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l Joe Feuchard, Nous servise'Eranch
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Divisian of Ptsblia Information, BQ Rodney L. Southwick, Assistant to the i
Manager for Public Infeemstion, SAM L
f-Saossa arrouswrs' marruevazz azaar om noomm MIsELS As meted in the final paragraph of og September A,1963, meme.
rendum, there is trapandtted heroustk a sepy of the relaaes of August 29, 1963, and report by Pierre Saint-Ammad om Bodega Say, i
with Saint-Amand's evaluettom of earthquake hasards at Bodega Bay.
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Certain pages de met appear in the copies becamas of difficulty in reproduaias photographs. As alae meted in my % : ' : 4, 1963, name, we have been promised tuo additiemal copies of the origiant saint-Amand report w h mest week, and one each util be sent to Feuchard and 1munostein, i
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GEOLOGIC AND.SEISMOLOGIC $TUDY OF BODEGA HEAD By Pierre Saint-Amand*
l 1.
INTRODUCTION l
1 1-1.
Field Work. The author visited Bodega Head on 6 and 7 April 1963 in order to make an examination of the geology of the Head. The area was traversed on l
foot in company with Oreste W. Lombardi. The study consisted essentially of l
mapping on both aerial photographs and on a topographic map. Special attention i
was paid to faulting, condition of the terrain as foundation material, probability
- i of landslides, and other aspects of engineering geology applicable 'to the con-struction of an atomic power plant on the Head.
1-2.
Previous Wod.
V.C. Osmont (1905) included a discussion of Bodega Head in a paper on the regional geology. Johnson (1934,1943) presents a general geo-
)
logic study of the region. Tocher and Qualde (1960) present the engineering geol-l ogy and seismology, and Housner (1961) discusses criteria for engineering design l
based on the work of Tocher and Quaide and on his own observations. Koenig (1963), of the State Division of Mines, has summarized the geology of Bodega j
Head. A study by Dames and Moore, concerned with foundational aspects, was not available -to the author at the time of writing.
2.
GEOLOGY 2-1.
Geography of Bodeoa Head. A general view of the region is shown in i
Fig.1, a close-up of the Head in Fig. 2, and the topography in Fig. 3 (from a map taken from Tocher & Quaide).
l The Head is an erosional remnant of an elongated ridge lying along a SE-NW line seaward of and parallel to the main San Andreas fault zone and in the general area of deformation marginal to the fault. It is connected to the main-l land by a tombolo of sand dunes lying over a mixed collection of littoral deposits and wind-blown sand. The long sand spit of Doran Beach State Park extends
-southwestward from the mainland to enclose the south side of Bodega Bay. A I
l protected channel has been built to prevent sanding of the entrance to the bay, which is used as a harbor by fishing and pleasure craft.
J l
2-2.
Stru cture. Bodega Head is part of a long, thin ridge of rock, interrupted-l ly connected to Tomales Point (4.5 miles to the southeast) by a line of sub-marine hills. The ridge bounds the western edge of the San Andreas fault zone, and is composed of a line of horsts, or uplifted blocks, squeezed up by move-ment on the fault. Similar ridges border the San Andreas and other strike-slip
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faults in many pl, aces. These ridges are slowly and continually being forced up-i ward at a rate somewhat faster than that at which the forces of erosion can re--
move the extremely crushed and broken rock. The presence of such ridges is a diagnostic aid for estimating the activity on a fault.
l
- Consultant in Seismology and Engineering Geology 602 A Essex Circle i
China Lake, California l
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. )RTHERN CALIFORN A ASSCCR(_JN TO PRESERVE BODEGA HEAD AND HARBOR TE: TH 1-6399 2714 Durant Avenue Berkeley 4, Calif.
PRESS REI. EASE FOR IMMEDRTE REI. EASE (Thursday, August 29, 1963)-
A-PIANT EARTHQUAKE HAZARDS REPORTED A new report on earthquake hazards at Bodega Head was released today by a delegate to the meeting of the International Union of Geodesy and Geo-Dr. Pierre Saint-Amand, who-will share the. leadership of the I'UGG physics.
tour of the San Andreas Fault with Dr. Don Tocher, is ' the author of the report.
He released it personally at a press conference in Berkeley this morning.
The report shows that the distance from the reactor' site to the edge of the San Andrea1 Fault zone is about 750 feet._ (AEC regulations require at least 1/4 mile separation.) At such close proximity a phenomenon known as
" fling," a whip-like action, occurs during major earthquakes.
The reactor, therefore, could conceivably be heaved 20 feet northward
,,,,, m in a few seconds during a temblor equivalent to the 1906 San Francisco carth-quake.
The report also shows illustrations of a potentially active fault through the exposed reactor excavation--in addition to several active faults crossing the reactor's cooling-water discharge pipes.
The 25-page illustrated study makes no bones that the site chosen by PG&E for a 325 megawatt nuclear reactor is exceedingly hazardous.
It notes, for example, th'at "a worse foundation would be difficult to envision."
"It is surprising l"' Dr. St.-Amand reports, "in view of the expert advice given by Tocher and Qualde, and by Housner (PG&E ' Consultants) that another site was not chosen and that construction has gon6 ahead...The location on Bodega Head is hazardous from a geological and seismic point of view."
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David Pesonen, spokesman for the Northern California Association to Preserve Bodega Head and Harbor, said at the press conference that the group had intended to submit Dr. St.-Amand's testimony in open public hearings be-fore the California Public Utilities Commission.
He added, however, that since the PUC turned down the Association's i
recent petition for re-hearing, they felt that the IUGG conference was an ap-propriate time to release the report publicly.
(William Bennett, President of the PUC, disagreed with his colleagues and recommended that hearings be re-opened.)
Dr. St.-Amand was appointed by the United States Ambassador to Chile and the Chilean Government '.to investigate the cause and extent of the disastrous Chilean earthquakes of May 1960.
He had been on loan since 1958 to the University of Chile in Santiago under the auspices of the U.S. State Department and the International Cooperation Administration.
His report ~ on the Chilean catastrophe has been published by the Naval Ordnance Test Station at China Iake, California, where he is head of the l
I Astronautical Sciences Division, engag ed in research on high atmosphere physics, optics, radio properties, seismology, tectonics, and structural geology.
Dr. St.-Amand has been a Fulbright research scholar.
He received his Masters degreelin geophysics from the California Institute of Technology in 1951 and his PhD in geophysics and geology from the same institution in 1953.
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GEOLOGY AFTER JOHNSON,1943 AND K0ENIG,1965 Geoloolc enop of Bodego Hood, Sonomo County, showing Son Androos Fault zone.
-(Reproduced from."The Geologic Setting of Bodega Head,"
.Koenig, J.B., Calif.'Div. of Mines and' Geolog/, Mineral Information. Service, July 1963.)
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Geologic and Seismologic Study of Bodega Read by Piene S4nt-Amand j
1 a
1 INTRODUCTION I
t-Dr. Saint-Amand's previous publications are many. The most recent, Los Terremotos De Mayo--Chile 1960, has been published as Tech-l nical Article No.14 by the Naval Ordnance Test Station, Michelson Laboratories, China Lake and 1
Pasadena, California. It is the first eye-witness I
account of a major earthquake by a recognized t'
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expert seismologist who was there to ride it out.
Dr. Saint-Amand's interest in Bodega Head i
+
began in late 1962, when David Pesonen of the l
Northern California Association to Preserve Bo-dega Head sent him several aerialphotographs of j
the area for study. Subsequently, Dr. Saint-Amand, accompanied by Dr. Oresti Lombardi,.al-i so from China Iake, made a two-day field trip to l
the headland. He enjoyed the advantage ingath-ering his data of seeing the proposed reactor site
" opened up" by Pacific Gas and Electric Compa-ny's initial excavation. In addition, the reports of the utility's experts (both close friends of Dr Saint-Amand), Dr. Don Tocher of the University of California and Dr. George Housner of the Cal-1 ifornia Institute of Technology, were made avail-able to him by the Association.
This report is Dr. Saint-Amand's analysis 1
and coriclusions concerning the hazards posed by the Pacific Gas and Electric Company's anticipa-ted construction of a 325 megawatt nuclear reac-tor at Campbell Cove on Bodega Head.
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TABLE OF CONTENTS page
- 1. Introduction.. e.........................1 2. Ge o log y.............................. - 1
- 3. Possibility of a Severe Earthquake at Bodega Head and the Possible Consequences..... s............
14 4. C on clu s io n s........................
2 0 5. Re f e re nce s........,...................
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i GEOLOGIC AND SElSMOLOGIC STUDY OF BODEGA HEAD By Pierre Saint-Amand*
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1.
INTRODUCTION q
l 1-1.
Field Work. The author visited Bodega Head on 6 and 7 April 1963 in orde l to make an examination of the geology of the Head. The area was traversed on J
foot in company with Oreste W. Lombardi. The study consisted essentially of I
mapping on both aerial photographs and on a topographic map. Special attention
)
was paid to faulting, condition of the terrain as foundation material, probability 3
of landslides, and.other aspects of engineering geology applicable 'to the con-l i
struction of an atomic power plant on the Head.
1 - 2.
Previous Work.
V.C. Osmont (1905) included a discussion of Bodega Heac in a paper on the regional geology. Johnson (1934,1943) presents a general geo-logic study of the region. Tocher and Qualde (1960) present the engineering geol q ogy and seismology, and Housner (1961) discusses criteria for engineering design i based on the work of Tocher and Qualde and on his own observations. Koenig 1
(1963), of the State Division of Mines, has summarized the geology of Bodega J
Head. A study by Dames and Moore, concerned with foundational aspects, was not available to the author at the time of writing.
2.
GEOLOGY 2-1.
Geography of Bodega Head. A general view of the region is shown in i
Fig.1, a close-up of the Head in Fig. 2, and the topography in Fig. 3 (from a map taken from Tocher & Quaide).
The Head is an erosional remnant of an elongated ridge lying along a SE-NW line seaward of and parallel to the main San Andreas fault zone and in the general area of deformation marginal to the fault. It is connected to the main-land by a tombolo of sand dunes lying over a mixed collection of littoral deposits i
and wind-blown sand. The long sand spit of Doran Beach State Park extends southwestward from the mainland to enclose the south side of Bodega Bay. A protected channel has been built to prevent sanding of the entrance to the bay, which is used as a harbor by fishing and pleasure craft.
2-2.
Stru cture. Bodega Head is part of a long, thin ridge of rock, interrupted-1 ly connected to Tomales Point (4.5 miles to the southeast) by a line of sub-marine hills. The ridge bounds the western edge of the San Andreas fault zone, and is composed of a line of horsts, or uplifted blocks, squeezed up by move-ment on the fault,. Similar ridges border the San Andreas and other strike-slip faults in many piaces. These ridges are slowly and continually being forced up-ward at a rate somewhat faster than that at which the forces of erosion can re-move the extremely crushed and broken rock. The presence of such ridges is a diagnostic aid for estimating the activity on a fault.
- Consultant in Seismology and Engineering Geology 602 A Essex Circle China Lake, California i+
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1 5.
2-3.
Bedrock. ('he bedrock is a medium to coar(~ -grained granodiorite,-
called "Bodegc dk.ite" by Osmont,(1905). It is prk., ably a part of the coast range batholith and is similar to other granitic rocks found along the coast and l
offshore. It is generally thought to be middle to upper Cretaceous in age; I
radiometric dating indicates an age of 80 to 90 million years (Curtis, et al.,.
1958). It is occasionally injected by acidic dike rocks. Rock crops out on the higher parts and along the seaward sides of the head; in one or two places it is exposed along the shore on the bay side of the head.
2-4.
Weatherino and Soil In newly exposed outcrops, where the sea is ac-tively removing material, the rock is relatively fresh but is broken into small l
fragments. Where erosion is less active the granodiorite is covered by a
)
deeply weathered clay-rich residual soil derived from we 4thering of the granodiorite. The residual soil is usually covered by a well-weathered sand, in part of aeolian origin. A sample of the upper sand from the reactor-pit excavation shows angular to sub-rounded grains of quartz and feldspar.
Horizons of the soil may be readily seen in the reactor pit and on the south side of Horseshoe Cove.
l Near Horseshoe Cove,1 meter of dark humic soil overlies 2 meters of coarse orange sand, whJc.h in turn rests upon 4 to 5 meters of light-gray arkosic sand. This material is somewhat different from that in the reactor pit.
None of the sediments are in any way cemented or indurated. They are in i
part Recent Marine or Estuarial deposits.
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Tocher and Quaide express the view that these are terrace deposits, l
pointing to the presence of mussel shells in the soil. Koenig (p. 6) describes
)
these sediments in some detail, concurring in general with the other observers.
They are probably correct, because although definitive evidence for these being terrace deposits seems hard to find, a series of what appears to be raised shore lines can be seen on the flat, grassy surface just west of the sand dune area (see Fig. 2, and Fig. 3, Point A).
hf.
Along the inlartd shore at Point D, Fig. 3, about 10 meters of coarse red l
sand unconformable overlies a blue-gray sand for the remainder of the exposure.
I Bedrock is not seen here. The sediments at this point are deeply gullied, are scarred by small slumps and ooze water from many small springs.
2 - 5.
Ground Water. The sediments are loosely consolidated, porous, and permeable. At the time of this study the sediments were saturated, and numer-ous small springs could be seen at the water's edge. At Point E, Fig. 3, a fault cuts the cliff and a good spring has developed. Here bedrock is just visible at about sea level. As Tocher and Quaide point out, the soils are saturated during rainy seasons and probably dry out somewhat in late summer.
2-6.
Fra cturing. Tocher and Qualde describe the rock of Bodega Head as i
extensively sheared and broken. Johnson (1934, pp. 24-25) reports the same.
The rock is indeed severely fractured, as Figs. 4 and 5 show, the fracturing being most intense near faults. In many places it is difficult to find sound rocks larger than a man's head. -
Often, in shear zones and in the vicinity of faults, the fragments are augen-shaped or rudely tabular and are aligned along the general trend of the faults. In such regions the "soi-disant" solid rock has the consistency of a vertically stratified alluvial deposit. Many of the surfaces have thin mylonite zones. In some places, the rock is a tectonic or cataclastic breccia. A combination of the fracturing, mylonitization, deep weathering, and an abundance of ground water render the bedrock far less stable than similar rock d4 7-g.q a
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would be had it'not been comminuted by the eons of shearing which it has undergone.
2-7.
Faulting.
Minor faults abound on Bodega Head and a few major faults can be clearly seen. The best way to appreciate' the degree of faulting is to walk along the sides of Horseshoe Cove.
The Cove itself is a rentrant cut by the erosive action of the waves j
from the zone of a large fault that trends about N450E. The crushed and broken i
rock of the fault zone has been more easily excavated than the relatively harder l
surrounding rock.. The Cove is bordered on both sides by faults parallel to its l
northwesterly and southeasterly; shores. The faults on the northwesterly side I
(Point F, Fig. 3) are very well exposed.
.A good-sized fault is expo' sed every hundred feet or so along the south-westerly side of the Cove. Short canyons have been elaborated along these i
faults. In the bottoms of some of these canyons are granodiorite boulders j
covered by alluvium and slope wash. Six steeply dipping faults of consider-j able size, measuring 1 to 5 meters wide, are found between Points C and G.
These faults trend between S45 E and S60 E, and are sub-parallel to the San 0
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Andre as. Occasional thmst faults, steep reverse faults and normal faults can be seen between the high-angle faults.
In fact, each rentrant along the coast is determined by a fault. One of t'ge pore notable ones (Point H) can be traced easily on air photos, and with only a little difficulty on the ground,,to the southwest of the hill crest marked 238 Between Points F and H, and as a matter of fact clear to Windmill Beach, faults sub-parallel to the coastline channel the wave-cut platform.
i The wave-cut platform that surrounds the seaward side of the Head dis-plays faulting very clearly. At several places the platform is at a different altitude on either side of a fault, indicating differential uplift across the fault, J
In places the platform is destroyed or completely. missing--for example, where the fault shown by Johnson enters the sea. At Point I the platform is at different elevations on either side of the fault,.and the 100 meters or so of missing platform h'eads against a cliff so fractured (Fig. 6) that it is actively lands 11 ding along the upper edge and face.
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Near Point K a rentrant channel has developed in the wave-cut platform (Fig. 7). At the head of this channel a fault zone--50 meters wide, consisting l
of septa of rock caught between shear zones and curtains of gouge--is exposed in the cliff. Figure 8 shows a portion of this fault zone. Horizontal stria may be seen by exposing any of the mylonitized surfaces.
4 At Point L a Japdslide is developing in the zone of a very large fault.
?$}M Material is being removed by mass movement, slope wash, and gullying.
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One spectacular fault is exposed on a bedrock high on the northwest
.Aface of the excavation for the reactor, the approximate location being at Point
- M; the original banks on the northwest and southwest sides of Campbell Cove h%)lif]ly in the rain..een removed by the excavation and precise location was diffic esp 6c The fault lies near a temporary road, a temporary drainage' culvert, and a stake marked " Top Bench 55.0".
The fault extends up and down the road for at least 50 meters, strikes N45 W, and dips vertically, 0
as nently as can be seen in the partial exposure. Numerous small fractures
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f 39, delineate the zone; these are interspersed.with septa of crushed rock and cur-tains of mylonite and gouge measuring 2 meters wide. The majority'of the stria indicate dexteral strike-slip displacement, but some clearly indicate vertical movement. Figure 9 shows a section of one wide gouge zone.
This fault alone might well concern the builders, and is ample reason to recommend against the use of the site because of the possibility of movement thereon during or following an earthquake.
2-8.
La nd slid ing. Numerous examples of soil creep and mass wasting are to be seen along the coast. The rentrant at Point D has been the scene of several small slides. The sediments here are now extensively gullied and show small slumps, terracettes, springs, and a topography indicative of an unstable terrain. An old landslide, on a gentle slope in " solid" rock, is found at Point N: the debris that flowed out is overgrown by gorse.
The water-soaked condition of the soil, the looseness and lack of com-paction, and the broken state of the rock all suggest that landsliding could be expected following a major earthquake during or just after a normal rainy season
--especially following extensive construction work that will redistribute masses of earth and change the ground loading.
The former Campbell Cove is now being filled to make a dock area and to dispose of the dirt removed in excavation. Unless an excellent sea wall is placed on the bay side this deposit will be unstable even in the absence of earth-quakes, and even with a sea wall the situation would be difficult in the event of an earthquake. A similar installation in Puerto Montt, Chile was destroyed in 1960 by liquefaction of the soil (Saint-Amand,1961, p.19, and Duke and Leeds,
)
1963).
eun 2-9.
Recent Uplift. Several lines of evidence point clearly to Recent uplift of l
Bodega Head:
Soil now found above sea level is in part a terrace deposit (Tocher and Quaide) that was originally emplaced below sea level, as indicated by the mussel shells. Absolute age of the deposits and hence a maximum age for the uplift could be determinable by Carbon-14 dating of wood and shells found in the de-posits. Carbon 14 dates for wood from the reactor pit, taken at about sea level, yield ages in excess of 42,600 years; dates are not yet available for shells and other debris from higher in the section. A careful study of these materials will be I
l Very important and interesting from an academic point of view.
Elevated shorelines, shown in Fig. 2 and in the plain at Point A, Fig. 3, are subdued but nevertheless visible in the field. The soil is soft sand, and al-though it is stabilized by grass cover one would expect erosion to have pro-coeded so rapidly, because of the abundant rainfall, that these marks would not be very old. This bespeaks a very recent uplift.
The wave-cut platform around the head is also diagnostic of Recent up-i lift. It is apparently a local effect, not caused by a eustatic change of sea level as postulated for uplift of the terrace deposits by Tocher and Qualde (page 7).
l This platform is quite young, is at different elevations in different places, and is clearly offset vertically by some faults.
NW A similar platform was found around Isla Mocha and near Lebu, Chile,
$11owing the great earthquakes of 1960. At that time Isla Mocha was elevated s.Ghc,(3 meters and Lebu 1.5 meters (Saint-Amand,1961).
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On the southeast shore of Campbell Cove there is a concavity cut by storm waves, probably before the mole was built. This is now elevated about a meter above the level at which it was formed. It was probably uplifted at the same time as the wave-cut platform.
Several of the canyons produced by erosion of fault zones now have old bottoms exposed at a height of 3 to 5 meters above present sea level (Fig.10);
they have wave-rounded boulders in the bottoms, and are filled with slope wash and soil. The same sort of boulders may be seen at the.present sea level in the heads of channels produced in the same fault zone, where the sea is now fash-loning them f[om joint-blocks and fragments. The soil in the old canyons is quite young, loose, and uncemented. This situation also indicates a Recent up-l lift.
l The rather surprising depth of alluvium reported in Campbell Cove is sug--
gestive of a tectonic origin for that bedrock depression. It is difficult to see how it could have been cut to that depth by erosion. Hence it is possible that it was produced by the down-dropping of a small graben; however, other explanations 4 could probably be found as well.
2 - 10. Recency of Tectonic Movement. Although fresh fault scarps are scarce, several were noted, and abundant other evidence such as cited above indicates j
vigorous Recent tectonic activity on the Head.
The fault lying along Points F and B, Fig. hon the northwesterly side of i
Horseshoe Cove, clearly offsets the pattern of the old shorelines, as may be seen in Fig. 2. Another fault, at Point C, leaves a faint scarplet running sub-parallel to the shorelines.
The relative lack of fault scarps is deceptive. This is almost certainly be-cause of the failure of the soft-soil overlying portions of the Head to reveal move-ments in the bedrock, and because rapid erosion caused by the heavy rainfall on the soft soil would quickly destroy any such scarps.
For example, in 1906 the San Andreas ' ault moved about 16 feet in the f
vicinity of Bodega Bay. Lawson (1908), p. 65, quoted by Tocher and Quaide, pp.1-3, clearly states that even this considerable movement was not visible in the sand dunes nor across the Doran Beach sand spit.
Fault movement effects produced by earthquakes have always been easier to follow over bedrock than over even the shallowest alluvium. Furthermore, the San Andreas fault and the majority of the fractures on Bodega Head undergo mostly horizontal movement, and this sort of displacement does not produce scarplets as conspicuous as an equal amount of vertical movement.
Scarplets do not long endure under the climatic regime at Bodega Head, l
nor in the type of soil found there. The trace of the 1906 earthquake is now quite l
modified. A similar example was shown in the Kern County earthquake of 1952 which broke a series of scarplets along the trace of the White Wolf fault (Buwalda and Saint-Amand,1955); today th' se scarplets are scarcely visible.
e Far less rain falls in Kern County than at Bodega Head, and the soil is much firm-l er. The scarp of the 1872 earthquake in the arid Owens Valley is now so smooth I
L and modified that it is in places difficult to recognize.
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Hence, one must observe considerable caution in estimating the state of present or Recent tectonic activity on the basis of vertically displaced scarplets l
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14, 3.
POSSIBILITY OF A SEVEkE EARTHOUAKE AT BODEGA HEAD I
AND THE POSSIBLE CONSEQUENCES l
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3-1.
Estimates of Frequency of Earthquakes. Tocher and Quaide, p.12, in regard to Bodega Head, estimate that "at least one and perhaps two major earth-quakes can be expected near the site within the next century. These may be at least. as strong as or even stronger than the California Earthquake of April 18, 1906." Housner, p. 3, says "It has been estimated that a large earthquake, suc as the 1906 shock, may be expected to occur along the San Andreas fault perhaps three or four times per 1,000 years. Less intense ground motion can be expected to occur with greater frequency, it being estimated that ground motions at least sufficiently strong to cause damage to poorly designed structures may be expectc at the site several times during the next hundred years." The author's personal guess would be almost the same as that of Tocher and Qualde.
l Small earthquakes do not often occur on the San Andreas fault. Large Sar !
Andreas earthquakes have occurred in the San Francisco region in 1838 (Louder-i back,1947) and 1906 (Iawson,1908). The 1857 earthquake (Wood,1955) toch i
place a little further south and is cited to indicate the general activity.
i A great earthquake in 1836 was tentatively assigned by Louderback to the Hayward Fault, Fig.11, a branch of the San Andreas fault at the western foot of the Berkeley Hills, and another great earthquake occuned on that fault in 1868.
Each time the San Andreas Fault has moved it has jumped a distance of 4 to 8 meters. Strain is estimated to be accumulating at a rate of about 6 to 7 meters per century across the fault. Hence one could expect at least one great I
earthquake per century.
Other similar fault systems have had a similar history. The great Yakutat Bay earthquakes of 1899 (Tarr and Martin,1912) were probably on the same fault as those of 1958 (Tocher,1960). In southemChile the Arauco Fault hac a great earthquake in 1835 and another in 1960 (Saint-Amand,1961).
Smaller earthquakes on nearby faults may be expected oftener. These wil probably cause no serious trouble unless they occur on or near Bodega Head as aftershocks of a larger event.
3-2.
Intensity to be Expected at Bodega Head. The intensity (severity of shaking) will be about the same as for other earthquakes of magnitude 8 to 8.4 at similar distances. Tocher and Quaide estimate a Mercalli VIII or IX for an earth-quake equal to the San Francicco earthquake of 1906.
The author's own observations made in other earthquakes would incline him to guess that MM IX would be the least expectable intensity. MM X might bc noted if landsliding of consequence were to occur, or if one of the faults on Bo-dega Head were' t'o move. If a large fault on the Head were to move during the main earthquake the intensity could easily reach XI. It is difficult to give a good opinion for intensities above MM IX because the scale itself is not too precise and the intensity assigned depends upon the presence of certain diagnostic structures and con,ditions.
Richter (1958, p. 353) gives the following table for average Me'rcalli in-tensities to be expected for metropolitan centers in California and for ordinary
_f ground conditions:
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These values are in good agreement with those observed in actual earth-quakes.
The author's own estimate for the severity of shaking at Bodega Head would be, for an earthquake of magnitude 8.2 - 8.4, something like an average maximum acceleration of about 0.4 0 for about 1 minute, with peaks in excess of 1.g and with a vigorous shaking continuing for perhaps 3 minutes.
This is somewhat larger than the intensity. predicted on the basis of the i
I strong motioh recorded in the El Centro Earthquake, as recommended by Hous-i The basic idea of using actual accelerograms is quite sound and is far ner.
superior to using Mercalli intensity alone. The El Centro earthquake is the only. j one for which such information is available for a position near a fault.
The earthquake occurred on the Imperial fault, a branch of the San Andreas system; it was originally estimated to be magnitude 6.7, but was subsequently upgraded to 7. 0.
The accelerograph was located about 5 miles to the west of the surface l
expression of the causative fault, or about 7 miles from the epicenter, the exact location of which is in some doubt. Not only did the thick alluvium alter the power spectrum by attenuating the high-frequency effects and probably aug-
.menting the lower and middle frequencies, but also it probably does not show clearly the effects of the " fling"--an effect discussed in the next section.
Further, the intensity at a point depends on the location of the point with respect to the fault and the direction of propagation of the faulting. The faulting begins l
at a point and progresses, usually in one direction. The shaking is much harder in the direction in which faulting progresses, with both the frequency and ampli-tude being changed (Benioff,1954, p. 201). Hence, the record of the El l
(
Centro accelerogram probably indicates a lesser intensity than it would have had i
it beeh located further south.
The historical record shows very clearly that higher intensities go with larger earthquakes, and thus the use of the El Centro record will not guarantee adequate design factors for the maximum accelerations produced by a shock of magnitude 8 or more.
For example, Gutenberg and Richter (1942, p.170 et seq.) relate In-tensity I,and maximum' acceleration, a,' for various earthquakes. They derive the semi-empirical relation:
log a = { - f This yiel s accel rations of half.
g at a Mercalliintehsity of Ten and one
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Accelerations in excess of I g have~ been hoted in severa11arge earth-quakes (Oldham,1899).,
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3-3.
Mechanics of Earthquake Motion. The movement near a fault is quite different from that at a distance; in fact, the oscillatory motion near a fault may well be a little less. There is one movement, however, that is at maximum near a fault and diminishes rapidly with distance--this is the permanent throw or fling.
During the inter-earthquake period the blocks of land on either side of a fault move continuously and slowly with respect to each other. It is most probable that the oceanward side 'of a fault undergoes more absolute movement than the continental side, but this makes little or no difference regarding the in-tensity of the impending earthquake as it affects opposite sides of the fault zone.
The gradual drift of the land deforms the blocks (see Fig.12) bending the rocks and storing energy in them in the form of elastic strain. The strain accumulates until the forces generated in the rock are great enough to overcome the frictional forces on the surfaces of a fault that prevent slippage. When this happens the rock on either side suddenly snaps back into its unstrained straightened condition.
In the zone near the fault, up to say 30 km, the land on both sides at this time undergoes a permanent sudden displacement. The total displacement across the fault zone"msy' be as.much as 5 to 8 meters. This takes place as a sudden high acceleration, followed by a slower deceleration; the time involved is pf th' order of seconds. Effects of the fling are most noticeable on or near bed-s roc}c, Most of the extremely-high-intensity effects in epicentral regions are the l
iesult of such action. The total fling may be ameliorated somewhat by drag in l
the fault zone, in the case of a verf large fault, but this carries the penalty of movement on a variety of subsidiary faults.
I freference line, for example, a fence
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Field cut by a strike slip fault, before strain accumulation.
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The Bodega Head reactor site is located in the zone of fling of the San Andreas fault and will probably undergo some 3 or 4 meters permanent horizontal displacement.
3-4.
Possible Fault Movement on the Head. The most serious cause for con-cern at the reactor site would be the possibility of movement on a fault passing either through the power plant area or across the cooling-water system. This possibility is quite high. When a fault such as the San Andreas moves, it moves -
not on a single plane but on many fractures. There is usually one main plane of movement, but in many major earthquakes faulting has been found to have occurred over a large area. Some of the faults move during the main event while others moyg.tlurjng $e aftershock sequence. Numerous examples could be given -
il 1,
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18.
where movement took place on more than one fault. Richter (195 8~, pp. 47 6-487), in discussing the 1906 earthquake, shows rather clearly that that event was much more complex than a simple fault movement. Although the 19 06 earth-quake was accompanied by displacement along a fracture to the east of the Head it must be remembered that during an earthquake energy is suddenly re-leased from a large volume of rock and movement can and does take place across many faults and fractures over a wide zone. Pre-existing faults, often considered " dead," joints, bedding planes and similar structures participate in the readjustments. The Chilean earthquakes of 1960 (Saint-Amand,1961) pro-duced movements on a plurality of faults spread over about 100 km of longitude.
The Kern County earthquake of 1952 (Buwalda and Saint-Amand,1955) had a zone of faulting of considerable width, with many minor faults and some clear-cut j
traces as far as 5 to 10 miles to the side. The list could continue indefinitely, j
The hazard from movement is carefully pointed out by Housner,. page 3, I
whereirt he says that "Since it is quite impossible to design a power plant to-survive without damage the large permanent ground surface displacements.that might occur if the earthquake fault slipp. age occurred on the site, this? possibility must be given special consideration."
While the whole slippage from a major earthquake will probably not occur on any one fault going through Bodega Head, it is quite likely that movement I
mqy occur on the big fault in the plant site, or on any of the several faults that cross the site of the cooling-water system.
3 - 5, Possible Chances of Iand Elevation. One phenomenon that has been ob-served following all large earthquakes, when an attempt has been made to notice it, is a widespread change in elevation on both sides of the causative fault, even when the movement is mainly strike-slip. The land on either side is-l either raised or lowered, usually a matter of a meter or more for the larger earth-l 1
quakes. Examples are the 1835 (Fitzroy,1836) and 1960 (Saint-Amand,1961) earthquakes in Chile, the Kern County earthquakes of 1952 (Whitten,1955, p. 79),
and the Yakutat Bay earthquake of 1899 (Tarr and Martin). This list could also be continued almost indefinitely. These changes are widespread and involve more than mere displacement on one fault. Many faults are involved, together with uplift and downbowing on a regional scale.
l This raises the serious question of th'e effects of a change of level on an installation that draws cooling water from a shallow estuary.
Attempts were made in the year following the 1906 earthquake to detect changes in the level of Bodega Head on the basis of barnacle growth, but these were unsuccessful. The study was largely confined to the eastern shore, where such evidence would be difficult to find or to assess. The negative findings do not indicate that the Head will not change elevation--it has done so in the past, as evidenced by the wave-cut platform and by the elevated shorelines. In fact, the very existenc,e of the promontory is due to continued uplift; no mass com-posed of such easily erodible rock could long withstand the assault of the sea were it not for continued rejuvenation.
3 - 6. ' Tectonic Movement in the Absence of a _ Major Earthquake. Even if no major earthqtiqKF occurs in the area concerned, the possibility of large-scale warping or s%ppage is present and should be considered. While there clear-cut evic ce at Bodega Head for other than catastrophic changes, it seems appropriate tp.recqunt one or two cases of this sort. Perhaps the most notable case is that of ti e W. A. Taylor Winery, near Hollister (Steinbrugge, et al., l 1960), wl)pra'demage has occurred from a slow slippage on a portion of the San andraac Mit Ene: rete floors have been broken and walls displaced at a rate h. 3
l { ( C 19. of one centimeter per year. f' A thrust fault in the Buena Vista Hills, near Bakersfield (Wilt,1958), has been moving without earthquakes for a number of years, deforming roeds and bending pipelines. l place in many places, such as across the San Andreas fau along the southern and southeastern coast of Alaska (Saint-Amand,1957, pp.13 60-1364). ChangeT%this sort are large enough to cause trouble, and a study to identify any such changes should be made before construction begins. j c m 3 '/. Foundation considerations. builci upon solid rocklhan upon alluvium.It is generally thought that it is better to from the causative fault This is certainly true at a distance l portant, rock may be as, bad or worse.However, at near-fault distances, where fling is Jo ibul}d upon a combination of the two. Clearly the worst possible situation is At the Bodega Head site the rock is severely crushed, broken, and mylonitized. It could scarcely be classed as good foundational material. It will transmit well high-frequency vibrations, and then elastically deform in response to regional readjustment of strain; it will probably also undergo mass movement due to its own weight during the long-period oscillations. In addition, the alluvium, a loosely aggregated clay-rich soll, will certainly yield at a different rate than the rock, subjecting the in-sta}lgtion to widely varying dynamic loads and permitting the several parts of { from changes in level and position to interconnecting i system, and to the reactor itself. i A worse foundation situation would be difficult to envision. s I 3-8. Transmission Lines. I The transmission lines which will stretch across the fault zone will probably be destroyed by the fault movement, after first possi-bly having been shorted by swinging together. While damage to the power lines could be repaired easily enough, such activity is certain to put a severe load on the power plant. 3-9. Ts una mis. Since California has never suffered extensively from tsunamis (seismic seawaves) produced by its own earthquakes, this does not seem a serious cause for concern. 3 - 10. General Observations. It is difficult to conceive all the trouble that a great earthquake can cause. Accounts of these events are always condensed, and hence a mere perusal of the literature, often after it has been'digeste' 'and edited in the interests of economy, does not convey a very graphic impression d of the extent of disruption of the normal human activities. Not only are there the usual concomitant' of fire and occasionally flood,.but widespread disruption s of water supply and sewage disposal occur. One of the most serious losses is that of electricity; this loss is invariably felt during the period it is most needed for emergency service. Loss of electricity leads to shortages of water, hospital facilities, elevator service, street signals, lights, etc. The most actious loss is in com-munications--especially radio, television, and the. press, which in turn leads to publig panic and the spread of groundless rumors and exaggerations. The latter tend to include misinformation regarding installations known to have a damage-qx
~ ~ i ( ( \\ producing potential such as dams ) atomic reactora--especially any know 20 , and in the future would inevitably n to be precariously located. plants be aq sitFor the above reascEs it is absolutely ve clude evpptual c, construction of seve s t extreme vulgarability of any plqnt on th e to disruption by i such an p8440t14} nerv)ca in auch a poorloc liunits installation in-e site, it seems highly imprudent to Eventual y the a ty. lta power raggire}meny.Uptte[ State ump 1 the safety of the (trat reacto accident 4004 pot op glarm, ral Reactors must be careful ergy to fulfill { spread mlatr4St Wil discourage, and dishearten the public t inge upon ; this ' nergy source.-} prpvea,t thp deliberate, carefuln. order that an i e at wide- , and competent development o (
- 4. CONCLUSIONS reasona;podeqq peac} $g 4 very poor locat
} o owing \\ over half a century ha)s elapsed since th4, The pro pected within the lifetime of the 1 s at least' one per centur e last one, hence another may be~ y; plant. b. i foundational material.The extensive faulting on the he d h .ex-i rock is especially unsuited to heaThe combination of an unstable a j vy construction. uvium and crushed i !*^ c. a region where exceptionally high ea thThe plant is lo quake intensities will developn r d. The large fault exposed in the pThe probability \\i high. and is itself sufficient reason to di resent excavation is of special c faults crossing the site of the c scontinue construction. j ing water system because of the pThe abun
- ncern, e.
i us aspect. with respect to the water source ossibi.lity of change in elevation of ) e plant i Quaide It.is surprising, in view I .has gon,e ahead.and by Housner, 'that anoth'er site wof the expert occupants is not a cau;re for great p blThe erection of a dev r and that in itself 1 builders have a.s. hazardous to others is a matter for publi u ic concern The erection of a deviceanger i not result from failure of the devicegrave moral responsibility to be c concern, and the from a geological and seismic poi The location on Bodega Head is hcerta nt of view. San Francisco northwBecause the San Andreas fault run i azardous ard, it seems that the nearest safe localis parallel to would lie ~n' rth of Point Arenas or south o selected should be carefully and prud m ently examined from all points o before,gginning construction ty near the sea e A( i b;
] f o ( .( 21. REFERENCES' l Eoq}off, ljugo (1955), Mechanism and Strain Characteristics of the White Wolf fault as Indicated by the Aftershock Sequence, Chapter 10, pp, l99, 202. Earthquakes in Kern County,1952, Bull.171, Cali-l Jqp}}q'pivision of Mines,1955. Bywa}d4, J,p' and Pierre Saint-Amand (1955), Geologic Effects of the Arvin Tehoc A,a Earthquake, Chapter 4 of Earthquakes in Kern County, O.glifcq furing 1952, Bull.171, California Division of Mines, San ?[ADA@P% PPfe641-56. s .. >... ' vo Curt}aai Q,,' J,J', Evarnden, and J. Lipson (1958), Age determination of some i granitic rocks in California by the Pottasium-Argon method. Calif. Diy. of Mines, Special Report 54,16 pp. py)ce, C. Martin and David J. Leeds (1963), Response of Soils', Foundations and Earth Structures to the Chilean Earthquakes of 1960, B.S.S.A., Vol. 53, No.2', pp. 309-357. i f}proy, R., Sketch of the Surveying Voyages of His Majesty's Ships Adventure j and Beagle, 1825-1836, Geograph. Journal, Vol. 6 (1836), pp. 311-14. .Jicusner, George W. (1961), Earthquake Hazards and Earthquake Resistant Design Bodega Bay Power Plant Site, Pacific Gas and Electric Company, Typewritten Manuscript. Johnson, F.A'..(1934), Ph.D. Thesis, University of California at Berkeley. Johnson, F.A. (1943), "Petaluma Region," California Division of Mines, Bull.118. I i Koenig, James B. (1963), The geologic setting of Bodega Head. State of l California, Division of Mines and Geology, Mineral Information S ervice, Vol.16, No. 7, July 1963, pp.1-10. l Lawson, A.C., et al. (1908), The California Earthquake of April 18, 1906, Report of State Earthquake Investigation Commission, Carnegie / '~ Institute of Washington, Vol.1,1908. l .k Louderback, G.D. (1947), Central California Earthquakes of the 1830's, b,. _ B. S. S. A., Vol. 3 7, pp. 33-74. 1 L Oldha$p.8.D., Report on the Great Earthquake of 12th June 1897, Member of ~ 29, 1899. ' Geological Survey of India, Vol. Osmont, V.C.,(19 S, a geologic section'6f'the Coast Ranges north of San Francisco. Bay, University of Calif., Department of Geology, Bull. Vol. 4, pp. 39-87. Richter, Charles F. (1958), Elementary Seismology, W.H. Freeman Company, L San Francisco, pp. 768. I9 . Saint-Amand, Pierre (1957), Geological and Geophysical Synthesis of the Tectonics of Portions of British Columbia, The Yukon Territory, and Alaska, B. G.S. A., Vol. 68, pp.1343-137 0. J i ilr
I I ( i 22. Saint-Amand, Plerre (1961), Los Terremotos de Mayo, Chile'1960, Technical l I Article No,14, U.S. Naval Ordnance Test Station, China Lake, Calif. Steinbrugge, Karl V, and Edwin G. Zacker, Part 1; Don Tocher, Part 2; and j C.A, Whitten and C.N. Claire, Part 3; 1960, Greep on the San Andreas i Tault, E.S.8,4,, Vol. 50, No. 3, pp. 389-415. i l Tarr, Ralph S, 4p4 Mwrence Martin (1912), Earthquakes at Yakatat Bay, Alaska l la SepfAU})Af }pSS U.S.G.S. Prof. Paper 69,135 pages. ] g parthquake of July 10,1958. A Collection of Tochpr, Don (IS69), The h}asicq}} ors is Included, B.S.S.A., Vol. 50, No. 2, Art}cleg by SpverAl but PR, 2}7-U q,, Tocher, Don and W} Power Plant Site, Pacific Gas and Electric Company Boc}eqq E4y Typewr}tten Manuscript, .j Whitten, C.A. (1955), Measurements of Earth Movements in California, Chapter Sg pull.171, Division of Mines, State of California, Earth-gt}4}ces fp Kerr) County, California During 1952, pp. 75-80. Wilt, James W. (}958), Measured Movement Along the Surface Trace.of an Active Thrypt Taylt in the Buena Vista Hills, Kern County, California, B. S. S. A., Vol. 4 8, pp, 169-176. Wood, Harry O.. (1955), The 1857 Earthquake in California, B.S.S.A., Vol. 45, No.1, pp. 47-67. l 4 9 s e 4 4 4 9 4 I O es n -2 $a
l ( I 23. FIGURES Fig. 1. General View of Bodega Head and San Andreas Fault Zone. Fig. 2. Bodega Head and Harbor. Note shoreline near Horseshoe Cove, also wave-cut platform. J Fig. 3. Topographic and Geologic Map of Bodega Head. Modified from Tocher and Qualde. Fig. 4. Crushed and Broken Rock Along Shore. Fig. 5. Portion of Wave-cut Platform, Showing Crushing and Minor Faulting. I Fig, 6. I,andsliding and Fracturing in Johnson Fault. Zone. I flg, 7. Rectangular Channel Cut by Waves From a Fault Zone Near Point K, 'I Fig. 3. Fig. 8 fortion of Fault Zone From Which Notch in Fig. 7 was Eroded. ) Fig. 9 My}cpite Zone in Large Fault in Excavation for Power Plant. Fig.10. Geomorphic Evidence for Recent Uplift. Note abandoned canyon above high-water line, l Fig.11. Locality Map for 1906 Earthquake. Taken from Richter 1958. Fig.12. Process of Strain Accumulation and Release. e 2
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