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REPORT ON EARTH 7 JAKE HAZARDS at the BODEGA BAY POWER PLANT SITE, PACIFIC GAS AND ELECTRIC COMPANY By Don Tocher Consulting Seismologist 27ko Derby Street Berkeley 5, California and William Quaide Consulting Geologist 125 East Eleventh Street
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Claremont, California Prepared at the request of J. D. Worthington Chief Civil Engineer Pacific Gas and Electric Company 2h5 Market Street San Francisco 6, California I-l 1
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Introduction...........................
1 Historical Record of Earthquakes Felt Near Bodega Head......
2 Seismographically-Located Epicenters...............
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Geology of Bodega Head......................
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Introduction 1
General Geology Quartz-Diorite Character of the Granitic Rocks at Campbell Cove and West of Campbell Cove on the Seaward Side of the Headland Terrace Deposits Western Margin of the San Andreas Fault Zone
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1 Geologic Conclusions Discussion............................
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Questions asked by George W. Housner
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McHuron's remark regarding earthquake epicenters Recommendations for Bore Hole Locations
't Conclusions.....................'...... e 12 References............................
13 Table I.............................. I-1 II-1 TEble II.............................
Photographs Following Table II Geologic Map I
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The Pacific Gas and Electric /Compary has. Acquired 3and on Bodega Head, y p' Sonoma County, California, and 140 contengdatidg crastruci,1on of a large electric-power generating plant'at that locatice. Bodega Head has loag been known to lie very close tv.the San Andren fault zone. This is a y.
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. major geologic fau14which is ac*,1ve at the p/aemt time, and haq been C active fcf aazgl4.111 cts of yesyr. in thapdst.
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cally speaking; a better dirm would be " San Andreas fault som".
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wit' tin the fault zone nhattered and pulverized during movements along the '
i fault, although at places ir./ths' fault zone, rather Jarge isinds of rela-(
are weaker and more easily p(fourvM In generali tW'Mcks of tAs f tively ur:aroken rock may ta l
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4' fault zone. This has resulted in a series of elongated valleys or trougha
I along the fault zone, such as the valley,between Bolinas and 01ema, which continuesias the, trough contaquing Tomalu L$. At Bodega Bay, the fault s
trough or zone is sbout a W24 end a half dde.
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This report, then, at W quakes at the proposed Bodega / pts to assess the possible hazards fros. earth-( j:
Bay pmter plant site. The following three
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sectionR of this report in turg. report the investigation e:(gesrthquake hazard from the points of vied oft,,
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, ul (1) The historical record of earthqua)ses iolt at or near Bodega Head./
(2) The distribution of epicenters of earthquaksh nec Bodega Head l
as determined from seismographic't-1ange]xtien, t4 (3) The geology of Bodega Head, with speciaVerphasis c:s indications of past gross ground movementh,
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~ KISTORICAL RECORD OF EARTHQUAKES FELT NEAR BODEGA Hii1D Central California, particularly in the Coast Ranges, is a highly seismic region, subject to frequent small earthquakes and occasional large
,,j and destructive shocks.- Before 1850 the whole region of the Pacific Coast J7 was nearly udnhabited by white men, and the region around Bodega Bay even d
todsy.is more sparsely populated than mag other sections. Mag earth-7
- quakes have undoubtedly occurred which could have been felt at Bodega Bay
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'had' there been'anyone living there. Doubtless still others were felt, i
but not recorded. Table I has been compiled from sources listed in
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.I4wson (1908), Louderback (19h7) port, especially from Holden (1898),
References at the end of this re
, Townley and Allen (1939), and for
. recent years, from the regular serial publications of the United States
. Coast and Geodetic Survey and the. Udversity of Califorda Seismographic
/ Station. Table I includes shocks which were felt at Bodega Bay, or at
' a v conmunity within 15 miles of Bodega He?d. A few shocks have been 7
included which were exceptionally strong at distances greater than 15 miles from Bodega Head, but for which no reports in the sources came from points within 15 miles of Bodega Head. For each shock Table I lists first, the date, second,the probable location of the epicenter, and third, informa-tion on the intensity of the shock.
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Seismologists differentiate between the two terms " intensity" and S.
" magnitude". The intensity of an earthquake is m' raunber which describes the damage done or the effeeus on people by the shock. Each different locality can be assigned a different intensity for the same shock.
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. Reports from persons experiencing a shock are assigned a numerical inten-sity rating based upon a standard intensity scale. Since 1931, the inton-(
sity scale in general use in the Udted States is the Modified Mercalli
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Intensity Scale of 1931. An abridged version of the criteria of this fj scale as used to rate intensities is given in Table II. Prior to 1931, f(
the Rossi-Forel Intensity Scale was ir general use in this country.
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rated on this scale. Table II also lists the approximate equivalent
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t intensity on the Rossi-Forel Scale opposite each intensity on the Modified
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'r, Marca111 Scale. In general, the intensities given in the " Remarks" column
'of Table I are those given by Holden (1898), Townley and Allen (1939), and g
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since 1928 by the U.S. Coast and Geodetic Survey.
t' The magnitude of an earthquake is a number assigned to the shock on the basis of the maximum amplitude recorded on a seismograph at a known
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distanes from the conter of the shock. Richter (1935) defined the magni-ttAs of av shock as "the logarithm of the a=W='= trace amplitude, expressed in microns, with which the standard short-period torsion seis-someter would register that shock at an epicentral distance of 100 km.".
~ As the scale is logarithmic, an increase of one unit on the magnitude scale.
isplies an increase of 10 times in the =M='=
recorded amplitude at a given r
distance.
s Although seismographic stations were established at Berkeley and Mt.
Hamilton as early as 1887, reasonably reliable instrumental data to permit the determination of epicenters only became available with the addition of
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stations and more modern equipment to the University of California network in comparatively recent years. Instrumentally located epicenters are given in Table I for shocks since about 1930 when the data were adequate. Epi-centers determined instrumentally are indicated by a (B) in the " Location of Epicenter".
Most of the earthquakes in Table I definitely had their centers at a considerable distance from Bodega Head. These are the strong, memorable shocks which central California experiences from time to time, and which are generally felt over wide areas. In the earlier years, these can be recognized from the stronger intensity at a distance from Bodega Head than at points close to it. In recent years, the instrumental determinations confirm that none of these strong shocks have had epicenters close to Bodega Head. However, there are 10 entries in Table I for shocks which were reported felt at only one point within 15 m'.les of Bodega Head, and nowhere else. Five of these 10 entries refer to dates since the installation of sensitive seismographs around 1930. None of these five was recorded on seismographs at Berkeyey, San Francisco, Palo Alto, or Mt. Hamilton, the closest stations to the Bodega Head area. It is the opinion of this writer (Tocher) that these latter five reports were not inspired by earthquakes, but by some other cause such as explosions, traffic, or the like. In all probability, this is also true of the older shocks for which there is only one isolated report in the literature.
The California earthquake of April 18, 1906 is unquestionably the most
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important shock in Table I which has affected Bodega Head. This shock may have been matched in importance by the major earthquake of June 1838, but the coastal region near Bodega Head was then uninhabited by whites except at the Russian Colony at Fort Ross, and unfortunately no reports of the 1838 earthquake have ever been found in the Russian records. The great fault break produced by the earthquake of 1906 has been well described by Lawson (1908). It began at San Juan Bautista in San Benito County and extended to Point Arena in Mendocino County, Surface fault breakage also was observed in Humboldt County between Shelter Cove and Upper Mattole. The description of the great fault break of 1906 as given by Lawson (1908) appears in Table I.
The breakage there occurred in the easily recognizable fault zone, and there is no mention of any fissures, fractures, or cracks in the surface of the ground on Bodega Head itself. The observed breakage was confined to the neck of land which connects Bodega Head with the mainland.
1 The earthquake of April 18, 1906 also was responsible for the greatest damage ever reported from the Bodega Bay area.
The Atlas accompanying report of the State Earthquake Investigation Committee (Lawson,1908)g the shows the intensity to have been I (R.-F.) at Bodega Bay. However this undoubtedly is based on the ground dislocations in the fault zone; if the ground breakage affects in the fault zone are everlooked, the 3906 intensity at Bodega Bay was only VIII or at most II on the Rossi-Forel Scale.
Since 1850, there have been no other shocks which exceeded intensity VI? (R.-F.) at any point l
within 15 miles of Bodega Head.
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SEISHOGRAPEICALLY-LOCATED EARTiQUAKE EPICENTERS j-i-
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In~T&ble I, epicenters followed by (B) have been determined by instrumental methods. None of these epicenters lies within 15 miles of Bodega Head. Although Table I contains only earthquakes definitely
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reported felt, tbs same statement can be made abcut instrumentally determined epicenters of shocks which vers not reported felt. This area sinqply is not the seat of small earthquakes, at least shocks large enough to be reporded at near by seismographs. Occasional shocks of anall to moderate size are located in the Cotati Valley from Petaluma to north of Healdsburg, Along or close to the San Andreas fault, locatable earth-quakes do not occur north of Bolinas Bay as far as Point Arena at least.
In assessing this last statement, it must be remembered that the epicenter determined by seismographic triangulation represents the point at which fault breakage commences. Thus, although the instrumental epicenter of the great shock of April 18,1906 was west of the Golden Gate, this shock is of great importance in the earthquake history of Bodega Bay.
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GEOLOGY OF BODEGA HEAD Introduction--The proposed Bodega Bay Power Plant near Campbell Cove in Bodega Bay lies near the San Andreas fault zone. The geology of the pro-posed site and bordering areas was therefore investigated with consider-able detail in order to evaluate the geological conditions at the site.
General Geology--Bodega Head is composed of quartz-diorite, ani intrusive granitic rock which has been transported to its present location by repeated movements along the San Andreas fault. The surface of the quartz-diorite has been eroded by sub-aerial and marine agencies and covered to a great extent by terrace deposits of sani a,nd silt. These sediments were probably laid down in a shallow marine environment during the Pleistocene Age. Sometime after deposition of the sands and silts, the land was ele-vated allowing for the removal of large quantities of these relatively uncemented materials, producing the present distribution of terrace depos-its as shown on the geologic map.
Quartz-Diorite--The quartz-diorite is a coarse-grained granitic rock. It is very similar to the rock exposed at Point Reyes and is probably part of the same intrusive body. Granitic bodies of similar character occur throughout much of the central California Coast Ranges. They all lie immediately west of the San Andreas fault and have been dated by Potassium-i from 83 to 91 million years old. The age of these Argon methods as being*it throws light on the amount of movement on the rocks is important for San Andreas fault. The rocks innediately east of the San Andreas fault are sedimentary rocks, mostly older than the granitic rocks, and they show no evidence of having been intruded by the younger granitic rocks anywhere q
in the Coast Ranges. This means that the granitic rocks of which Bodega j
Head is a part have been transported great distances to their present loca-tions. Curtis, Evernden, ani Lipson (1958) have proposed a displacement
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with a horizontal comporant of at least 300 miles since Upper Cretaceous j
time (during the last 80 million years), for a mean rate of 0.023 feet per j
year. When this rate of movement is contrasted with the present rate of 0.2 feet per year, the great distance does not seem at all unreasonable.
i The quartz-diorite at Bodega Head is extensively fractured, sheared, and jointed, as are all the other granitic rocks lying west of the San Andreas fault system. The rock is not penetratively sheared but is cut by innumerable discrete shesi surfaces with breccia (broken rock) and mylonite (milled rock) zones ranging from a feather edge to a foot thick.
j The most common thickness is about one-half inch. Some of the dislocation surfaces are nea-ly planar, whereas others are curved and branching. Those faults with the greater displacements tend to have the more planar atti-tudes. The magnitude of the displacements of the faults can not often be measured, but in those cases where dikes are present in the rock, dis-placements can be seen and, in general, have displacements of only a few inches. Many cases were observed, however, where large dikes were cut off
_3 by faults and not observed again on the other side of the fault, indicating movements of tens of feet or more. The intensity of the faulting and joint-ing in the rock is so great that the formerly massive rock is now broken into a mosaic of blocks with average dimensions of approximately one foot.
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soet Character of the Granitic Rocks at Campbell Cove and West of Campben Cove on the Seaward Side of the headland--The granitic rocks at these two localities were examined in greater detail in order to establish the character of the foundation rocks at the proposed plant and tunnel sites.
The rocks on the seaward side are well exposed on wave-cut platforms.
The quartz-diorite exposed there is strongly sheared and jointed as des-cribed above. Faults strike in an compass directions, most of them dipping steeply. Many eastward trending faults were observed, but there is no indication of a preferred eastward strike paraneling the sediment filled vaney which trends eastward across the headland. Nor is the brecciation or displacement on the eastward trending faults greater than on the others. Although the evidence is not conclusive, it is suggestive that the vaney trend is not controlled by faulting.
The granitic rocks in the vicinity of Campben Cove are poorly exposed.
The contact between the quartz-diorite and the overlying sediments gener-any lies but a few feet above sea-level on the bay side of the headland, and below sea-level in Campben Cove itself. Exposures are mostly plant covered and determination of the rock character there is difficult. A number of faults are visible, however, and attitudes in space of 18 dif-ferent shear surfaces were measured. These measurements indicate that there are two recognizable trends, approximately N 50' W and N 80' W.
The fault surfaces with the more westerly trends are variable in dip. In lp those few cases where slickensides were observed, movement was dip slip (down the dip). The more northwesterly trending faults are an steeply dipping. In the few cases where the slickensides were observed, both strike slip (along the strike) and dip slip movement pictures were seen.
Both of these fault trends are probably subsidiary systems of the main San Andreas zone.
The fact that there is a regularity of trends on the cove side (pro-vided the number sagled is representative) and none on the seaward side of the headland can be explained. It is reasonable to expect the faults closer to the San Ardreas zone to have more regular trends, more or less compatible in direction with the main fault system, and faults farther from the main zone to have a more random pattern. The intensity of shear-ing and brecciation of the rock, however, is no greater near the San Anireas zone than on the seaward side of the headland.
Terrace Deposits--The distribution of the terrace deposits is shown on the geologic map. The deposits consist mainly of silt and sand. A few, 1
thin plastic clay beds were observed in the vicinity of Horseshoe Cove, l
but this type of sediment is rare. The sand is usually coarse grained, I
and all the deposits are ve n bedded. The sands and silts are generany weakly cemented, but a few horizons are firmly cemented with iron oxide which occurs as concretionary bodies in the sand. The sands generally l
stand wen on the steep cliff exposures. A few slumps can be seen on the over-steepened sea cliffs, undoubtedly occurring when the sediments were water saturated.
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shallow marine enviernment of deposition. The deposits are similar in aspect to many deposits of Pleistocene Age along the California Coast, i
arxi it is reasonable to assume such an age for them.
The contact between the deposits and the underlying quartz-diorite can be seen in may exposures. The sediments were deposited on an irregu-A, lar surface. The contact is marked in may places by a firmly cemented conglomerate horizon varying from a few feet to ten feet thick. The quartz-diorite beneath the sediments is deeply weathered in some places, fresh in others. At Campbell Cove the rock appears fresh, but on the seaward side of the headland it is wsatbercd to depths of 20 feet in some places.
The sediments contain a large content of water. They are porous and permeable and numerous springs are present in cliff exposures. The water i
table is probably variable, high in the wet season and low at the end of the dry season.
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The terrace deposits are not deformed. They have been elevated above l
the sea, but not by movements along the San Andreas fault. This was a wide-spread elevation which affected the entire coast of California.
.pareful examination of the terrace deposits failed to reveal a single
" fault cutting them. In the " Reconnaissance Engineering Geological Report, Bodsga Bay Power Plant Site, California" by Clark McHuron,1958, a fault contact between the quartz-diorite and the terrace deposits was indicated l
,T at Horseshoe Cove. This fault contact was represented by McHuron as being
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the western margin of the San Andreas zone. Repeated trips to Horseshoe j
Cove were made, but the fault could not be found. The feature described j
by McHuron is the sedimentary contact between the terrace deposits and the quartz-diorite. May faults were observed Armediately beceath the sedimentary cover, but no disturbance whatsoever could be observed in the overlying thin-bedded deposits. This means that there have been no recog-nizable surface breaks on the headland in the last few thousarxi years, although at least one and perhaps more major breaks occurred in the San Andreas zone during this interval.
Western Margin of the San Andreas Fault Zone-The San Andreas fault in this area is not a single dislocation but a zone of dislocations a mile 3
and one half wide. The rocks in the fault zone can not be observed, for j
they are crushed arxi broken so that they are easily eroded. The position i
of the zone is marked by a belt of low relief, covered here by sand dunes and recent marine deposits. The western margin of the fault zone is indicated on the geologic map as a straight line separating granitic exposures on the southwest from the dune-covered low relief area to the northwest. The western margin may not be as exactly straight as indicated, but evidence for a more detailed location is lacking. Very straight trends, moreover, are characteristic features of large strike slip faults
,such as the San Andreas.
Conclusions--Bodega Head is composed of quartz-diorite, an intrusive rock
d which was transported may miles along the San Andreas fault to its present location. The San Anireas system is still active and movements can be expected to continue. The surface of the quartz-diorite was eroded to hills and valleys before Pleistocene time, when it was submerged beneath J
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- the sea. During the submergence a thickness of at least 175 feet of I
sands and silts was deposited on the eroded quartz-diorite. Later, regional uplift elevated the headland and erosion has stripped part of 7
the sediments from the granitic basement. Movements along the San Andreas fault which brought the quartz-diorite to its present location have continued since the deposition of the Pleistocene sands and silts, i
an inexactly known period of time of several thousands of years, but no evidence could be found to indicate that any displacements have taken place on the headland during these breaks.
The quarts-diorite bedrock on which the proposed power plant would be constructed is strongly sheared and jointed, resulting from the forces
.j involved in the long continued transport of this rock unit along the fault.
Major faults in the quartz-diorite could not be recognized, but minor faults with considerable displacements do exist there. There is no evi-dence indicating that the location of the east trending sediment tilled valley at Campbell Cove is controlled by a large fault. The power plant and tunnel sites there do not appear to straddle large faults.
The terrace deposits consist of some weakly and some strongly cemented horizons. They generally stand well in steep exposures, but some slumps on over-steepened sea cliffs were observed. These undoubtedly occurred during a period of maximum saturation. The sediments are porous and
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permeable. The water table is probably quite variable, high in the raig season and low in the dry season.
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DISCUSSION I
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l Questions asked by George W. Housner. In a letter dated June 30, 1960, George W. Housner requested that four specific questions be dealt with in this report. These questions are repeated below numbered the same as in that letter; each is followed by a discussion and answer.
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1.
What is the likelihood of active faulting occurring on or near the site?
j As Within the probable lifetime of a large power plant (on the order of a century) there is a strong likelihood that active movement will
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occur in the San Andreas fault zone near the site on Bodega Head.
Such fault movement occurred in 1906. Similar movement in the San Andreas fault zone is known to have occ'urred in San Mateo County in 1838, and at that time the movement may have extended as far north as Bodega Bay or beyond. No historic records exist to suggest the dates of earlier fault movements, but the geologic evidence clear 4 indicates that the San Andreas fault zone has ruptured repeatedly in the past. The power plant site lies just west of the western boundary of the active fault zone.
No evidence was found in 'the geologic examination to indicate O
the existence of a large fault beneath the plant or tunnel sites.
Chances of disruption of the sites by breakage along a large fault are therefore small.
2.
What is the likelihood of ground movements occurring at the site during an earthquake because of fissuring or fracturing of the rocks?
A: Probably quite small. The quartz-diorite upon which the power plant would be built is strong 4 jointed and faulted, resulting from con-tinued deformation over the past 80 million years. However, the geologic evidence suggests that there have been no fault breaks, or fractures or fissures, in the quartz-diorite of the headlarri in the past few thousand years. In view of the lack of recent fracturing or faulting, it seems reasonable to expect none during the relatively short life expectancy of the proposed structure. Complete absence of fracturing in the quartz-diorite cannot be predicted with certainty, however.
3.
What is the likelihood of ground movements of the landslide type occurring?
As Mass movements of the Pleistocene sands and silts could be expected in the event of a large earthquake, especially where those sediments are exposed on steep slopes. Such movements would be much more extensive if the earthquaki occurrod when the rocks were saturated 3
with water. No landslide danger to the plant site (on quartz-l diorite) would be expected if the cut elopes around the plant were kept lov, and especially if proper drainage of the sediments were effected.
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- h. what intensity of earthquake ground motion may be expected at the site L"
as compared with a similar site 15 miles or so from the San Andreas fault?
As As the proposed site is not in the fault zone itself, and is on hard rock (quartz-diorite), a maximum Modified Mercalli Intensity of about a
VIII should be expected at the site when the next shock occurs which is comparable to that of April 16,.1906. Faulting comparable to that of 1906 would probably justify assigranent of intensity I in the fault' L
sono itself, but not on the quartz-diorite of Bodega Hesd.
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. quartz-diorite, but 15 miles from the San Andreas fault, would be L
only alightly lower, say VII to VIII; the energy released in a major earthquake emanates from a source that is distributed in extent both horizontan y and vertically along the fault plane, to that near an extended fault, the strength of shaking dies off mr.ch less ra with distance than from a point source (such as an explosion)pidly McHuron's remark regarding earthquake epicenters. On page 7 of his report, McHuron (1955) states
" Seismic records show that Bodega Bay has not been the epicenter of a recorded earthquake of the Richter intensity magnitude greater than three."'
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" Epicenter" can and has been defined as the point on the earth's surface determined by seismographic triangulation to be directly above the point of initial fault rupture, and under this definition, McHuron's statement is completely true. Ecwever, it should be qualified by the remark that prior to about 1930, the sensitivity of instruments and dis-tribution of seismographic stations in California was such that shocks of magnitude less than 5 or 6 north of the Golden Gate could not be " pin-pointed" instrumentally.
In considering McHuron's statement quoted above, it also should be emphasized that, although the instrumental epicenter of the 1906 earth-quake was von to the southeast of Bodega Bay, the faulting extended through Bodega Bay and for some distance in either directien. The effects at Bodega Bay, both from chaking and from gross movements cf the ground, were overy bit as severe as if the epicenter had been in the corder of the Bay.
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Recommendations for Bore ble Locations. One copy of P0&E Drawing SK 0095-7 4 is appended to Copy 1 of this report. Recommended locations for six bore holes, four on land and two under water in the Campben Cove area, are marked on the drawing.
The primary purpose in boring the two holes under Campben Cove (Phase 3 of Dames and Moore's investigation) is to confirm the existence gv.
of quartz-diorite at shanow depth--under the cove and adjacent beach.
The quartz-diorite is exposed up to about five feet above sea-level along the base of the cliffs both northwest and southeast of Campben Cove, but
- only Recent dune and beach sands and Pleistocene sands and silts are exposed
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suggests that the top of quartz-diorite bedrock is at or a few feet below sea level along the seismic line. The seismic data and geologic information suggest that the bedrock will be reached at shallow depths in the cove area, but this should be directly confirmed by borings.
The primary purpose of the recommaMed borings on land also is to confirm the conclusions drawn from available geologic and seismic evi-dance regarding the shallow depth of the top of the quartz-diorite under the proposed site of initial construction (Unit #1, as shown on Drawing SK Bo98-7-A).
It is important from the standpoint of ability to withstand strong ground shaking that the buildings and arge other large appurtenances be constructed on foundations resting on the hard quartz-diorite bedrock.
Should the borings reveal that bedrock will.not be reached.at practicable depths where it is proposed to erect structures, serious consideration should be given to alternate sites.
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CONCLUSIONS Our investigation of seismological and geological conditions at the proposed power plant site near Campben Cove on Bodega Head leads to the fonowing conclusions:
i 1.
The proposed site lies a short distance west of the western margin of the zone.
1 2.
No evidence was found of aqy major faults under the site. Although the evidence is not conclusive, it suggests strongly that the trend of the sediment-fined vaney or saddle which trends westward across Bodega Head from Campbell Cove is not controlled by faulting.
4 3.
The quartz-diorite thought to underly the site near sea level win provide a much better foundation than any other geologic formation on Bodega Head. If borings fail to show quartz-diorite near sea level at the site (as suggested by seismic and geologic evidence) the characteristics of the unexpected material should be studied I
carefuny to determine if it is suitable foundation material for massive structures so close to the San Andreas fault zone.
h.
The quartz-diorite is strongly jointed and is faulted on old minor
~
)
faults. However, there have been no movements on these minor faulta j
q in the past few thousand years. Lack of recent movements strongly
./
implies, but does not guarantee, that there win be no movements i
throughout the life-expectancy of a power plant.
A 5.
The region near Bodega Bay is remarkably free of sman to moderate-l sized earthquakes. The region is subject to infrequent major earth-quakes generated by large fault movements in the San Andreas fault At least one and perhaps two or more major earthquakes can zone.
be expected near the site within the next century. These may be as strong or even somewhat stronger than the California earthquake of April 18, 1906.
m 6.
All power plant buildings and appurtenant structures should be designed to resist an earthquake of Modified Mercalli Intensity VII,
or to provide a margin of safety, II.
(The quartz-diorite should provide as good foundation with respect to the hazards of ground I
shaking in earthquakes, as any rock formation to be found in the A
Bodega Bay region. This fortunate circumstance is largely counter-balanced by the immediate proximity of the San Andreas fault zone.)
7.
Slopes cut into the Pleistocene sands and silts around the site should be kept low and well drained to eliminate any danger of mass movements in the expected large earthquake (s).
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Appendix IV-
'\\ /
REFERENCES I
.Curtis, G.
H.,'J. F..Evernden, and J. Lipson 1958.. Age Determination of Some Granitic Rocks in California by the Fotassium-Argon Method, California Division of Mines (Special Report 54),16 pp..
Dames and Moore (Soil Mechanics Engineers) 1960. " Report of. Seismic Survey, Proposed Nuclear Power Plant, Bodega Bay, California," Private report prepared for the Pacific Gas and Electric Company by John F. Stickel, Jr. and Robert T.
Lawson(January 25,1960).
Holden, Edward S.
i 1898. Catalogue of Earthquakes on the Pacific + Coast, 1769 to 1897.
smithsonian Institution Miscellaneous Collections No.1057. -
Lawson Andrew C., Editor 190b. The California Earthquake of April 18 1906 (Report of the EEte f.arthquake InvestigattTon Co ttee) Carnegie Institu-tion of Washington, Vol. I, Parts 1 and 2, plus atlas.
Iouderback, George D.
l 19h7
" Central California Earthquakes of the 1830's," Bulletin of
^
the Seismological Society of America 37: 33-7h.
McHuron, Clark E.
1958. " Reconnaissance Engineering Geological Report, Bodega Bay Fower Plant Sites, California," Private report prepared for
.the Pacific Gas and Electric Company (May 15, 1958)..
k Richter, Charles F.
1935.
. An Instrumental Earthquake Magnitude Scale," Bulletin of the Seismological Society of America, 25: 1-32.
~
Townley, Sidney D., and Maxwell-W. Allen 1939. " Descriptive Catalogue of Earthquakes of the Pacific Coast of the United States,1769 to 1928", Bulletin of the Seismological Society of America. 29: 1-297.
United States Coast and Geodetic Survey.
Since 1928: United States Earthquakes (annual publication).
1 Since 1930: Abstracts of f.arthquake Reports for the Pacific Coast ed the Western Mountain Region (latest title; title has varied slightly through the years) Irregular pub-j lication 1930-33. Regular quarterly, MSA Series, since i
MSA-1,1st Quarter 193h.
University of California
<)
Since 1911. Bulletin of the Seismographic Stations.
(Title has varied slightly through the years.) Regular quarterly since Vol. 6, No. 1, 1st Quarter 1936.
Wood, Harry 0., and Frank Neumann 1931. " Modified Mercalli Intensity Scale of 1931," Bulletin of the Seismological Society of America, 21 277-283.
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I-1 Appendix IV TABLE I EARTHQUAKES FELT AT OR NEAR BCDEGA }EAD, 1838-1960 l
l Includes shocks reported felt or probably felt at points within 15 miles of the i
proposed site for a P0fcE electric generating plant on Bodega Head. In the remarks.
H column, names of such connunities are underlined. Where no community is under-lined, the shock probably was strong enough to be felt within 15 miles of Bodega Head, but. confirmatory reports.are lacking.
1 Location of l
Date --
Epicenter Ramarks i
1838 Central California-Maximum intensity I.
Conqparable with the earthquake 4
Late in
- June,
'of April 18, 1906. Originated in the San Andreas fault zone. One of
(
just the five strongest shocks to affect the San Francisco Bay area since after its settlement by white men. Louderback (19h?) concluded that sec-noon tion of the fault active in the 1838 earthquake extended at least from a point near San Francisco to the latitude of Santa Clara. The j
fault rupture may have extended throughout all or most of the line active in 1906, but north and south beyond the limits mentioned above, the fault lay in water or in country. uninhabited by whites (except at 3
the Russian Cology at Fort Ross, from which no reports are available).
[-
y-
.1851 Central Califorzda Eleven shocks felt along the coast from Santa Cruz Nov.
(?)
~26 to Mendocino.. (This may be an incorrect reference to strong shocks in southern California on Oct. 26, 1852 or'Nov. 27, 1852. None of
{
the eleven is known to have been destructive.
1853 central California Moderate shock in San Francisco, Bodega.. and Shasta Jan.
City. (Probably 2 separate shocks.)
2 1855 Sonoma County Intensity VI (R.-F.) in Sonoma County. Violent
]
Aug.
shock.- (No details available.)
27 1856.
Central California Intensity IV (R.-F.) at Bodega (VIII at San Feb. 15 Francisco. Felt over a wide area of central California. )
1857 Central California Felt at San Francisco, Oakland, and Bodega.
Nov. 8 i
L, 1858 Sonoma County Intensity VI (R.-F.) in Sonoma County. Two shocks, Aug.18 first light, second heavy enough to waken sleepers.
(Also reported felt in San Francisco.)
m
\\
1865 Sonona County Severe in Santa Rosa, and more severe in Bennett
(
Mar. 8 (possibly Napa Valley. Intensity VIII? (R.-F.).
VI at Petaluma.
County)
" Smart shock" at San Francisco. (No reports from places near Bodega Bay.) " Heavy shock"at Napa.
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'I-2 Appendix IV t-606r L
/D' f
Location of Date-Epicenter
- Remarks.
1868 Hayward fault Maximum intensity I (R.-F) near the epicenter. Damage in Oct. 21. from San-varying degrees at Hayward, San Leandro, Oakland, San
.Leandro to Francisco, San Jose, as far. south as Gilroy and Santa Warm Springs Cruz, and as far north as Santa Rosa. (At Santa Rosa, nearly au brick buildings were more or less injured, and many chimneys were thrown down.) Severe shock but little damage at Guerneville. Sebastopol. Bodega.)
1876 Sonoma County Most severs at Fulton and Freestone, where bottles and.
Jan. 3 crockery were upset and the wans of at least one house were cracked.
j 1878 Sonoma County Intensity VI? (R.-F.) at Forestville. " Severe shock Aug. 1 aroused the neighborhood." (Not reported felt elsewhere.)
g i
1888~
Probably Maximum intensity VII (R.-F.) at Petaluma. VI at Santa
- Feb. 29 Sonoma County Rosa and Martinez. " Light" at Healdsburg. (No reports from places near Bodega Bay.)
1891 Sonoma or.
M== intensity VIII to II (R.-F.) at Sonoma and Napa.
Oct. n Napa County
.(No reports from pla'ces near Bodega Bay.)
h' 1892 Sonoma County Shock felt at Forestvine.
(No details; not reported Feb. 23 felt elsewhere.)
1892 Solano County Maximum intensity II to I (R.-F.).
Heavy damage at April 19 Vacavine, Dixon, and Winters. Felt as far north as Healdsburg. Holden (1898) estimated VII at Santa Rosa.
1892 Solano or Maximum intensity II (R.-F.) at Winters. -Strong after-April 21 Yolo County shock of April 19 Felt as far north as Red Bluff.
Holden (1898) estimated VII at Santa Rosa.
1893 Marin County?
Tomales, Marin County. (No detailsj not reported felt July 2h elsewhere.)
1893 Sonoma County VII to VIII (R.-F.).
Sonoma County. Santa Rosas Chimneys Aug. 9 feu, windows broken, plaster in the courthouse exten-sively damaged. Petaluma (VI to VII, R.-F.), six shocks felt.
Heaviest was the first, duration 15 seconds. Crockery thrown from
)
shelves, plaster cracked, clocks stopped. Sonoma: " Heavy." Also felt Napa, Healdsburg, San Rafael, San Francisco, Alameda, and Sacramento.
1898.
Near Mare M== intensity VIII at Mare Island. In Sonoma county, Mar. 30 Island reported felt at Fort Ross, Petaluma, Lytton Springs, Peachland, and Santa Rosa. - (No reports from places
]
closer to Bodega Bay.)
1899 Sonoma County Felt at Guernevine. (No details; not reported felt Jan. 2 elsewhere.)
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Date Location of Remarks Epicenter 1899 Sonoma County Intensity VII to VIII (R.-F.) at Santa Rosa. Some t' '
.Oct. 12 damage at Petaluma. Also felt at Peachland. (No reportsfromplacesnearBodegaBay.)
1906 Central and San Francisco earthquake of 1906. '(See Lawson,1908.)
Apr. 18 northern.
The epicenter of this shock as determined by Harry 4
California, Fielding Reid (in Lawson,1908, Vol. II) was on.the San l
San Andreas Andreas fault about five miles west of the Golden Gate.
fault The point so determined represented the place at ubich l
rupturing on the fault commenced. Tbs fault rupture progressed in both directions, southeast to San Juan Bautista and,
northwest to southern Humboldt County. ' The mariana displacement across the fault was measured on the road which runs southwest fron l
Pt. Reyes Station across the head of Tomales Bay. The region south-I west of tbs fault was displaced 21 feet relatively northwest at that point. The fonowing description of the fault trace at Bodega Bay,is taken from Lawson (1908, p. 65):
"The location of the fault across the neck of land which connects I
Bodega Head with the mainland was determined by Prof. J. N. LeConte.
i He reports that on the south side of this neck the main earthquake fissure was found passing about 50 yards vest of a house occupied by
!')
Mr. Johnson. It could be traced as a multitude,of sman cracks in the swasqpy land from the bay to the road, then as a wen-defined fis-sure up the aman depression west of the house for 200 yards to where it disappeared in the sand dunes. No trace of it could be dstected in the sand dunes, which reach from this point entirely across the peninsula. Only one fence crosses the fissure and this had been repaired so that no measurement of the displacement was possible.
The movement was evidently northward on the west side as was shown by the direction in which the bushes were bent. The vertical movement was about 18 inches, the uplift being on the west side. The sand spit which closes the bay on the south was examined for evidence of movement, but nothing could be detected in the drifting sand."
The fonowing description of effects of the shock (other than fault trace phenomena) was made by Prof. J. N. LeConte and Mr. A. C. Wright (Lawson,1908, p.191):
1 "From this point (Davis Mins at the mouth of the Russian River) the road along the bench above the sea was fonowed 12 miles to Bodega Bay.... The country is sparsely settled. Only three or four houses were past, and these were uninjured except for broken chimneys.
Near Bodega Head the bridge over Salmon Creek was somewhat twisted.
Just beyond this a good-sized hotel, previously used as a summer resort, was badly wrecked by the earthquake. It was moved on its foundations, and rendered unfit for habitation. The building was
, -]
close to the sand dunes and probably rested on sandy deposits. The barn was completely wrecked. A few hundred yards beyond this a sman l
l mud-flat extends from the sea up to the road. Curious mounds of mud, l
shaped like truncated cones, were thrown up by the earthquake. Subse-quent examination showed that the line of the earthquake fissure must have past near this spot."
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6-Date Epicenter Remarks 1906 Probably Guernevines Articles thrown from north to south; cracked May l' Sonoma County smch plaster.
.1910 San Benito or ' Reported felt at Graton. (Strong shock, heaviest at
.Dec.-31 Monterey Honister and Salinas.)
p County 1913 San Mateo Light near Sebastopol. (Felt from Sebastopol to Palo Alto.
Oct. 25 Count;y Highest intensity v (R.-F.) at Stanford University.)
1915 Sonoma County Sebastopol: Intensity III (R.-F.).
North-south trembling felt by several lasting five seconds.
(Not reported Oct. 7 feltelsewhere.h 1915 Hayward fault Reported felt from Sebastopol to Santa Clara. Wr4==
Oct. 7 3 to 5 miles intensity VII to VIII (R.-F.) at Piedmont, where a few SE of Berkeley chimneys were thrown down. (No details from Sejastopoli probably very light there.)
1916 Sonoma County Sebastopol Intensity n I (R.-F.).
Up and down bumping Jan. 15 motion lasting four or five seconds. Felt by several.
(Not reported felt elsewhere.)
I) 1919 Sonoma County May4== intensity VI (R.-F.) reported at Napa, Petaluma,
,i Feb. 25 Santa Rosa, and Point Reyes. Motion " trembling" at J
Sebastopol.
I 1932 Humboldt Felt over an area of 50,000 square miles. Dv4== inten-June 6 County sity VIII (R.-F.) at Eureka, Arcata, and vicinity. III at (Probab Bodega, Tomales, et al.
offshore 1932 Between San Felt in coastal region of central California from Watsonvine June lh Jose and Mt.
to Tomales. Marinnun intensity IV (R.-F.) at many locali-Hamilton ties, including Tomales.
(Bed =4ng with next shock, intensities are rated on the Modified Mercani Intensity Scale, Wood and Neumann,1931.)
19 %
Northern San M*M== intensity IV (M.M.) at San Francisco. III at Feb. H Francisco Bay Sebastopol.
area 193h Sonoma County, A series of highly localized shocks felt with intensity V
]
Feb.
very near (some only IV) at Santa Rosa, but only slightly in surround-1h-16 Santa Rosa ing communities. One or more of the series felt lightly at Bodega, Forestville, Jenner, Monte Rio, Sebastopol, et al.
'-)
193h Sonoma County? IV at Dillon Beach.
(This report probably refers to the June 20 following shock.)
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I-5 Appendix IV 6 oar Location of Date Epicenter Remarks 193h 6 miles west Felt generany over the San Francisco Bay area. Maximm June 20 of San Mateo intensity V to VI. IV at Tomales, et al. II at Dillon 49s f (B)
Beach, Forestvine, et al.
193h Near Lake Mar 4== intensity Vn at Colma, San Francisco, and South Oct. 2 Merced (San San Francisco. III at Tomales, et al.
Francisco).
1 (B).
l 0=12:21 PST i
(lat shock),
I 0=12:31 PST (2nd shock) 1937 Marin County?
Dinon Beach: Several tremors reported felt.
(Nothing Feb. 26 recorded on any Bay area seismographs to correspond with thisreport.)
j t
1937 North Berkeley, W r4= = intensity VI-VII at Alba g, El Cerrito, and north Mar. 8 1 mile north of Berkeley..,IV at Bodega, Guerneville, Sebastopol, et al.
5 University III at Occidental, et al.
II at Forestville, Oraton, 1
Campus.
(B).
Tomales, et al.
j 0=02:31 PST Q
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(
1937 Marin County?
Tonales: Light tremors reported.
(Nothing recorded on Mar. 10 Bay area seismographs to correspond with this report.)
4 19h2 Sonoma Jenner: "One sharp concussion felt, apparently from the
{
1 Feb. 28 County?
southwest." (Nothing recorded on Bay area seismographs j
to correspond with this report.)
i 1
19h2 37' L3' N, 120' Felt widely in the San Francisco Bay area. Richter Magni-Dec. 29 h7' W (B).
tude h.3.
Wr4== intensity VI at Cowen and San Leandro.
0=10:18:1h PST II at Sebastopol.
19h3 37' 268 N, 121' Felt widely in central California. Richter Magnitude Oct. 25 h1' W (B).
h.9 Fm7== intensity VI at a number of Bay area com-4 Santa Clara munities. IV at Bedega g Jenner, et al.
County.
0=20:50:33 PST j
l 19h?
37' 00 ' N, 121' Felt over an area of 7,000 square miles--north to Santa June 22 h6' W (B). Near Rosa, east to Stevenson, and south to San Ardo. Richter Watsonvine.
Magnitude h.7.
Maximum intensity VI at may commnities 0=15:29:33 PST in southern Santa Clara and Santa Cruz counties and northwestern Monterey County. IV at Forestvine, et al.
19h?
ho.h' N, 125.2' Felt generally over an area of LOOO square miles of north-
,_)
Sept.
W(B). Off western California. Richter Magnitude 5 3.
Wr4==
i l-23 Cape Mendocino. intensity VII at Punta Gorda. Isolated reports stated 0=05:52:55 PST that the shock was felt slightly at San Francisco and Occidental.
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- L Appendix IV I
Location of
~ p)
Date Epicenter-Remarks s.
j 19h9 Sonoma County?
'Jenner
. Trembling motion felt by three. (Nothing cor-l Mar. 8 responding to this report recorded on av Bay area seis-mographs.)-
19h9 37' 018 N, 121' Felt over area of 20,000 square miles from Santa Rosa Mar. 9 29' W (B). Near to Paso Robles. Richter Magnitude 5.2.
Maxima in-Gilroy tensity VII at Hollister. Y at Din on Beach et al.
)
0=0h:28:39 PST IV at Sebastopol (2 miles west of), et al.
19hp Sonoma County?
Janner: Light shock felt by three persons. Two other-Aug. 27 shocks felt in a.m. of daylight hours. (Nothingcor-responding to this report recorded on av Bay area seismograph.)
1952 '
37' h5 8 N,122' n' Felt over an area of 3,500 square miles. Richter Oct. 12 W(B). Oakland.
Magnitude k.2.
Maximum intensity V at may localities,-
0=16:3h09 PST including Dinon Beach. IV at Bodega Bg, Jenner, et al.
195h 36' 56' N, 121' h18 Felt over an area of 12,000 square miles. Richter Apr. 25 W(B). Near Magnitude 5 3.
Maximum intensity VIII east of Watsonvine.
Watsonvine. I to In at Tomales, et al.
0=12:33:28 PST h*
195h 37' h3' N, 122' 08' Felt over an area of h,500 square miles. Richter Dec. 16 W(B). East of Magnitude h.5. W r4-= intensity VI. IV at Tomales, San Leandro.
et al. I to III at Marshall, Sebastopol, et al.
0=23:08:58 PST 1955 37' 22' N,121' h7 ' Felt over an area of 12,000 square miles. Richter Sept. h W(B). East of Magnitude 5.5.
M*v4-= intensity VII at and near San San Jose.
Jose. I to III at Marshan, et al.
0=18:01:18 PST 1955 37' 58' N,122' 03' Felt over an area of 12,000 square miles. Richter
)
Oct. 23 W(B). Between Magnitude 5.h.
F=v4=m intensity VII near the epi-Walnut Creek and center. IV at Bodega, Bodega Bg, Dillon Beach.
Concord.
Forestvine, Jenner. Marshan, et al. In at Oraton.
0=20:10thh PST.
Guerneville, sebastopol, Tomales, Vaney Ford, et al.
1956 38' 32' N, 122' 318 Felt over an area of h,500 square miles. Richter Apr. k W(B). Near St.
Magnitudes h.h and h.2 respectively.
Av4== intensity Helena.
VI in the Napa Yaney area. IV at Bodega Bay 0=20-29-13 PST, and Quernevine, et al. 3 to III at Marshall. et al.
0=20:29:32 PST 1957.
37' LO' N, 122' 298 Felt over an area of 12,000 square miles. Richter Mar. 22 W(B). Near Mussel Magnitude 5.3 Maximum intensity Yn at Colma, Daly Rock, San Mateo City, Westlake, Westlake Palisades, San Bruno, San
, _^
)
County.
Francisco, and Sharp Park. VI at (inter alia) Bodega v
0=11thh:21 PST Ba where the shock was felt by an, frightened mag.
l 1 objects shifted; books fe u. Y at Camp Meeker, Dillon Beach Duncan's Mills Fanon, Forestvine, Querns e, Jenner, Mars
. Occidental, Sebastopol.
Tomales, Vaney Ford, et al. IV at Oraton, et al.
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I-7 Appendix IV 7
Location of Date.
Epicenter Remarks 1958-37' 29' N, 121' h7'
. Felt in scattered localities our an area of 1,500
. Oct. 30 W (B). Northeast
. square miles. Richter Magnitude h.2.
Maximum of San Jose.
intensity VI it Milpitas. I to III at Bodega By 0=16:26:1h PST.
(6 miles north of), Marshall, et al.
. )
j 1958 37* h18 N,'122* 32' Felt over an area of 6,500 square miles. Richter Dec. 11 W (B). Southwest.
Magnitude h.7 Maximum intensity VI near the epi-of San Francisco.
center. Y at Dillon Beach, Fallon, Marshall, 0=01:52:27 PST.
Tomales. et al. IV at Bodega, Bodega Bg, Jenner, Monte Rio, Valley Ford, et al.
1 N.
j 1959 36' 59 6 12136' Felt over an area of 8,000 square miles. Richter Mar. 2.
W(B). Near.
Magnitude 5.3. Maa-t intensity VI. IV at Marshall, Oik oy.
et al. I to III at Dillon Beach, Jenner, Sebastopol, 0=15:27:17 PST.
Tomales, et al.
1959 38.7' N, 122 7' W Telt over an area of 600 square miles. Maximum June 27 (B). Near Mt.
intensity IV at CaMstoga, Forest.ille, Healdsburg, Saint Heleria.
Middletown, a:xi Sair.t Helena.
C=23:58:01 PST.
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J II-1 Appendix IV soar TABLE II O
MODIFIED MERCALLI INTENSITY SCALE OF 1931 s
(Abridged)
Approx.
{
equiv.
intensity.
q Rossi-Forel J
Scale l
I.
Not felt except by a very few under especially favorable I
circums tances.
j II.
Felt only by a few persons at rest, espe.-ially on upper floors of buildings. Delicately suspended objects may swing.
I to II III. Felt quite noticeably indoors, especially on upper floors of 4
1 buildings, but many people do not recognize it as an earthquake. III Standing motor cars may rock slightly. Vibration like passing truck. Duration estimated.
IV.
During the day felt indoors by snany, outdoors by few. At night some awakened. Dishes, windows, doors disturbed; valls made IV to V creaking sound. Sensetion like heavy truck striking building.
Standing motor cars rocked noticeably.
V.
Felt by nearly everyone; many awakened. Some dishes, windows, d
etc., broken; a few instances of cracked plaster; unstable V to VI objects overturned. Disturbance of trees, poles, and other tall objects sometimes noticed. Pendulum clocks may stop.
VI.
Felt by all; many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster or VI to VII damaged chimneys. Damage slight.
]
VII. Everybody runs outdoors. Damage neglibible in buildings of l
good design and construction; slight to moderate in well-I built ardinary structures; considerable in poorly built or VIII-badly cesigned struef:ures; some chimneys broken. Noticed by persons driving motor cars.
VIII. Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse; great in poorly built structures. Panel walls thrown out of frame structures. Fall of chimneys, factory stacks, columns, monu-VIII+ to IX i
ments, walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Disturbed l
persons driving motor cars.
IX.
Damage considerable in specially designed structures; well designed frame structures thrown out of plumb; great in sub-stantial buildings with partial collapse. Buildings shifted IX+
off foundations. Ground cracked conspicuously. Un'derground pipes broken.
X.
Some well-built wooden structures destroyed; most masorry and frame structures destroyed with foundations; ground badly cracked. Rails bent. Landslides considerable from river X
banks and steep slopes. Shifted sand and mud. Water splashed (slopped) over banks.
XI.
Few, if any, (masonry) structures remain standing. Bridges l
destroyed. Broad tissures in ground. Underground pipe
}
lines completely out of service. Earth slips and land slips in soft ground. Rails bent greatly.
' - -)
XII. Damage total.. Waves seen on ground surfaces. Lines of sight and level distorted. Objects thrown upward into the air.
i l
This abridged version was published together with the complete version by Harry O. Wood and Frank Neumann in " Modified Mercalli Intensity Scale of 1931,"
Bulletin of the Seismological Society of America, 21: 277-283 (1931)
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Seaward side v
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tl GEOLOGY OF BODEGA HEAD wn...
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l EARTHQUAKE HAZARDS AND EARTHQUAKE RESISTANT DESIGN-BODEGA BAY POWER PLANT SITE 1
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George W. Housner 919t 1201 East California Street Appendix V Pasadena, California n-t January, 1961 l
Mr. J. D. Worthington Chief Civil Engineer Pacific Gas and Electric Company 245 Market Street San Francisco 6. C411fornia EARTHQUAKE HAZARDS AND EARTHQUAKE RESISTANT DESIGN BODEGA BAY POWER PLANT 51TE I
PACIFIG GAS AND ELECTRIG COMPANY I
The proposed site for a nuclear reactor power plant is on Bodega Head, Sonoma County, California. The proposed location of the plant I
(PG k E Scheme VII) is shown in Fig.1. This site is close to the San Andreas fault mone which passes a mile or so to the east. Bodega Head is formed by twin hills of granitic (quartz-diorite) rock, as described in Report on Earthquake Hazards at the Bodega Bay Power Plant Site, by D. Tocher and W. Quaide. The valley between the two hills is filled
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with alluvial deposit as shown in Figs. 2,3 and 4 and is described in the j
1 report of Tocher and Quaide and the reports of Dames and Moore dated l
i January 25, 1960 and December 2,1960.
1.
Possible Earthquake Hazards.
A discussion of earthquake hazards at nuclear reactor power plant l
sites must answer the following questions:
a).
What is the frequency of occurrence of earthquakes in the general area?
b). What is the probability of occurrence of earthquakes of large
.)
Magnitudes in the general area?
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1.
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4 g]( c).1 What is the likelihood of active earthquake faulting occurring I
ca the site and thus producing gross bodily movement of one part g
of the site relative to another$
d). What is the likelihood of rock fissures or movements, not I
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the earth tremors of a strong earthquake produce a failure of a l
't stru.cturally weak geological formation?
e). Is t.here any danger of permanent displacement of alluvial deposits resulting from consolidation (settlement) or resulting I
from slumping or landslide?
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s f). What intensity of ground shaking is likely to be experienced j
1 at the site?
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IO g). What design procedures should be used to protect the plant against earthquake damage?
b). Are any special precautions called for because of special conditions pertaining to the sitet These eight items will be discussed in the following paragraphs.
2.
Frequency of Occurrence of Earthquakes.
l l
A general discussion of the frequency of occurrence of earthquakes j
l I
in California is given by Gutenberg and Richter and a more detailed discussion of the occurrence of strong earthquakes in California is given by NousnerN. The proposed site is in a region of high seismic activity.
I TT t
Gutenberg, B., and C.F. Richter, " Seismicity of the Earth Princeton University Press,19 58.
'2)Housner, G. W., " Spectrum Intensities of Strong Motion Earthquakes,"
Proceedings of the 1952 Symposium on Earthquake and Blast Effects on
., Structures. Earthquake Engineering Research Institute,1952.
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The 1906 San Francisco earthquake (Magnitude 8.2) centered some twenty miles from the site. It has been estimated that a large earthquake, such as the 1906 shock may be expected te occur along the San Andreas fault l
in the Bodega Bay region perhaps three or four times per 1,000 years.
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Less intense ground motion can be expected to occur with greater frequency, it being estimated that ground ractions at least sufficiently strong to cause
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l damage to poorly designed structures may be expected at the site several J
times during the next hundred years.
3.
Probability of Large Earthquakes.
1 Since the 1906 San Francisco earthquake was caused by slipping of the adjacent San Andreas fault, it is established that the site is subject to the ground motion produced by very large earthquakes at close distances.
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4.
Earthquake Faulting at the Site.
Since it is quite impossible to design a power plant to survive witnout damage the large perman. nt ground surface displacements that might occur if the earthquake fault slippage occurred on the cite, this possibility must be given special consideration. It is well-known that the adjacent San Andreas fault zone has experienced many permanent horizontal shear-type displacements in the past, and this raises the question as to whether there may be a branch of the San Andreas fault passing through the site. Tocher and Qualde in their report give special consideration to this question, and conclude that there is no evidence of any active faulting having occurred at the site during the past several thousand years.
A second question that arises is whether a fault that has lain dormant wj for several thousand years could oecome active in the future. While it is
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possible for dormant faults to become active, it is extremely unlikely that there could be such a reactivation of a fault at the Bodega Head site, 3
During the past several thousand years there must have been many I
slippages along the adjscent San Andreas fault zone so that this is a well-established plane of weakness. There appears to be no reason to expect that future slippage should go through the stronger rock formation underlying Bodega Head rather than to continue to be concentrated along the weaker San Andreas fault zone.
5.
Permanent Displacement of Alluvium.
The alluvium between the granitic hills is well consolidated as described in the Dames and Moore reports and Tocher-Quaide report.
There appears to be no danger of any gross movement of the alluvium n
4 during an earthquake. Any open-faced cuts made in' the alluvium must, of course, have a sufficiently flat slope to avoid danger of landslides.
- 6.. Intensity of Ground Shaking.
The most intense ground shaking at the proposed site will occur when a large earthquake, say, of Magnitude 8.2, originates on the adjacent San Andreas fault. Since no strong earthquakes have been recorded under the special conditions of this site, it is necessary to estimate the intensity of ground motion. In estimating the intensity of ground motion the following conditions are of special importance:
a) The Imrge Magnitude of the shock. Strong ground motions produced by Magnitude 8.2 shocks have never been measured. There have i
been measurements of Magnitude 7 and Magnitude 7.7 shocks. If a
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O-Magnitude 7 shock were to occur on the San Andreas fault it would be associated with a fault slippage extending perhaps 40 or 50 miles in length and some 20 miles in depth so that the source of the seismic waves would be r
f approximately a rectangle measuring 50 x 20 miles. The fault slippage associated with the El Centro,18 May 1940 shock was approximately of this extent. For a Magnitude 8.2 earthquake, such as the 1906 San Francisco shock, the fault slippage can be expected to be several hundred miles in leng th. If we compare the intensity of shaking of a point on the surface of
' he ' ground near the center of the Magnitude 7 shock with the intensity near l
t the center of the 8.2 shock, it is seen that the extra areas of slippage of the 8.2 shock are at distances greater than 25 miles from the point of measure-ment'of ground motion. It can be concluded from this that the intensity h
of shaking at a point near the center of the 8.2 shock will not be much more severe than for the Magnitude 7 shock.
.i b) The proximity to the fault. The proposed site is only about a mile or so from the fault so that the guestion arises whether this proximity means more intense ground shaking. The center of the slipped fault area for a large shock will be at a depth of some 10 miles beneath the surface of the ground. This center is not significantly closer to a point one mile from the fault on the surface of the ground than it is from a point five miles from the fault. So that from this point of view, the intensity of shaking can be expected to be approximately the same over a distance of perhaps five miles on each side of the fault. In fact, from the anti-symmetric throw on each side of the slipped fault it could be expected that the intensity of ground motion might even be somewhat less at one mile from the fault than at five miles. This diminution of intensity very near the fault has,
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in fact, been observed during some' earthquakes. For the design of the power plant it will be reasonable to assume that tho intensity of shaking will be approximately the same over a strip extending five miles on either side of the fault (see also the Tocher-Qualde report).
c) Intensity of shaking on the granitic rock. None of the past recordings of strong ground motion have been made on granitic rock. Some seismologists 3I, on the basis of measurements of very weak ground motions, have suggested that the intensity of shaking on granite will be very much less than on alluvium, perhaps only 1/4 to 1/8 as intense. The writer does not I
agree with this and recommends a reduction of intensity of no more than 1/2 for granite as compared to alluvium. (The alluvium on the proposed site has dimensions, that are small compared to the wave lengths of the seismic waves so that its motion can be expected to be essentially th'e same as the underlying granitic rock).
d) The ground accelerations. Figures 6, 7 and 6 show recorded ground accelerations for three large earthquakes. The strongest so far recorded is that of the El Centro,18 May 1940 which is shown in Fig.6.
This motion was recorded approximately 5 miles from the fault at a point near one end of the slipped area. This war a Magnitude 7 shock. If the Gutenberg, B., " Effects of Ground Shaking in Earthquakes," Transactions of American Geophysical Union, Vol. 37, No. 6. December,1956, 4)Housner, G. W., " Behavior of Structures During Earthquakes," Journal of Engineering Mechanics, American Society of Civil Engineers, EMS, h
Octobe r, 1960.
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7.
slipped area had extended another 50 miles beyond the point of measurement the intensity of shaking would have been increased. It is estimated that the intensity would have been approximately 40% greater ([5 ) in this event.
At El Centro there is a deep sandy alluvium and it would be a reasonable estimate to suppose that the underlying rock had an intensity approximately one-half as great as measured on the surface of the alluvium. We thus arrive at an estimate for the intensity of shaking at Bodega Head for a Magnitude 8.2 shock on the adjacent San Andreas fault that is approximately 75% as great as the ground motion shown in Fig. 6.
Since the foregoing method of estimation is subject to inaccuracies, tha writer recommends that the maximum ground shaking to be expected on Bodega Head be taken i
equal in intensity to the ground motion shown in Fig. 6.
The various (D
intensities of ground motions recorded in the Ur.ited States are given in s
Appendix A to this paper (page 121).
7.
Recommended Design Procedures.
The main reactor building will be seated in the granitic rock as shown in Figs. 3 and 4.
Subsidiary structures will be located around it.
For purposes of earthquake design, the structures of the power plant may be classified in the following three categories:
Class 1 Structures. These are the buildings and the pieces of equipment vital to the operation of the plant which, if damaged during an earthquake, might cause a nuclear incident. See Appendix B for a more complete discussion of this class of structures.
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Class 2 Structures. These are the structures important to the operation of the plant but whose failure would not entall any hazard of a i,y...,,_,,,,;,,,,,j.m.m,3-. y.,7,m7. a.3 y,-
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8.
nuclear incident. $se Appendix B for a more complete description of these structures, j
Class 3 Structures.
These are the structures that are not essential to the operation of the plant and whose damage or failure would be an in-convenience but would not necessitate a shutdown of the plant.
It is recommended that Class 1 structures be designed on the principles set forth in Appendix A.
The design should be based on the average a
spectra shown in Figs. 9 and 10 with the ordinates multiplied by the factor
- 2. 7.
As explained in Appendix A, this is designing against ground motion
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I of the intensity of the El Centro,18 May 1940 ground motion. The design j
of the Class 1 structures should be such that the stresses do not exceed
,p the usual allowable stresses.
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It is recommended that Class 2 structures be designed for the sams f
intensity of gror.nd motion as the Clasal structures, but the design may utilize energy absorption by stressing oeyond the yield point, etc. The factor of safety against failure should be not lese than 3.
It is recommended that Class 3 structures be designed in accordance with the earthquake provisions of the Uniform Building Code.
8.
Special Precautions. The only special condition at the site is the bedding of the reactor building in the granitic rock with 20 to 40 feet of uvium surrounding the upper part of the structure. Because of this, s
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during an earthquake the alluvium will bear agt. inst the structure, and in the design consideration must be given to these forces. This will not pose
,,1 any serious design problem.
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9.
- 9. Summary.
It is concluded that the proposed Bodega Day power plant (P.G. k E. Scheme VU) can be satisfac'torily designed to resist earth.
l quake ground motion. Because the main reactor building (Figs. 3 and 4) is a concrete box structure very little, if any, additional cost will be required to make it earthquake resistant.
GEORGE W. HOUSNER i
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Appendix V-v..
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Appendix A Housner, G.W., " Behavior of Structures During Earthquakes",
Journal of Engineering Hechanics, American Society of Civil Engineers, October 1959.
Appendix B Housner, G.W., " Design of Nuclear Power Reactors Against Earthquakes", Second World Power Conference on Earthquake Engineering, Tokyo, July 1960.
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