ML20127D775
| ML20127D775 | |
| Person / Time | |
|---|---|
| Site: | Vallecitos File:GEH Hitachi icon.png |
| Issue date: | 09/04/1992 |
| From: | Pate R NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION V) |
| To: | Cherb J GENERAL ELECTRIC CO. |
| References | |
| NUDOCS 9209150169 | |
| Download: ML20127D775 (14) | |
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General Electric Company Vallecitos Nuclear Center P. O. Box 460 Pleasanton, CA 94566 Attention: Joe Cherb, Manager Nuclear Safety
SUBJECT:
SEISMIC POTENTIAL Of THE NORTHERN CALAVERAS FAULT Subsequent to the licensing of your facilih. the U. S. Geological Survey
-(U.S.G.S.) conducted a study of the seismic potential of the northern Calaveras fault. This study is a part of the Proceedings of the Second Conference on Earthquake Hazards in the Eastern San Francisco Bay Area and was delivered at that conference during March 1992.
-We have enclosed a copy of that study for your consideration. We request that you take any actions that you determine are required to ensure that your safety analyses remain valid.
Should you have any questions feel free to call me at (510) 975-0246.
E Robert J, Pate, Chief _
Safeguards, Emergency Planning, and Non-Power Reactor Branch Enclosure L cc:-
'Seymour Weiss, NRR Marvin Mendonca, NRR L
G. Cook, RV
' P.Quall s, .; RV
.C. Hooker, RV V ,L
.RV/gmd ,/ V, UEST COPY REQUEST C0P SqDTOPDR ///
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PQualls RPate 9 3 /92 9/54/92 , ,
9209150169 920904 REGS SUBJ PDR
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ne Potential for Earthquake Rupture of the Northem Cahveras Fault by David 11. Oppenhehner and Allan G. Lindh' AUSTRACT Since 1969 virtually no seismicity has occurred on the Calaveras fault north of the Calavtras Reservoir and south of Danville. By comparison with the seismic behavkar of the Calaveras fault south of the resenoir and tie fault segrnent of tlw San Andreas fault that ruptured during the toma Prieta :arthquake, we infer frem this absence of canhquakes that the northem Calaveras fault is hxted and accmnuhting chstic strain er.crgy. %e slip ute of this segment of the fault is approximately 6 mmlyr, and there is no indication of surface creep. He sels nic gap k approximately 40 km long and the width of the fault infened froin tle muimtun depth of earthquakes tanEes from 10 to 15 km. We consider the potential of an earthquake acuning on this segment over the next 30 years for two equally phusible rupture f.cenarios. If the entire northern Calaveras fault ruptures at one thne, the expected magnitude (M) would be approxirnately M 7, but there is insufficient infonnation at present to calcubte reliable probabilities. Allematively, the fault could rupture in two or more M 6 earthquakes, as suggested by the occunence of three modente earthquakes between 1858 and 1864, with a probability of one or inore evtuts of M > 6 of 0.?1. A10iough the probabilities have large uncertahities, we believe the geologic and geophysict1 data indicate that the noithe a Cahveras fault is a significant seismic hazud for the San Francisco Bay region.
IN'lRODUCTION ne canh science community is not i,b' at present to provide precise forecasts of the time, place, and inagnitude oflarge canhquakes. Yet earthquakes present considerable risk to society, and despite our limited undentanding of the earthquake process, we are continually tuged to m:6 educated assessments of canhquake hazards. Publications by the Working Group on Califon.ia Earthquake Probabilities (WGCEP)(1988) remhed consensus ophdon on the probability of large e.arthquakes in Califomh through a fonnal process that considered the availabh infonnation on known, active faults. A subsequent report was released in 1990 that considered the probabilities of earthquakes h1 the San Francisco Day region in light of the occurrence of the 1989 magidtude (M) 7.0 loma Prieta carthquake and newly published htfonwition on the Rodgen Creek fauk (WGCEP,1990). %at report considered the carthquake potential of the liayward, Rodgers Creek, and San Francisco Penhu.ula segment of the San Andreas fault. We will not revisit those topics in this repoit; rather, we consider the potential c:uthquake hazard of a fault segment not considered by the WGCliP - the Cahvens fauh north of the Calavens reservoir ("northem" Cahveras)(Fig.1). We will review tl.e geologic and geophysical data that bear on the likely size of the ruptw e 2cne, the interval of tbne since the last earthquake, and the slip rate of the fauh.
We will then u9: this infonnation to forecast the probability of an c:uthquake occuning on the northem Cahveras fault over the next 30 years.
' Doth at ll.S Cnciegical survey,345 Middkfield Road MS 977,Menlo lit,Cahl.mia 94025 1
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MICROSElSMICIT( AND MAIN S!!OCK RUPlVRE AREA Maps st.owing fauhs with late Quatemary or llolocene dispherment an one of the prhnary tools .
In forecasting the locations of hrgo earthquakes (Schwartz and Copperunith,1984). Ibese maps, however, depic4 only die two-dhnensional aspect of fault stnicture; the thint dhoension, depth, has remained more difficah to quantify until the advent of earthquake networks beglaning in the late 1920's. Several features of the background selsmicity teflect conditions in the crust that control main shock nipttue,1he depth intaval in which this seismicity occus indicates the width .
of the brittle, or t.elsmogenie, region of the crust, and consequently the mashoum width of faults .
that could dynamicaDy nipturt during large earthquakes. I In addition, analyses of recent easthquake sequences in Califomia indicate that the location panems of badground miermeisruJcity can also delineate the lateral extent of the rupture zone of ,
the expec1ed unin &!m(1. For example, studies of selsmicity on the Calaveras fauh south of Cahvens reservoir (* southern
- Calaveras) and of the segment of the Saa Andreas fault that ruptured during thelema Prieta earthquake show that the region of the fauh that slipped dining the main r. hock was aseismic both before and af er the mahi shock (llartzeD and IIcaton,1986; Opperdichner er al.,1990; Oppenhchner,1990). Couversely, Dakun et af. (19 % ave h shown that the cmnulative amount of slip produced by microscismicity b much less than the amoimt of slip expected from long term slip rate estimates; consequently, they concluded that regions of a fauh rc>ne that ext ibit micsoseismicity slip primarily through aselsmic creep processes. Thus, 3
continuous monkoring of seismicity over extended time periods can hange selsmic ' gaps" that may be sites of potential main sl ock oxurrence. These gaps provide an estimate of the dimensions of the mptun zone, and hence, the magnitude of future carthquakes. ,
If the above modelis appropriate, creeping r.ections oi .; alt should have microcarthquakes that locate on the plane of the fault. Likewir.e, dieir focal meclanisms should alt.o be consistent with the sense of slip anticipated for the main shock. Conversely, locked faults cotdd exidbh background schmichy, but any microcuthquakes should primarDy occur w!jacent to,t ;ot on, the fauk anticipated to nipture during a main shock. Because of the prevailing state of fault nonnal compressive stress in central Californla, the focal mechanisms of off fauh seismicity is fmiuently different imm the inechanism of the anticipated main shock. Thus, to discriminate
' locket' fauh zones from creeping fauh zones, it is necessary to examine the locations and focal mechanisms of the background scismicity.
11IE NOR111ERN CALAVERAS SEIShuC OAP Seismicity has been recorded in the east San Fnmcisco Day legion by the U,S. Geological Survey (USGS) since 1969 (Fig. 2). We esthnate the epicentral location securacy to be approximately 1.5 km in this segion due to a selsude station spacing of 715 km and the use of locally specific velocity models (F. Klein, USGS, written conumm.,1991) The locations of earthquakes recorded duiing the past two decades image several seismic gaps throughout the t region that could potentially rupture in M > 6 cuthquakes. We do not believe our understanding l of the slip rates and historical rupture for most of these gaps is sufficient to enable us to provide l meaningful esthnates of the likelihood of rupture for each gap. Nonetheless, the sudden onset of l the 1986 Mt. levis (Afg 5.8),1980 Livennore (Af 5.9), t and 1989 Loma Prieta carthquake l sequences detnonstrates that it is not unusual for main shuks to occur on fauh segments which l
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.. l are not well knaged by background seisadcity, but which are known to be active either through geologic investigations or by the occurrence of seismicity on adjacent segments.
In contrast to the fauh segments descric l above, tie alip nte and Idstoric record of main shock occunence for the schmic gap on the av em Cahvens fault is perhaps adequate to attempt a preliminary estimate of the probability of rupture on this r.egment. De gap extends approximately 40 km from the Calaveras Reservoir to the vicinity of Danville (Figs. 2 and 3); the vertical width of the schmogenic zone app.ars to increase from 13 km near Cahveras Reservoir to 18 km near Danville. By comparison with the similar size of the rupture area of the Loma Prieta carthquale, the northem Calaveras seismic gap is capable of sustaining an earthquake with a magnitude as brEe as M 7, ne etxlpoints of the seismic gap also coincide with complications in the fault geometry. De southem end of the segment occurs at the intenection of the southem Calaveras and the Mission fauh.s; the latter is poorly expressed in the surface geology ahbough a trend of seismicity is observedm' the vicinity of the mapped fauh (Wong and llemphill.llaley, 1992; Andrews sJxt Oppenheimer,1992). De northnu end of the segment mincides with the northern tennination of the Calaveras fauh (Simpson et al.,1992), as manifest in the surface geology, where the dispbeement of the Calaveras fault is probably transfened to the Frankliu and'or Conwid fauhs. We believe that rnost of the Calaveras displacement is taken up primarily by the Concord fauh because the step-over region that cormects the two fauhs is schmically active (Oppenhehner and Macgregor-Scott,1992).
Slip Rate No creep has been detected on the surface fauh tace of the northan Cahveras fauh (Prescott and Lisowski,1983; Galehouse,1991), but the slip rate recorded on short (<51m) baselines is approximately 2.5 3.5 nmdyr (Moder,1977; Prescott et al.,1981; Durford and Sharp,1982; Prescott and Lisowrli,1983).1mmediately south of the segment, the slip rate nueasmed across the Cahveras Reservoir geodetic uctwork (7 km aperture) is approximately 6 mmlyr (Prescott et al.,1981). North of the rap, creep obscavations across the Concord fault indicate slip iates of 2.7-4.0 nun /yr (llarsh and Burford,1982; Gakhouse,1991). Because of the proximity of the llayward s xl Calaveras faults, k is dimcuh to model how stain accumulation is partitioned as a function of depth, but it is liety that these short base line observations reflect shallow slip.
Ilowever, strain s,ccunmh!)on at seismogenio depths on locked sections of the northen Calaveras fauh is not prechiedby these observations, and the shallow slip rates of 2.54 umvyr can be considered a minimum estimate of the slip rate at &pth.
Another estimte of the slip rate on the northern Calaveras fault can be obtained ir:directly by consi& ring the regional slip bedget. 'Ihc 17 nun /yr slip nte acioss tha southem Calaveras fault (Savage et al.,1979) is partitioned into 9 mnVyr across the llayward fauk (Lienkaemper et al.,
1991), perhaps a few mm/yr on the Mt.12wis-Greenville fauh system, leaving about 6 ImWyr on the northem Calaveras fauh If we further assume that approximately 2 tom'yr of slip occun across the San Gregorio fauh and 19 mm/yr across the San Andreas fault (WGCEp,1990), then the wtal strp rate across the central San Francisco Bay area is approximately 40 mudyr. His total agrees well with the DeMets et al. (1990) NUVElel model for the San Andreas system and is consisteut with the Lisowski et al. (1991) observation of 38 mm/yr across the t .ab Day. De Lisowski et oI. obseivation is a lower bound value since their tri.lateration arny did not extend far enough to the east or to the west to be assured of detecting all the disphcemem Moreover, Kelson et al. (1992) obtain a geologic slip rate estimate of 813 mm/yr at the Leyden Creek trench 3
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' i site on the northern Calaveras fauh. Considering all the avaihble data, we esthnale that the slip rate ice the northem Cahynas fauh is 612 mmlyr.
Microsehmicity ne focal mechankms of the background schmicity indicate that the canLquales are occuning on a variety of fauh types and orientations (Fig. 4), conshtent with the hypothesis that the northern Calaveras fauh is locked. At the southern end of the segment near the Sunol Valley, Quee fault types predom* mate reverse faults and vertical, right-lateral, strike slip fauks, both of which strike porallel to the Calaveras fauh, and vertical, right lateral, strike-slip faults trending north south. He few reliable focal me hankms that could be detennbed for the central ponton of the gap hulicate that seismicity b occurring predominantly on vertical, right.lateml, strike-slip faults parallel to the Cahveras fault. Ilowever, these earthquakes are distributed within a region 5 km from the trace of the Cahveras fauk, far wider than could be attributed to earthquake location enor. His diffuse nicentral dhtribution suggests that these carthquakes are occuning on small, subsidiary fauhs adjacent to the dalaveras. At the northem end of the segment near Pleasanton, reverse and strike slip faulting is occuning in a fashion shnitar to the fauhing near Sunol, and the epicentral distributions are too diffuse to be able to correhte with the focalinuhanisms, in nunmary, the mechanisms indicate that some of the background schmicity may be occuning on the Calaveras fauh, assmning that the fauh is vertical, but the hck of definitive epicentral tremis and the variety of other methanisms suggests to us that ruost of these earthquakes occur off the Calaveras fault and that the Calaveras is locked.
In support of this hypothesh, the recent geologic investigations by Kelson et al. (1992) at Leyden Creek indicate that several slip episodes occurred within the past 1800 years. In general, surface rupture is not observed for earthquales smaller than M 6 in Califomia. Rus, the presence of a mapped fault with evidence of discrete slip events at the s.urface, historical seismichy awxiated with the fault (discussed below), the bok of observable creep, a slip rate of 6 mmlyr, and the lack of microseismicity are all consistent whh the hypothesis that the northem Cahveras fault is locked and acctunuhting chstic strain energy at depth that coukt be released during moderate to-brge canthquakes.
RUPTURE PROBAUlllTY To assess the likelihood for a main shock on thh segment, it is necessary to know the date of the hst main shock. Seismographs were installed in the San Francisco Bay itclon beginning in the early 1900's, and infonnation on earthquake locations and magnitudes before that time a.e based on published accounts of the severity and location of damage. nese isoseismallocations are prt>bably accurate to 25 km at best (Toppozada, et al.,1981), except where smface nipture is cherved, and the magnitude imcutaintles are about 0.5 units. In 1861 an carthquale occurred at the nodhem end of the noithem Calaveras fault (Fig.1), producing a fisswe 13 Lan in length.
Toppozula es al. (1981))tas assigned the earthquake a M 5.6, but if the fissure represents surface rupture rather than shaking effects or post-seismic creep, the 13 km length indicates that the earthquAe probably exceeded M 6. Earthquakes aho occurred in the vicinity of this gap in 1858 (M 6.1) and 1864 (M 5.7), but it is not known whether they ruptured the Calaveras fault, the nearby llayw.u d fault, or a blind fault that is not currently generating microseismicity. De historical record indicates that the northem Calaveras fauh has probably not ruptured in a # 7 earthquake since 1830, and certainly not after 1849, 4
We have argued above that the selsmological evidence Mnnits the interpretation that the noithem Calaveras fauh consists of one locked 40 km long segment, capable of failure in a single # 7 earthquake, llowever, there is no independent geologic evidence for such an interpretation. The fauh geomorphology (Shopson et al.,1992) suggests, in+tead, that the northern Calavens faih in more frequent M 64 events, two or more of which would be seguired to fill the 40 km gap. This nrpttue scenario is equally plausible in consideration of occurrence of the three earthquakes between 1858 and 1864. We have no basis on which to distinguish between Gese two hypotheses, and so consider them both in the following discussion of nrpttue probabilities.
Because the slip history of the fauh is not yet known, we must male sevent asstunptions in order to calcuhte the probabilities,in particular, we require an esthnate of the expected recunence inteava) and the time of last rupmre. For the fbst hypothesis of a single M 7 on the northern Cahvcras fanh, we know only that the time of last nipture precedes at hiest 1649.
Moreover, shar we do not yet have any geologic evidence to provide a recurrence interval, we must esthnate the repeat time frorn the slip rate and the annount of slip expected to occur during a M 7 earthquake,llere too, we have little data on the amount of slii pu event. We can assume that the rupture length, L, of 40 kra would generate slip, D, of appulmately 1.1 m, using the relation: D = 2.8 X 104 L of the WGCEP (1988). Ahematively, we can anticipate that the slip would be nearly 2 m, consistent with estimates of the total displacement from geodetic data (Lisowski et al.,1990) in the Loma Pileta casthquake. Combinet with the slip rate estimate of 6i 2 3rmVyr, this resuhs in an expected recurence intaval of either 333 or 187 years, depending on which slip value is assumed.1hese uncertainties enter into probability calculations through the coefGcient of variation (WGCEP,1990), and we would assign a value of 0.4 to the coefficient as 4 the WOCEP did for the two llayward segments. IIowever, without a reliable date oflast mpture, the probability cakatlations would probably not be reliable. Qualitatively, we would esthnate the' probability itat a M 7 on tie neithern Calaveras will occur in the next 30 years to be less than th estimates of 0.224.28 on the four other faults in the San Frandsco Bay region (WGCEP,1990).
Jf the northem Calaveras fails in more frequent M 5 e /ents, we can cahtthte the probabilities based on the Lindh (1988) statistical model that assumes that earthquake recurrence can be described by a Gaussian probability distribution with a mean reatrrence time and coefficient of variation of the recturence time. Unlike the # 7 scenario, we know when the last M 6 earthquake occurred on the northern Cahveras fank.11owever, we have no direct informrition on recunence.
On the southern Calaveras fanit, the M,6.2 Morgan 11111 segnxst and M2 5.9 Coyote Lake segments have recurrence times of 73 and 82 years, respectively (Oppenheimer et al.,1990), and '
the WGCEP (1990) esthnated its slip rate no be 14.4 mudyr. Adjusting this ~80 year repeat tune !
for M 6 events for the 6 mudyr slip rate of the northern Cahveras segment, we edimate the recurrence interval of the northern segmait to be about 192 years. If we further assmne that the '
entire noithem Calaveras failed between 1858 and 1864, we can use 1860 as a reasonable j
estimate of the thne of the most recent event for both sub segments. The resuhing esthnate of the time of the next event is then 2052158, asstuning a coefficient of varbtion of 0.3 to reflect the better esthnate of the thne of the last event under this hypothesis. We esthnate a probability of 0.18 that a M 6 event will occur on each segment within the next thirty years, and a probability of 0.33 that an M 6 will occur on one or more segments in that same interval of thne.
I A discussion of rupture likelihood, however, should be cor.sidered in the context of how failure of neighbonng faults may influence the state of stress on the Calaveras fault. Since 1836 four 5
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M57 earthquakes have occured in the Day area the 1836 and 1868 events on the northern and southern llayward fault, the 1906 San Andreas earthquake, and the 19891ana Prieta earthquake. De amount of stres change is a fimction of the amotnit and distribution of slip postulated for these historic events and the distance from the Calaveras fauk. Consequendy, the 1836 and 1868 earthquakes had a significant effect on the state of stress on the Calaveras fauh due to their protiraity, but the 1906 earthquake also had a significant effect due to its greater amount of slip. The first order effect of these canhquakes was to knpose left lateral shear strtss on the northern Calaveras fault segment, budbiting the ocanrcuce of earthquake activity on the fauk foe perhaps as long as a century, depending on the slip-rate assumed for the fault (R.
Simpson, USGS, written commtm.,1992). He Inod: ling also indicates that the northem Calaveras fault has probably recovered from the stress reduction of these earthquakes, coraistent
- with the oceturence of moderate seismicity sunounding the northern Calaveras fault since 1955 (solid stan, Fig.1). Ellsworth et al. (1991) reached shnilar conclusions in their analysis of the schmicity following the 1906 earthquake. Recendy, Oppenbehner et al. (1990) discumd the northward progression of moderate events on the Calaveras fault since 1974 (solid star labeled 1988 in Figure 1 is northertunost of these events) and speculated that this progression might continue with a #7 event on the 11ayward fauh. It is unknown whether tids northward progression willlead to a failure of the Ilayward fault or pe haps the northem Calaveras segment, but the halo of large caithquakes in the East Bay region suggests that the level of stress Giroughout the region may again be high enough to produce significant earthquakes on either of these faults.
CONCLUSIONS From our analysis and interpretation of the seismic, geodetic, and geologie data along the northem Calaveras fault, we believe a seismic gap exists between the Calaveras Reseivoir and the city of Danville. Several studies indicate that this section of the fault has a slip rate of approxhnately 6 nun /yr. By comparison with the trdcroseismic characteristics of the southem Calaveras fauh and the lana Prieta segment of the San Andreas fault, we propose that the northern Calaveras fatdt is loded and accumtdating clastic strain energy that could ruuh in the occurrence of an earthquake as large as M ?~ % lindted 162 year history of earthquakes along this seismic gap indicates an altemative ruptme scenuio in which this gap could rupture in a series of smaller (M 6+) earthquakes. Until geologic investigations reveal the rupture history of the northem Calaveras fault, h is not possihte to detennine reliable pmbabilities for the occurrence of a y 7 wthquake on this fault segment.11owever, we calculate the probability of one or more # h . thquaes occurring in the next 30 years to be 0.33. De asswnptions required to calculate these probr.bilities result in considerable uncertainty, but we believe the evidence is sufficient to consider the noithem Calaveras fault as a sigrdficant seismic hazard to the entire San Trancisco Bay region.
ACKNOWLEDGMENTS We thank Fred Klein, Billlettis, David Schwartz, and Ivan Wong for their careful reviews of the manuscript, and Bob Simpson for providing us with the results of his dislocation modeling. We also acknowledge the many people who have recorded and processed the earthquake data reconled by the Northem Califomia Seismic Network of the U.S. Geological Survey since 1969.
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Andrews, D.J., and D.H. Oppenheimer,1992, ne Mission fauh: A magnitude 64 thrust or reverse. slip earthquake nu be expected, this vokune.
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Simpson,0.D., W.R Lettis, and K.I. Kelson,1992, Segmentation model for the northem Calaveras fault, Calaveras Reservoir to Walnut Croek, this volume.
Topporada, T.R., C.R. Real, and D.L. Parke,1981, Preparatioa ofisoseismal maps and stumnaries of reported effects for pre-1900 Califomia carthqtnkes, Califomia DMslon of Mines and Geology Open File Report 81 11 SAC,182 p.
Wong, LO., and M.A. liemphill lialey,1992, Seismicity auj faulting near the If ayward and l MissJon faults, this vohune.
Working Group on California Earthquake Probabilities (Agnew, D.C., C.R. Allen, L.S. Cluff, J.H. Dieterich, W.L. Ellsworth, R.L. Keeney, A.G. Lindh, S.P. Nishenko, D.P. Schwartz, K.P.
Sieh W. 'niarcher, R.L. Wesson),1988, Probabilities of large earthquakes occurring in l
t California on the San Andreas fault, U.S. Gr. logical Survey Open-File Report 88 398,62 p.
8 -
1 L'
- _ = - - - - _- - _ _ . . - . .- . - -
)
WorUng Group on Californis Earthquake Probabilhies (J.!!. Dieterich, C.R. Allen, LS. Cluff, C.A. Cornell, W.L Ellsworth, LR. Johnson, A.O. Lindh, S.P. Nishenko, C.ll. Scholz, D.P.
Schwartz, W. Thatcher, P.L Williarns),1990, Probabilities of large earthquakes in the San Francisco Bay Region, Califonda, U.S. Geological Survey Circular 1053,51 p.
FIGURE CAP 110NS Figure 1. Location of Quaternary fauhs in the eastnn San Francisco Day vegion (Jennings rt al.,
1975) and earthquakes (sbrs) above M 5.0 since 1830. Instnunentally )ocated earthquakea are shown as solid starn Darthquake locations and magnitudes (in parentheses) are from Topporada et al. (1981), Bolt and Miller (1975), and the USGS. Rectangular box corresponds to region shown in Figure 4. CCF-Concord fault, CVF-Calaveras fauh, GVF-Green Valley fauh.
IIWF liayward fauh, FNF Franklin fauh, CR-Calaveras Reservoir, DV Danville,
. LV-Ilvennore, M1-Mount Lewis PI Pleasanton, SD Suisun Day, SFB San Franelsco Day.
Figure 2. Seismicity erecmbl by the USGS from January,1969 through January,1992. The earthquale locations (Klein,1989) have an RMS travehime error icss than 0.3 s, at least 6 -
P wave anival tirnes, and horizon'al and vertical uneestainties of 2.5 and 5.0 km, respectively.
- Ibe dashed polygon from A to N depicts the selection region for the cross section shown in Figure 3. Itachured region indicates extent of hypothesired gap. See Figure 1 for fauh names and locations of cities cited in text.
Figure 3. Cross section of seismicity whhin polygon of Figure 2 along the Green Valley - I-Conoord - Calaveras fauh system. There is no vertical exaggeration. Seismicity gap extends from Calaveras Reservoir to approximately Danville (~40 km). Comparison with map view of l seismicity in Figure 3 shows that most of the seismicity whhin the Esp occurs wfjacent to the Calaverts fault. In parilcular, the seismicity cluster near Danville ocaus within a step +ver regkm to the Concord fauh, the chister at 82 km occus east of Calavesas fauk, and the selsmicity from 86 90 km ocaus in the vicinity of the Mission fauh. IIachured area indicates area of fauh expected to rupture in a moderate-to large carthquakes.
Figure 4. Lower hemispkre, equal area focal mechanisms calculated from P-first motions (Reasenbesg and Oppenheimer,1985) for earthquakes within polygon shown in Figure 2 and rectangular regkmhown in Figure 1. All mechanisms have at least 25 Fast-motion readings.
Compressional quadmnts are shaded solid. Earthquale. locations are depicted by .unall dots.
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