beds in that face probably lie between 60 and 70 feet above MLLW.
- 92 J,. :,i i .
The layer contains remnants of what aw believed to be tree roots yC& in pitee, though the tree limbs, which reach a diameter of 1 foot, now , a.: x'. lie parallel to the bed. Although this bed could not be fonowed
, i contianously besanoe it was concealed by vegetation and soil, it may
, e,. y
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mw
. s - -.. .. . . , , , .% < +. . - ,
4
,' ,.,! , +h, &5, &hN&s%.W$ i ~,5k%ff,h'.N, Y W
- h. 7" 5?.l*&&&. .,? ' j'.g'^i.m5 , p 5. N.., &iYi Nf. ,00$.w,N$$,$.c?$. N..YY&
;- .o . m .
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-s.
. .s w:;: :~r;- erw.y m:n:: m,n;wpewywycy :. n- m~r
. ,d .
I i extend 550 feet southeastward to the sharp bend of Campbell Cove, where it may be represented by a 1-foot-thick black organic layer adjacent to a layer containing two tree trunks, eac'a shout 2 feet in diameter and 1
. at least 6 feet in length lying parallel to the bedding.
f';h The organic layer is believed to represent either a forested tidal s'?n . l
' 'M marsh deposit, possibly vbers trees were inundated by rising sea level,
, or a fresh water lake in a fo. rested area. In either situation the T
original bedding must have been virtually horizontal. The deposit now l . dips fram 50 to 80 to the southeast. Dips of 40 to 50 were measured l from this bed'to the tree-bearing layer in Campbell Cove. Except locally, most beds exposed in the north face show a g6neral dip between 2 0 to 100 to the south and east. Some of the dip of these beds is the original depositional dip because the materials were transported and accumulated
,.1 .
.I on the southeast slope of hill 238. Nevertheless, the dip shown by the organic layer is proof that all the beds have been tilted southeast-l l vard since they were deposited.
! Age of the deposits I
Fossil wood from the black peaty organic layer at elevation 49 2 c feetaboveMLLW(sampleno.1279)wasanalyzedforitsradiocarbonage
~
(fig.12). Results received fras Meyer Rubin of the U. S. Geological Survey's Washington, D. C. laboratories, indicate an age in excess of k[.. I N
. ?.c
- 38,000 years. Isolated tree limbs are also found scattered at several 4 horizons in the seM= ants of the north face. The highest and, presumably, 3? -
byf R . the youngest one was found at an elevation of T5 feet MLur. It and
..y g another piece of wood frca eieratica 55 feet MLur have been submitted
. :y "..
4 Ay m . 31 & 32 ,
,.. ~~~a =e-==== ==a-e = * - - '= = * = = = = .
' bh Y I h f bb ,
hki -[. N ' d fb Y .
. , ? ,. v .,. ; ::::y; , y- Tr n a p 2:p . = r;,, y c 7 ~? m m m :
w wwms.ow m wwaw, a -~w J. , . 4} . i i for radiocarbon dating with the hope that an estimate will be derived from them on the rate of acetamulation of the terrace deposits. Bone
.I fragments as yet unidentified were found on the quarts diorite surface
.I
.. j in the north face.
t - Structures in the Pleistocene deposits at the site
,.1;
- ) .
Pleistocene beds in the north and west faces and temporary faces 150 to 300 feet south of the north face were examined for possible dis-ruption of bedding due to faulting. No faulting was found, though some disturbance of bedding from other causer was seen. If faulting took l l place in the area of the north and west faces during the last 38,000 1 . . l years, the sediments in these faces should show disruption by such 1 l faulting. Lack of disruption by faulting strongly suggests that none i took place at these faces during the last 38,000 years. The walls of 1, the largest gully, a few feet above quarts diorite, seen on figure 8 reveals joints in friable beds 1/4 to 1 inch thick of medium to coarse i sand and aparse 1/4- to 1-f.nch-thick layers of 1/4-inch size quartz l diorite debris. About sit joints were seen in the sediments of this 4 gully. Most are 4 to 6 inches from each other, although three 1 foot fl apart were seen. They are about 2 sua vida and are filled with fine-grained sand. Most of thema dip southward from 15' to 80*. Their strika 9 orientation is difficult to measure, although one was exposed and had a h. strike of W. 80' W. and a dip 70' to 80' S. One that could be foll'oved
- v. 6
';l for 5 feet varied in dip from 15' to 40' souttrrard. Beds are not dis-J
.., 1 . rupted across thema.
y ni .
.. i - -
; 33.
i
.; l '
. 5,UM'k h i .V.y. Q f,Q, p .fjh h!, EN ". ,7.}[hAkib Idin$h.M N $[jsl
4~ A,"?
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.e. I x d~. . "
- l e
l Alang one clean break that had an approximate strike of due west, dip 50' 8., beds were displaced about 1 inch vertically through an
~
interval of about 1 foot. One foot above the offset beds, however, the 1
;1 J beds were not dicturbed and therefore this displacement is believed to 4.
w . be caused by compaction of . sediments. g.; .
.n.
f Joints with no offsett%g of beds are present in the bedded sediments ! l that are about 5 feet above the ge. arts diorite surface and appear dark gray in figure 8 from 20 to 30 feet west of the largest gully, _i 1 Faults in the quarts diorite and details of the quarts-diorite sur- ) face in the north face were revealed by washing down with water. The l j faults were traced to the top surface of the quarts diorite, but none of
.1 them showed fault offsets in the overlying sediments. In many places the
,, surface of the quarts diorite was found'to be very irregular. In about four places the top of the quarts diorite was a stripped fault surface.
i The righthand side of the area labeled "2" in figure 7 is an exmeple. It is clear that in all of these places some quarts diorite adjoining j a fanit surface was detached and removed, possibly by wave action at the shore, before sediments were deposited above the fault.- In one place, i where about 2 feet of quarts diorite was removed from one side of a fault
.i
; before sedissats were deposited, a bed 6 inches above the quarts diorite ,
was offset by about 3/4 inch, although the beds about 3 inches
.j stratigraphically higher and above were not disrupted. The offset here
- e; . , , .
7j is interpreted as due to differential cease 11dation of sediments, for 2 - M sediments en the doun-dropped side'of the slight break are about 2 feet
,q .
( thicker than these deposited sa the other side of the fantt-line scary
~
la the quarta diorite. . . i . . [ $4
~
I . . 1 - j u; an.emamnann--m.ml 4 ,.. . y -x.. : ::. w ,. .e. + -
. . . . . l
, . . m . ,-, . . ~ , u . , .. . . . s . . a.m m. r y. , .. ...m .
e ,-, ... . . - - ~ -- - 1
-6s maA6&pi.D$$APM/%$. . m..;.m ,J. Na,.c,p.L , :,,6s;,3, .;,,x21W'ME OhIEU$PWEU$ffiE&f~ "#-
l
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.i The possibility that fault sevement of I or 2 inches in quarts ,
l l diorite can be accommodated by and disappear in a few inches of unconsol- )
> \
i idated sediment cannot be precisely evaluated, but it is believed to be
- -i highly unlikely.
]-
-j 3 Nearly vertical portions of the quarts diorite - sediment contact 4<y .
.j
. in the serth face are similar.co contacts of sediment against former ses
]
N q , stacks seen along the, western and southern shore. Furtheres:d, similar i relationships are being created today as anodern sediments are deposited offshore against vertical er even overhanging cliffs of quarts diorite. 1 1 l
; l i
I l l 9 4 1 1 i i l l l e
.i
.i ,
i .
,e t
'6
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1 i e e [ t
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- + . 3:17.y. .. q . .g g g' . g ~., g> f g ;y. g.,. ggy
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m:. w .%%mesny2.a m am -w - >. .. .
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1 POSSIBM PAULT COW 25t0L OF. EE 8005 SHORE OF CAMPBELL COVE The nearly straight south shore of Cangbell Cove and the nearly 1 71, . coincident alismaant of the buried valley cut in guarts diorite at the
.4 l- ,,3j site suggest possible control by a fault. No evidence for such a fault )
'i+;l l
~^l1 - was seen in the quarts diarite and overlying sediments alcag the south '
4
*',v shore of Campbell Core, although the exposuns are poor and it would be
-2 a t y difficult to see such evidence there. Neither was evidence found in quarts diorite exposed cm the western shore of Bodega Head on a line extended vestward tztus the south shore of Casqpbell Care. No faults vem seen la the sediment covering the quarts diorite at the south and of the q
vest face of the site excavation, ibare such a fault would be expected i i
'[j to pass. l 1 .
i l l
.;i ,
+4 l .
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f l * = 1
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% s {. ;.,
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DNdsfMi~dN,!b,Ad2MM^p%p^IMEI %.ru ,y.,s._,.,.g % 4.- _L.. ,.z e.e . - pp ,fd MSd*g;-.' ;h ~ di r _ '1
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.- . . . . . . - - - ' . . . - - .+ =
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+
,. ROCK-CUT TERRACE i
; A terrace cut in granitic rock was observed at several places along
.i
'1 the west and south shore of Bodega Head. It appears to be absent along -
- d. d, ,
M 7:p the eastern shore, though it may be at or below sea level there. It ~ .
".1
- slopes seaward fma an elevation of 10 to 20 feet above NLLW at the base
^6
. of the sea cliff to about 5 feet above NLur about 50 to 100 feet from
~ [.1 .
j the sea cliff. Where it is highest and closest to the base of the ses
! cliff, it is generally overlain by sand and gravel beds several feet in i 5 l thickness. Inasmuch as the terrace was cut at or near sea level, its 1
presence indicates an uplift of Bodega Head since it was cut; however,
- i
, no data were obtained on the age of the cutting. David Hopkins, geologist, U. 8. Geological Survey, suggested to us that the terrace was
.;o ,
A probably cut at a high stand of the ses~during the Sangamon Interglacia-tion, about 100,000 years ago, and that the sediments that lie on it may have been deposited during the following 01aciation, the Wisconsin. It may have been uplifted when tho' sediments, at the site were' tilted, but i no data on the age of the tilting were obtained. Tilting any also account for the absence of a wave-cut terrace on the east side of I Bodegh Head, although it any be obsent there because wave attack on the 1 east side of the Head was weak. l
, Uplift of the rock-cut terrace and tilt of the deposits lying on
- l. , quarts diorite at the site, both of which any have been caused by the same crustal moyensat, are believed to have been gradual over the entire
.y site.
'Y'4 0 .
l . e ge , ; > .-- '
-.+.--. ._s j, . . ., ,
{p.t.p, ,..w p a g w ; w ; w .w.v g g s y w w m a g g u u w g p #.w:. w;,.s x w ,y,, w . x q, p fa. g ; g eg g
_f,Why__y_&fghjAm b 9 h N k $ Q $ l,* S y f k 4 ??I f E 2 lli b ? $ M U G T U $ $ ' h $ N?? Ky '
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th4 -1 ik 1 r 1 l FALERING TIDtotEE 'IEE SITE DURIBO FUTURE EARTBqlAKES 1 i q' Al h ugh the portions of the San Andreas fault sono that were active -l 5
'A in 1906 are more than a mile from the site according to published reports
.j -
j
..@},l (Lewson and otherp,1906), it is not possible to say that future faultingI
,,' will take place only in the scoes of recently active faulting. The l
~ . - . \
entire fault some must be considered highly susceptible to active fault- l 1 ing. The fault sone has been a scos of weakness for a long period of. time,perhapsaslongas80millionyears.jPresentlyexistingorfuture differential tectonic stresses on the two principal blocks of rock on I
, either side of the fault are most likely to be relieved by rupturing samsvbere in W fault scoe, rather than by rupturing of the rock on ;
either side of the fault some.
'Although it can be argued that the fault has, over'the period of I its active life, graduauy grown from a single fracture of infinitesimal
, width to a zone of crushed and broken rock a mile or more in width and must, therefore, have incorporated blocks of sound rock during some episodes of faulting, it is nevertheless extremely improbshle, now ht the fault sone is well established, that future major faulting would pass through the rock of Bodega Head in preference to the weak crushed ,
material of the fault some itself. On the other hand, if the fault scarps
. ..l reported at the line of the 1906 earthquake south cif Mount Wittenberg
}
4 - (discussedearlierinthisreport)actuallycutthequartsdiorite;it opens up the possibility that such a :aJ - ----- could occur on Bodega
-t
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s . 4 + *
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i CONCLUSIONS i - On the basis of field investigation, a review and appraisal of i t l l reports en the site and published scientific literature on the area,
;. and an ===4=mtion of core samples from boreheles at the center and
?
- near the south side of the proposed reactor escavation, the following eseclusions are dravn
- 1. The sediments overlying quarts-diorite bedrock at the site, i
; the oldest of which were deposited more than,38,000 years ago, j have not been displaced by faulting where they are exposed in l ' ' 1 the north face of the site escavation between 30 feet above 1 i.
sea level and 115 feet above sea level. Thus no faults active in the past 38,000 years and roughly parallel to the San Andreas fault sone pass through the parts of the site excavated as of June 6, 1963. 1 j i
- 2. Quarts diorite bedrock beneath the proposed excavation for the l
reactor is virtually the same as that exposed on the western and southern shores of godega Head. The rock is jointed and
- faulted, and in places it probably would be necessary to remove 1
- .; some crushed and weathered rock from tabular sones in the quarts
\ g: t
?,
i . diorite and veplace it with concrete or other suitable filling,
} but the rock in general is structurally sound.
- 3. The site is west of the San Andreas fault sona. Coupe 111ag.
;-e geologie evidense is lacking for any major faults through both
- i t
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-i the Head such as the fault mapped by Johnson (1943). ]
41 4. On the basis of information available on June 6, 1963, the l
'I western margin of the San Andreas fault sone is at least l c j
.3 1,000 feet from the center of the proposed reactor. Gravity l
- .;.;n \
17 5 measurements suggest that' the western margin of the most yo .
-.'. intensely disturbed and crushed rock of the San Andreas fault sono is abent 3,000 feet east of the proposed center of the
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RECOGENIATIWS FOR FU2UIM HVESTIGATIGIS . i -,; Recently active, low. angle faults that do not' show up in the faces t A d
@g of the site excavation could transect the proposed' reactor excavation, iM
'.a4i 9t $h or high-angle faults that die out between the reactor excavation and C:,tc .
M the faces of the site excavation could be present. It is therefore Vd'1 recommended that the excavation for the reactor be studied down to the c , contact between the sediments and the guarts-diorite bedrock. A careful
. study of this type could prove or disprove the existence of recently l active faults through the proposed reactor and adjacent structures.
I Investigations of the fault cracks reported by Gilbert (see p.14
.l . of this report) on Point Reyes Peninsula should be made to determine I whether'the cracks extend into the quarts diorite or are landslide or
, other surface features. A' study of the stnactural relations of .these l
l i features might indicate the probability of faulting through the site 1 I l
, during some future earthquake.
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l
'.,,:..,4 EEFERENCES CITED q
Crowell, J. C., 1962, Displacement along the san Andreas fault, p California: Geol. Soc. America Spec. Paper. 71, 61 p. ) h Curtis, G. N., Everndon, J. F., and Lipson, J. I., 1958, Age determination e Y$ . of some granitic rocks in California by the potassiumr-argon method: y,'
- n. ;
California Div. Mines spec. Rept. 54,16 p.
%c ,
Dames and Moore, consultants in applied earth science, 1960, Preliminary soils investigation and Seismic survey.. proposed nuclear power plant, Bodega Bay, California: Unpublished report for Facific Gas i l
, and Electric Company, December 2, 1960, 4 p., 17 platos. '
1 g, 1962, Foundation investigation, Bodega Bay Atomic Park, Unit No. 1,
- m. -
;Y Bodega Bay, California: Unpublish&d report for Facific Gas and I t
g
./p
..i m Electric Campany,' April 30, 1962, 25 p., 26 plates, appendix A, 4
@ l 7 p. appendix 3, 2 p.
- s 1
Galloway, A. J.,1961, Geology of the Point Reyes Feninsula: p 30-34 l in Land use survey, proposed Point Reyes National Seashore, National 1 I q Fark Service, 57 p. l I 1962, Field trip 3--Point Reyes Feninsula and San Andreas fault d ra sone California Div. Mines and Geology Bull. 181, p. 391-398. ! dN yw Biggins, C. G.,1961, San Andreas fault north of San Francisco, . w- . l %,' California: Gool. Soc. America Ball., v. 72, no.1, p. 51-68. m om Johnson, F. A., 1943, Petalans region, California: California Div. m c. c: . -
. w, . Mines and Geology Ball. 118, p. 622-627.
J
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( l'@w$
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.s-9 lawson, A. C., and others,.1908, The California earthquake of April 18, 1G j 1906. Report of the State Earthquake Investi,gation Commission:
N
%g Carnagie Inst. Washington pub. 87, v. 1,'pt. 1, 254 p.; Pt. 2, l
%. w ,..
. aM p. 255-451; atlas, 25 maps and seismograms.
J. " t :f;;" Osmant, V. C., 1905, A geologica1'section of the Coast Ranges north of ag' .t M. s ;.# The Bay of San Francisco: California Univ., Dept.: Geology Bull., '
- v. k, no. 3, p. 39-87
;p 8potts, J. E., 1962, f.ircon and other accessory minerals, Coast Range .
4 1 batholith, California: Geol. Soc. America. Bull., v. 73, no.10,
, p. 1221-1240.
.- Tocher, Doo, and Quaide, Willies,,1960, Esport on earthquake hasards W
/ .
at the Bodega Bay power plant site: Appendix k of report issued
- ..$m -
#M: hsi by Pacific Gas and Electric Company titled, Preliminary hasards
,". - ,;.:< E ,
] summary report Bodega Bay Atomic Park Unit Bo.1,13 p., 2 tables,
.: 3 photographs, 1 geologic map.
I l Travis, R. B.,1952, Geology of the 8ebastopol quadrangle, California: California Div. Mines and Geology Bull. 162, 33 p.
*( Weaver, C. E.,1949, Geology of the Coast Ranges immediately north of 9k ,
' R.;
the San Francisco Bay region, California Geol. Soc. America
.:: .n.;
- v..,u Ag Nam. 35, 242 p.
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i ; n n, PART II--SEISMIC HAZARDS EVALUATION e
]
, 7: by Jerry P. Eaton ~
3.t ddE v$y$ The planned construction by the Pacific-Gas and Electric CPy ' (E'y
- of a nuclear electric power plant at Campbell Cove, Bodega Head,
. ,c n.y^ .
5 California, has precipitated considerable controversy over tho' seismic g.e, !
, n. .
hazard posed by the nearby San Andreas fault zone to the proposed plant. Several seismologists,-geologists, and engineers have prepared.
)
reports dealing with various aspects of the problem, some as consultants
., 4 1 to P. G. and E. and some in othsr capacities. The prasent report reviews pertinent data and judgments provided by these earlier writers and offers
.n
':,n a reevaluation of the seismic hazard to the proposed plant.
i h@ It is hoped that this review will illustrate the tenuous nature of 3 some of the scientific judgments that must be made, these judgments then 1 3 I serving as the body of " fact" on which the engineering design of the j plant will be based. The primary difficulty'is that the seismologist is i i i called upon to make judgments that require large extrapolations beyond 4 1 his personal professional experiences and even beyond those of the science
..g.. ,4 ., he serves. When such seismological judgments are shorn of qualifications
- A,x p,'y and condensed to a convenient statement for engineering guidance, they A;O .
P
. &p . , take on an unwarmnted ring of cerW.nty that belies their shaky founda-M
,M , tions. The thread of responsibility is broken at this step, the seis- ,
.gp mologist believing that he has handed it to the engineer, who reasonably l
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,N There is general agreement that about once per century the
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San Andreas fault can be expected to shift in the vicinity of Bodega
' .u .
- g. Head and to generate an earthquake comparable to that of April 18,
&g q 1906, which had an estimated magnitude of 8.2.
~
W.',3 The hazant posed by M D# - such an event to a power plant at Bodega Head has been estimated very 3x
- MlWR differently by Tocher and Quaide (1960) and by Saint-Amand (1963).
Two sources of danger must he considered: (1) the intense shaking of
.r .
j
; the ground at the power plant site caused by seismic waves propagat- ;
~
ing outward from the fault and, perhaps, from the actual pomanent displacement (fling) along the fault of the site itself, and (2)
,- possible dislocations within tha site due to rupture.by the main {
v l J[ ' fault, a branch of the main fault, a minor auxiliary fault, or to { f
@r landslides precipitated by shaking, etc.
n, Tocher and Quaide estimate that such an earthquake would pro- ( duce a maximum Modified Mercali Intensity of about VIII on the quartz
! diorite bedrock at the site and an intensity as high as 10R in the
.i j fault zone itself. They conclude that the power plant structures should be designed to resist an earthquake of WVIII, or to provide 9 a margin of safety, WII.
M Saint-Amand estimates a mMaum intensity of WII on Bodega
,, /k .
f Head for an earthcpake comparable to .that of 1906. If important' q a 3 u9:y .
, landslides were to occur during the main earthquake he would antici ,
'l
- g.
- pate'an intensity of WK, and if major faulting on the Head were to
((k5$: AD
; occur during the main earthquake, ExI. In support of thue utsmata A,4f f~ he cites Richt*A (1958) average expected maximum W intensities for
, s ,. '
average ground conditions in metropolitan centers of California
- 1 t
W VII to VIII for magnitude 6, W II to 1 for magnitude 7, and WII
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] for magnitude 8.
Because recorded max 4m- intensities of the 1906 quake near Bodega
/
j Bay appear to have been somewhat smilar than Richter's averages vould !
,, suggest and because the plant would presumably rest on the quartz diorite
. .w . .., ,
u.4 - bedrock, M4IX appears to be a reasonable estimate of the 1906 intensity
- .g . i l 1
,'l at the site. With the normal variation in intensity slong the fault yj (
>j .
resulting from variations in local conditions, amplitude of slip, etc., I intensities as large as MMX at the site from earthquakes no larger than magnitude 8.2 cannot be ruled out. t Feak ground accelerations that should accompany the foregoing i l 1 1 intensities can be estimated from Outenberg and Richter's (1942) semi- l
~ . -
.y i empirical relationship log a = 1/3 - 1/2. MM VIII, IX, and X yield l m !
; V accelerations of .15 g, .32 g, and .69 g, respectively.
Housner (1961) approached the question of =av4== expectable i i acceleration more directly. Estimating the probable length of fault break on the San Andreas required to produce earthquakes of agnitudes , l i of 7.0 and 8.2, and assuming the same depth of break i both cases, he deduced that the intensity of ska1Hng at the center of a mgnitude 8.2 earthquake fault break should not be appreciably greater than that at the i i center of a magnitude 7 0 earthquake fault break. This argument assumes j . that the energy liberated per unit area of fault ' surface is the same in both cases and notes that the extra faulting required for the magnitude
] 8.2 earthquake lies more than twenty-five miles from the center of the -
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Saint-Amand pointed out that the foregoing result conflicts with
, the common observation that larger intensities are indeed observed for i 1 l l 1
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- larger earthquakes. I su. gest that the very definition of magni-
- f. tude and the seismologist's ability to co=pute magnitudes from relatively j short period body waves indicate that the amplitude of waves radiated 1
-0 from relatively smal.1 areas of the fault surface increases appmximately
'fa.i
"]q
.. ten times between magnitude 7 and magnitude 8.
f1 An accelengram of the magnitude 7.0 E Centro earthquake of 1940. 1 .
! was obtained about five miles' from the end of the fe. ult break at a station -
1 on deep allirrium. Housner estimated that if the fs. ult had bmken another i fifty miles beyond ;ne point of measurement the me.asured accelerations ; i would have been fa,rty per cent greater. He also estimated that ground
,. 4
[. motion on a grsah.a outcrop would have been about one half as great as that measured on 't.he alluvium. From these considerations Housner con-cluded that the El Cents accelerogram, with a =mv4== acceleration of l 0.3 g, was a reasonable match for one that might be recorded at the Bodega i Head site during a magnitude 8.2 earthquake originating on the nearby San Andreas fault. It appears that Houanor's probable underestimate of ' the increase in ==v4==_ intensity betweUn earthquakes of magnitude 7.0 and 8.2 is largely compensated by the underestimate of ground motion l t amplification in the sediments beneath n Centro.
; Thus, we see that estimates of expectable earthquake accelerations j at the Bodega Head site rango from less than 0.2g (Tocher and Quaide) i
/.. jj ., to 13(Saint-Amand). It appears to me that an earthquake similar to
' .] that of 1906 would paduce peak accelerations between 0 3 and 0 7g, and -
f-
- values appramahia= 1g are not impossible.
/ c ,
l The second aspect of seismic hasard to the site, possible disloca-I s. tions in the site or adjacent parts of Bodega Head during a major saMh- ; 1 "
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, :l w"' .j quake, is even = ore difficult to. evaluate, and widely different opinions 1 R t
~
have been expressed. In the more common appmach to this problem'(Tocher
,(, .i j and Waide, Schlocker and others) it is assumed that the site is " safe",
u
%, despite its immediate proximity to the San Andreas fault zone, if no m.,j ;l JN @ i .
positive geologic evidence for recent faulting on Bodega Head can be found. ' It is generally agreed that the quarts diorite bedrock of the Head f'C.e S q . is intensively fractured, sheared, and offset along dinor faults. ' Saint- -q e] i A and argues on the basis of aerial photographs and a brief field trip to the area, that several faults large enough tu exert visible control l 1 over topographic features traverse Bodega Head. He considers these faults l
..l to be a real hazard to the pmposed power plant. In possible vertical
, .,. offsets of a recently elevated wav4 platform and possible horizohtal off-
'l ~
.y sets of some subtle linear soil features (elevated shorelines?) onsened
, . . s. .
on aerial photographs, he sees evidence for recent movements along the
) faults.
f From somewhat more detailed fieldwork, first Tocher and Quaide, then 1 Schlocker and others, found no compelling evidence for major faults travers- ) ing Bodega Head. They made detailed examinations of the quarts diorite
., in sea cliffs where the pmposed major faults should appear and found the 4
rock there to be no more penasively faulted than at other localities. M.f.l
? .
Field evidence suggests that most, if not all, of the fracturing, shead
.} .
. .. 9
~l ing, and faulting in the quarts diorite is very ' ancient. Fault off,- .
[$ .< sets followed up through the quarts diorite have not been found to off-g;
-]
set pleistocene marine terrace deposits that appear to be several tens of
,G -
thousands of years old.' O sp weathering of the top of the quarts diorite Yq ,
.j 3cq severely restriots ones ability to follow small offsets up to the base of
. ' i:) the pleistocene cover in maar places, however.
More intensive geologio, studies have been made in an excavation at ,
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6 - l the site itself. Careful examination of the quartz diorite - terrado deposit contact around the periphery of the excavation failed to reveal q faulting that extended upward into the terrace deposits. Detailed map-pin. pf minor contacts and bedding features within.the pleistocene de-s.
- i. . , posits being exposed in the excavation for the reactor pit is now under-0 way. Results of this study should be followed very closely.
I believe that, absence of demonstrably recent faulting in those . l parts of the site and Bodega Head where field relationships are reasonably clear is not an adequate criterion for establishing the safety of the site. Though necessary, it is not sufficient. In the maze of old' faults and fractures cuttin6 the quart,z diorite, minor recent offsets could be
-l easily overlooked. On portions of the Head where the quarts dioritepleisto-cone contact is badly weathered or otherwise obscured small recent offsets I
I pabably could not be traced from bedrock into the sedimentary cover. The site excavation itself samples only a fraction of the region'in which j faulting might prove disastrous to the plant. The coanon occurrence during large earthquakes of offsets-on a num-ber of minor faults in sympathy with a large displacement / &n the causa-I tive fault cannot be disregarded (Fig.1). Displacements in bedrock- ' l west of the San Andreas fault on the Point Reyes Peninsula at the time of the 1906 earthquake (Gilbert,'in Iawson and others,1908) indicate _. that faulting does occur outside the San Andreas fault sono in sympathy s . with large displacements within the sone. The occurranoe.cf. such off-N.l -
'! sets on Bodega Head during future earthquakes is a definte possibility.
y Because reliability limits on any estimate.of skismia hasard to the _. , pmposed plant are so breed,'the statement of tolerable risk must be " very general. The magnitude of possible human damage that would result S"' '
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i from the destruction of the plant by an earthquake suggests that..it : d j should be built only it' there is no reasonable doubt' that' it would' ' i 1
,e survive : er.rthquake likely to occur on the nearby San Andreas fault. ;
,j;g It appears to me that the site does not meet this test and that it is .
e not an adequately safe location for a nuclear power' plant. . Few places on the earth are exposed to more certain earthquake risk than are those along the San Andreas' Fault in northern and central'y California. the case arguing the safety of the Bodega Head sit,e rests ' . i i largely on the confidence that " granite" is a good foundation material j and that it minimizes groud shaking due to earthquakes and on the . l jutament, supported but not pmved by geologic investigations of Bodega l I' Head, that no faulting has occurred there during the past several
; .thousand years. 'The case against the site stresses seismology's lack i
l of detailed information on events and conditions in the epicentral tract i of a major earthquake. Because we cannot prove that the worst situation ' j will not prevail at the site, we must recognize that it might. - Acceptance of Bodega Head as a safe reactor' site will establish a a precedent that will make it exceedingly difficult to re, ject any pro-4 J posed future site on the grounds of extreme earthquake risk.
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Additional References l l Housner, George t'. (1961), Earthquake Hazards and Earthquake Resistant
~ ;1 Design, Bodega Bay Power Plant Site, Pacific Gas and Electric Company, Typewritten Manuscript. -
i Gutenberg, B., and C.F. Richter (1942), " Earthquake magnitude, intensity, f . energy, and acceleration", BSSA, vol. 32,1942, pp.163-191.
. , Richter, C. F., (1958), Elements--r Seismolory, W. H. Freeman Company, .
'l j San Francisco, pp. 768 .
i l
' Saint-A=and, Pierre, " Geologic and Seismologic Study of Bodega Head, Typewritten Manuscript Schlocker, J., M. G. Bonilla, and A. Clebsch, Jr., " Geologic investiga-
- tions of a proposed nuclear power plant site on Bodega Head, Sonoma County, California, U.S. Geological Surysy report TEI--837, July,1963 4
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