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.: SEISMICITY AND TStJNAMI REPORI l
]' BODEGA HEAD, CALIFORNIA l l
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The Division of Licensing and Regulation of the Atomic Enercy Conmincion, j Washington, D. C. , requested the U. S. Coast and Geodetic Survey to report ]
q on the seismicity and tsunami condition of Bodega Head, California. This ,
M report contains. an evaluation of the seismic condition of Bodega Head as i
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.l defined by the Pacific Gas and Electric Company in numerous documents I
) submitted to the AEC. In addition, the Survey presents an independent
- evaluation of the earthquake frequency pattern along the San Andreas fault, l the most probable ground motions masured in acceleration and displacement
. e for a > magnitude 8.3 on this fault near Bodega Head and the tsunami hazard at the same location. The Survey is in a unique position to perform this j service because it has either the original documents or a complete file of historical data for ee.rthquake seismology, engineering seismology, and tsunamis and has made studies in these fields for approximately 30 ho years.
1 j In this report and all other geologic and seismic reports submitted to l the AEC relative to the proposed reactor at Bodega Head, frequent reference t
I is made to geological faults and in particular, the San Andreas fault. This l
a is to be expected in describing earthquakes because a fault is the only {
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] I surface manifestation of earthquake occurrence. Geologists refer to faulto
' i or earth fractures as active or inactive, depending upon the recency of '
- movements. Active faults are associated with recent earthquake activity, i i
,j such as the earthquake belt around the perimeter of the Pacific Ocean and across Asia and along the Mediterranean Sea to the Atlantic Ocean. An
'l j example of an inactive or relatively inactive belt is the Appalachian i system in eastern North America where there are extensive fault systems but only minor and infrequent tremors.
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! The San ' Andreas fault, which is of principal interest 'due;to its proximity - 1 1-
.! -(within 1000 feet of the limits of the.westernisone) to the proposed site of.
Bodega Head reactor, is considered by geologists ~ and' seismologists to be an ;
l -,g 1' active earthquake source. This fault extends southeasterly from a point
.( under the ocean about 300 miles from the' Oregon' coast (approximately. h5' L .t
. north latitude,130' vest ' longitude) L across Califomia and under the'Oulf of California to a point across from the southern tip of Lower California.
It is a right hand strike ' slip fault (i.e. imetion is predominately hori-zental) and it has been the source of two great earthquakes in historie-i l times (1857 and 1906). Other faults trending nearly parallel .to this master j :
4 fault show evidence of right hand slip or horizontal displacement. Faults '
with trends roughly at right angles to the San Andreas are predominately 9
left hand type.
t In Southern California there is evidence for accumulated shift of 1
about 25 miles along the San Andreas fault since mid-Tertiary time. Some investigators believe much more horizontal motion has taken place. ' Movements
] along the fault have been measured by the Survey in the vicinity of Point I
- Reyes to Petaluma, San Francisco to San Jose, Hayward, Hollister, San. Luis i
Obispo to Avenal, and others. At each location several geodetic lines were measured across the fault and where movements were noted, they approximated i
, j an average of 2 centimeters per year. The plative motion indicates th,e i
j vest side of the fault is moving northward and the east side southward.
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' The maximum horizontal shift observed after the 1906 earthquake was '21-l i
,j . feet at Tomales Bay and the vertical motion was no greater thari 3 feet.
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'!his is typical of the earth motions resulting from California earthquakes,
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il 1.e. , gnater horizontal than vertical displacements.
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i i Even though California and specifically the San Andreas . fault are 4 I considered earthquake prone areas, they experience a surprisingly lov
' 1, number of magnitude 6 and greater earthquakes.' There have been two " great" earthquakes (1857 and 1906) on the fault and approximately 20 earthquakes
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h from 1800 to 1950 with magnitudes 6 to 7+ vithin 75 miles of this fault.
Many of the largest California earthquakes have been followed by swarms of l 'l strong aftershocks. Usually the strongest aftershocks have magnitudes at l
l least one unit lower than the main shock. The frequency of the aftershocks increases with a decrease in magnitude.
As noted in the report submitted by Tocher for earthquakes felt at or near Bodega Head, 1838-1960, none of them were centered at Bodega.
Of the 58 listed earthquakes, ik were reported felt at Bodega or along Bodega Bay 4 2 caused little or no damage; and 1 in 1906 caused appreciable damage and some surface fissuring.
Studies of the seismicity along the San Andreas fault for the past 57 years show that a magnitude 6 to 6.9 earthquske occurred every 7 years on l the average and a magnitude 7 to 7.9 earthquake occurred once during this l +
period. These occurrences agree with the pattern determined by Gutenberg
{ in " Seismicity of the Earth" which predicts a magnitude 8 earthquake about
. once every 100 years. In discussing the frequency of high magnitude '
i earthquakes, it should be noted that many reoccur in the same epicenter 1
areas . Richter mentions 3 earthquakes occurred in Honshu, Japan at 39'
. j north, lh3' east, in 1897,1898 and 1905 with magntidues from 7 9 to 8.3.
, A number of areas in Italy experienced repeated damaging earthquakes, for
.i. .j example, cirifaleo (1626,1659,1783,1906); Monteleone (1659, 1783, 1905);
}j Gerace (1720,178h,1791,1907). According to Davis, Valparaiso, Chile was destroyed in 1822 and again in 1906. Skopje, Yugoslavia, of recent memory, was totally or partially destroyed in 1963,1921,1555 and 518. J
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. ,' To design and construct earthquake resist' ant structures it is 'necessary.-
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to know not only the above-mentioned saismicity information about destructive l earthquakes, but . also the: displacement , velocity and acceleration of ground
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jg,y motions and the response' characteristics of structures to these motions.
. Since 1933, tho' Coast and Geodetic Survey has made such measurements of.
, strong earthquakes in the Westem United States and in Latin America. All 4
interested parties in this investigation are aware of this work and the data collected have been used extensively by all. There is general uniformity in the interpretation of the ' direct. recorded strong motion data at El Centro,
- j e. San Francisco and Seattle. However, there is 'some ' dispersion in the computed i .
I results when attempts are made to extrapolate' from magnitude 6-7 earthquakes to magnitude 8.3 (San Francisco,1906). Extrapolations are necessary sin:e no magnitude 8 or greater earthquakes have ever been recorded by. strong i 1 motion seismographs. '
i l The strong motion seismographs or.erated at a number of locations in 1
l California since 1933 have recorded several intermediate magnitude earth-l 1 quakes. Among those recorded were the Long Beach of 1933. El Centro of, l 19k0, Kem County of 1952 San Francisco of 1957, and Olympia-Tacoma, I Washington of 19h9. The greatest recorded accelerations vere of the order of 1/3 g in, Seattle, Washington, and the wave periods for the greatest .
'q acceleraticos were 0.2 - 0.h second. Most of the recordings have been m made by instruments en unconsolidated material with the exceptions of the ma s.
% fj Golden Gate recording of the March 1957 earthquake which was on granite.
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j - [, To date, no recordings have been made within 7-8 miles of the epicenter Y so there are no experimental data for accelerations or displacements of ground motions within a mile or two of an earthquake epicenter.
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- Through the use of these strong motion data in computing response i I
, spectrym and in correlating intensities of strong earthquakes, the Survey '
.T i has estimated acceleration factors for Bodega Head. . In addition to these y;
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>, a earthquake computations , the Survey has performed similar experiments in connection with nuclear explosions. . Even though the source mechanism for )
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.,(. - the release of energy by earthquakes and explosions differ, it is interesting l . to note that the accelerations of both are of the same order of magnitude.
Results of the Survey's studies show that en rock', a maximum probable J
ground' acceleration of 2/3 g at periods from 0.2 - 0.6 second should be j expected at Bodega Head and that ground accelerations as high as 1.0 g in the same period range could occur and should be taken into account in the
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Relative to the possible dislocations in the site or adjacent parts l ;
of Bodega Head during a major earthquake, thers is a scarcity of data upon l vhich to estimate a probable displacement. The geology of the site was l
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' i studied and reported by the Geological Survey based on excavations of the
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i shaft. It must be emphasized that this study covered an infinitesimal l I l
8 segment of the geological structure associated with the San Andreas fault.
Therefore, if such concentrated surveys were extended to other areas even
'l very close to the proposed site, it is our opin' ion that evidence of recent 1
1 j faulting would be found. Moreover, the occurrence during large earthquakes i
of offsets on minor faults in sympathy with a large displacement on the i ,
causative fault cannot be disregarded. Certainly in the case of the Point
.y Reyes Peninsula during the 1906 earthquakes the displacements in bedrock indicate that faulting does occur outside the San Andreas fault zone in sympathy with large displacements within the zone. Such an occurrence of offsets on Bodega Head during future earthquakes is a definite g ti-
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} possibility. The Survey believes that the permanent displacements at 3
- Bodega Head could be as large as 2-1/2 feet, as experienced at Point Reyes !
- a during the 1906 earthquake.
i .i (j Another factor for consideration in evaluating Bodega Head as a 3
possible site for a reactor is the probability of a' damaging tsunami. It is-well established that the San Andreas fault extends into the ocean off the northern coast of California. Since 1898 there have been two submarine earthquakes along this fault that could have generated a tsunami. ._Moreover, j l -
there is a possibility that ' the next earthquake of magnitude greater than 8 'f !
1 along this fault could occur off-shore and generate a tsunami. )
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. Now it might be in order to digress briefly and any something about 'la tsunami causes and propagation. For the most part,' tsunamis are generated {
j by submarine earthquakes or earthquakes located close to coastal aress.
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1 However, it is well established that caly a small percentage of submarine earthquakes generate measurable water waves. . The most common explanation j
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is the vertical displacement of submarine blocks of the earth's crust.
Since it has been observed on land that great earthquakes have caused
! uplifts of 30 - 50 feet and affected crustal blocks hundreds of miles long j
- I and up to a hundred miles wide, it is easy to conceive of such a crustal
.i 1 movement under the ocean generating a huge water wave. Slides along the 2
coasts are also thought to be sources of tsunamis. In the case of great
- h. earthquakes originating on the sides of deep oceanic troughs, huge masses A
'of unconsolidated material may slide into the depths , displacing a great amount of water. It has been suggested that there is a possible coupling ]I mechanism between tsunamis and great seismic surface waves with periods over a minute.
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4 7-It is not surprising that the tsunami peril was not mentioned in connection with the applicant's report because, with the exception of a vave reportedly generated by a local earthquake on December 21, 1812, there j
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4 is no record of a destntetive tsunami being generated along the California j f
The 1812 vave reportedly reached land elevations of 50 feet at coas t.
"l Gaviota, 30 - 35 feet at Santa Barbara, and 15 or more feet at ventura. j i
. l Inasmuch as historical records for locally generated tsunamis are so 1 !.
l l sparse, the dimensions of tsunamis that have been established through j
, relatively frequent occurrences in Japan should be considered. Iida has
, done considerabic work in establishing statistical relationships from i l available Japanese data using both earthquake magnitude and focal depth.
3 His formulas show a small tsunami vill be generated for a shallow earthquake of magnitude 6-1/2 - 7 and disastrous tsunamis for shallow earthquakes with magnitudes of 7-3/h or greater.
i Based on Iida's formulas, a tsunami classified as destructive vill hav a height of about 33 feet or greater. The earthquake of March 3,1933, off the Sanriku coast of Japan, had a magnitude of 8.3 and was of shallow focal
,! depth; the wave rose to heights of 77 feet on the coast. The recent Alaskan 4
earthquake had a magnitude of 8.43 maximum waves of 30 - 35 feet were l reported at Kodiak and may have been exceeded elsewhere. Iocal waves )
. .; I i were reported of 50 - 60 feet for the chile tsunami of May 1960 (earthquake magnitude of 8-1/4 1/2). These support Iida's formula as being L! reasonable (even though far from being rigorous evidence) and suggest that
~his conclusions for the Japanese area may apply approximately in other areas.
1 It is not possible at this time to cite values for specific areas such as the Bodega Head site. However, for the California coast as a whole, there is one justification for differentiating between runup due i
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, [ to locally' generated tsunamis (epicenter within several hundred miles) and. -)
>. l that associated with distant earthquakes. For tsunami runup from nearby ,
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,I severe marine. earthquakes, a site should be protected to a vertical height y
.:,j . of 50 feet above mean lower low water. There appears to be sufficient. )
experience along.the coast of. California to limit'the required protection:
j's ! against tsunmais front distant generating areas to. 30 feet. ab'ove the same datum.
The seismological risk of. a local tsunami-generating seismic event may .
! vary from place to placei but the numerical height' that must be assigned for the subsequent runup must be the same. Future refraction studies may indicate
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' ! that it is possible to lower somewhat the 30 foot limit at' this site for :1 tsunamis from distant generation areas. The Coast and Geodetic Survey expects to complete an electronic computer program describing tsunami propagation in the Pacific in about one year. This program will calculate 1 l the relative convergence of rays of energy for different coastal zones i
from various source areas and thereby permit some degree of differentiation l in the required elevation for protection of special sites. However, this I
differentiation will be significant only' if it is decided that the danger of a locally generated destructive wave is so small that it may be disregarded.
1 I CONCLUSIONS' (1) The seismicity study of the San Andreas fault area near the ;
i proposed Bodega Head reactor shows the possibility of a magnitude 8.3 earth '
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1 j quake about every 100 years. For this reason, the- Coast and Geodetic Survey q believes that an earthquake of magnitude 8.3 should be expected during the. .;
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CR lifetime of the reactor plant.
1 (2) The Survey believes that a maximum probable ground acceleration i
', of 2/3 g at periods from 0.2 - 0.6 second should be expected at Bodega .
, . , Head and that ground acceleraticas as high as 1.0 g in the same period 'i l ;pJ .. i k [ f f M M :. @ 2 - hHnW7 %s .p-@4,.+ ic.:
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range could occur and should be taken'into account in the design of the i facility.
i (3) The Survey believes that relative ground displacements of 2-1/2.
feet should be considered in the design of the facility.
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(k) . For tsunami runup from nea@ severe marine earthquakes, a site should be protected to a vertical height of 50 feet' above mean lower low q
water. There appears to be sufficient experience along the coast of 0
California to limit the required protection against tsunami from distant
' q generating aress to 30 feet above the same datum.
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j U. S. Coast and Geodetic Survey )
l Washington , D. C. .{
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, , October 196h
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