ML19310A253
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| Site: | Vallecitos File:GEH Hitachi icon.png |
| Issue date: | 05/23/1980 |
| From: | Ellsworth W, Marks S INTERIOR, DEPT. OF, GEOLOGICAL SURVEY |
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| 80-515, NUDOCS 8006060473 | |
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APPENDIX C 9
UNITED STATES DEPARTMENT OF T*4E INTERIOR GEOLOGICAL SURVEY Seis=icity of the Livermore Valley, California Region 1969-1979 by W. L. Ellsworth S. M. Marks Open file Report 80-515 l
l This report is preliminary and has not been edited or reviewed for conformity with Geological Survey standards and nomenclature.
8006060443y
2 Su==:ry Seis 10ity of tne Liver = ore Valley, California, region for the 11-year peaiod from 19t9 througn 1979 reveals a cocplex pattern of seistic strain energy release. Earthquake epicenters an: focal mechanis:
solutions confir: the presence of numerous active faults in the region, all of which move in response to taa regional stress field. A systematic evaluation of the relationship between the seismicity for this period and candidate faults leads to the classification of the Calaveras-Suncl, Concord, Greenville and Hayward faults as active faults. The Las Positas, Pleasanton and Verona faults are identified as probably active faults. The Livermore fault and the segment of the Williams fault located to the south of the Las Positas and Verona faults are classified as possibly active. However, a northern prolongation of the Williams fault said to exist to the north of the Las Positas and Verona faults is unsupported by seis=0 logical evidence. Additionally, the connection between the Livermore fault and northwest trending fault said to exist to the south of the Las Positas fault is also unsupported by seismological i
evidence.
Earthquake focal mechanism solutions for events near Vallecitos j
Valley de enstrate that this region is a none of active thrust faulting.
Available seis:: logical data indicates that some of these thrust faulting i
1 events are in probable association with the Verona fault. Setter cierocartnquake instrumental coverage and i= proved knowledge of crustal structure in the Valle:it s-Livermore area should allow this relationship to be de:Onstrated.
3 Introduction The Liver = ore Valley of central California, apprcxi=ately 60 k: east of San Francisco, is an east-west trending valley located between the northern end of the exposed Franciscan core of the Diablo Range to the south and the Mount Diablo diapir to the north. The region lies within the eastern third of the 100 km broad zone of active faultin6 and seiscicity which comprises the San Andreas fault system in the San Francisco Bay region (Figures 1 and 2).
Earlier studies (Lee et al.,
1971; Bolt and Miller,1975) of the seismicity of this regien indicate a East of highly cc plex and spatially diffuse pattern of strain release.
the Hayward fault this activity is poorly correlated with known faults.
The absence of a clear correlation is, in part, a reflection of the volu=etrically distributed nature of strain-release within the region.
It is also related to the inadequacy of seismographic station coverage for cost of the historical perod, and to an incomplete knowledge of crustal structure.
This report presents the results of a co:prehensive and systematic re-analysis of the seismicity of the Livermore Valley region for the years 1969-1979 The basic earthquake phase data upon which this report is based comes fro: earthquake data collected and analyzed from records of the U.S.G.S. central California Microearthquake network. Within the Livermore region, this network of high-gain, short-period, vertical seismometers has an inter-station separation of 10 to 20 km (Figure 1, Table 1).
Network instrumentation and analysis procedures are described in earthquake catalogs for the years 1969-1976 (Lee et al. 1972 a, b, and c; Wesson et al.,1972 a and b,1973 a,1974 a and b; Bufe et al.,
4 1975; Lester et al., 1976 a and b; Lester and Meagher, 1978; McHugh and Lester, 1975 a and b).
Data fec: the period 1977-1979 are derived from i
earthquake catalogs that are in preparatien.
The re-analysis of seiscicity reported herein restricts its attention to the period for which microearthquake network coverage is adequate.
Use of an improved crustal model significantly enhances the precision of the earthquake locations and improves the resolution of focal mechanism solutions. Over 3900 earthquakes from the period 1969-1979 have been relocated, and their hypocenter soutions have been reviewed for accuracy, i
i a
4 1
)
5 Table 1.
Seiscograph station coordinates, travel time delays in seconds and operation dates for stations used to locate errthquakes in the !.ivtr: ore study area.
S i r.
LAT L O f.
ELEV LELAY CPE E A T ICr4 C4;Es t.
- 7. 55.57 121. 45.e2 74 0.61 731026 - FRESENT crESENT ce!
- 27. 51.53 1224 25.77 223
-G.c3 ccCFDE S F E : E *.
- 17. 27..l7 121 L. 47.95 265
-0.11 e.71
~C C t. ;
2 7..
- 2. 0c 121. 31.96 t2t
-0.29 671:10 - cEE;ENT CEF
- 27. 4t.77 122.
I.72 61C O. J1 c C C F. 2 2 - F F E 5 E r. T CE.
27n 53.45 12 2.c e.40 221
-C.02 71 C 4 2 E - P R E ; r *; T C C f.
- 37. 47.41 1 21.1 56.59 219 0.37 760205 - PoESCNT F R E S E r. T Ctc
! 7 :. 15.ec 1 21.: 40.35 366 0.13 671013 760205 CCf 3 7 *. 4 7. ?]
121. 5 7. C i.
155 0.45 700729 PWESENT CCv 27? !!.12 122s 5.45 67
-0.37 75CSC:1 C?n 2 77. 33.54 122.;
5.e2 38
-0.39 66*215 - 75ne::
Ctc 17 :. 43.50 121. 50.12 19E C.43 70C729 - PRE:ENT C :. 5 I 7 :. 5 7.h 122. 15.17 109
-0.27 731115 - P o E S E *. T Ctu 3EN 1.78 1224 0.05 169 0.29 710 4 2 S - P R E S E *. T PRESENT CLC 3 7:. 44.23 122A 3.t3 312
-0.02 721121 c v. C 27* f.o.ss 122s 10.55 90
-0.28 710720 - P R E S E *. T c u E S E r. T Cvh 3 77. 21.57 121s 45.3E 51 ti 0.02 69C3C4 FEEEE'T CvJ 17: 31.25 121. 52.23 405 0.34 72C7 1 cv0 2 7:.
- t..d. 6 3 121. 4E.15 792 0.30 69C417 - r R E S E.T CPF I ? *. 35.63 1216 3t.22 500
-0.02 6 : C 1.17 - F C E 5 E '. T C;L 2 77. 37.?
1214 57.!7 463 0.30 e9Ct27 r S ES E.T CE*
I 7 t.
46.05 121s 56.25 171 0.24 76CG02 - PRESENT CAP 37:4 54.75 1 21 54.13 331
-0.15 70C913 - PRESENT.
CSC 3 7 :. 17.11 1 21. 45.35 12C C.43 67C610 - PcESENT C5n 3 7 t; 3C.Ed 122.
2.57 17C
-C.10
- S R E S E r4 T /
CTL 3 7:. 30.44 121.: ?S.c!
455 0.23 77C623 PR:5ENT 760911 C r.,
37. 14.52 1226 7.E2 607
-0.07 7311 C*
Mtc 3 7 t.
52.90 121 ri 54.E5 117
-0.15 67C 5 2 7 - 7 4 0601
%0 3 7 tJ 52.04 1 21. 55.09 719
-0.15 7C C 5 27 - 700720 vHC 3 7 t.
20.50 1214 36.50 1232
-0.25 1387 FCE5ENT PRESENT JAL 3 7t4 9.50 121. 50.82 244
-0.22 651016 JSC 2 7 f:
9.c2 1 22's 1.57 660
-0.03 690521 PRESENT APESENT Jv(
37 f; 3L.22 122. 2E.43 201
-0.65 710',31 PRESENT JCF
! ? ti 47.70 122. 23.4 137
-0.46 760406 PRESENT/
J54 37N ~ 4. "J 5 122. 25.C3 207
-0.49 JSC 37:4 '7.07 122s 7.42 357
-0.22 661223 - F R E S E'. T c:EEENT JSF
! ? i; 2 4.. ! 1 122. 10.55 143
-0.21 661213 J5C
',7 t; 16. %
122.
3.03 lic C.25 75:*22 c:ESENT J5J 2 7 f.
2C.C3 122s 5. 4 ',
122 0.13 ct 223 - P:.ESE*.T J55 3 7 t.
10.1/
111. 55.54 04s
-0.13 750122 - PPESENT J !. T
~ 7.4 12.41 121s 47.84 149
-0.15 751002 - P P E S C f. T PRESENT J.E
" 71; 25.rd 122. 10.33 250
-0.24 et1221 c R E S E r. T t: F1 3 h t:
9.23 121s 48.02 e5 G.7b 710429 Station
..s intermittent di. ring tt.is time period.
=
/
- t. c installation sis t e availaele.
6 Analysis Procedure Routine earthquake locations reported in the U.S.G.S. catalogs for central California for the period 1969-1979 were deter ined using a simple crustal model (Wesson, et al.,
1972a). Owing to the diverse asse blage of geologic provinces within the area spanned by the network, this model necessarily represents a si=plistic first attempt at characterizing the average velocity structure of the region. Detailed studies of crustal structure de=enstrate that wnile this =odel produces acceptable earthquake locations for many purposes, it fails to provide locations as accurate and precise as can be obtained from the same arrival time data using localized structural =odels (Mayer-Rosa,1973).
Consequently, we have developed a velocity model appropriate for sources in the Liver = ore region and have used that =cdel to syste=atically relocate all events detected by the U.S.G.S. network during 1969-1979 This model is specified by a sequence of homogeneous, plane, horizontal layers overlying a half-space. The velocity within each layer is ddrived from explosion and earthquake travel time data using a least-squares adjustment procedure developed by Crosson (1976) and Aki and Lee (1976). A tctal of 1494 P-wave observations for 9 explosions and 57 earthquakes were inverted to deter =ine the veocity model given in Table 2.
This model is very similar to previous models for the Diablo Range (Byerly, 1939; Stewart, 1968) in that its most i=portant feature is a shallow crustal refractor with a P-wave velocity between 5.6 and 5.7 k=/s.
The principal refinement of the codel in Table 2 is the i= proved resolution of the average vertical velocity gradient in the seis:cgenic r
r i
t
7 par: Of the crust (2-12 k ).
The velocity of 6.8 km/s adopted for the half-space below 13 k= depth, although not well-constrained by the data, is consistent with both of the earlier studies.
Structural details near the surface cannot be adequately resolved by the available data. Because of the relative importance of near-surface structural features in the ea: thquake location proble=, a set of station travel time adjust =ents (Table 1) was developed using the same procedure e: ployed to deter =ine the velocity structure.
Comparisons between earthquake locations routinely reported by U.S.G.S. and those obtained with the model described above show that the new model greatly i= proves the relative precision of the locations. Use of this =cdel typically reduces the residual travel time variance by 5:
to 75%. Systematic differences in epicentral locations generally show the relocations to define narrower cones of seismicity, especially along the Calaveras and Hayward faults. However, the sytematic displacement of j
the epicenters from these faults (Figure 3) strongly suggest that the locations may contain a syste=atic biase of 1 k= or more.
A small calibration explosion located near San Ramon (Figure 1) provides an independent data set for evaluating the absolute precision of the models. Relocation of this event using the refined crustal structure gives a surface focus epicenter located 770 m north-northwest of the true shotpoint which is within the 955 confidence ellipse of the epicenter.
Relocation of the shot using the standard crustal structure gives a location 1.5 km NW of the shotpoint, which lies outside of the 95%
i OOn[1ddGOG interval.
j
S Tatie 2.
Crustal velocity structure of the Livensore region deter ined fr : j0 int inversion of explcsion and earthquake travel ti=e data.
Velocity (;:n/s)
Derth to ter layer f%nl 3.4 0.0 4.7 1.0 5.2 30 5.6 5.0 5.7 7.0 5.8 9.0 6.0 11.0 6.5 13.0
9 Eartheuske Locations in the Liver :re Eerien Eclocated epicenters for earthquakes in the greater Livermore region confir: a cc: plex pattern of seir=ic strain energy release in the northern Diablo Range (Figure 3).
Seismic activity within the region is sharply bounded on the southwest by the nearly continuous zone of sein:icity aligned along the F.ayward and Calaveras-Sunol faults.
Its eastern boundary is diffuse in character and approxi=ately corresponds to the western edge of the San Joaquin Valley. Earthquake focal depths within the study area extend fro: near the surface t0 over 15 k: depth, the normal range of focal depths in central California (Wessen et al.,
,,b,.
as e
Between the Hayward fault and the San Joaquin Valley, earthquake epicenters for: randomly distributed clusters which do not display an obvious systematic relationship to =apped faults. The densest concentration of activity in this zone lies 10 km south of Mount Diablo near the town of Danville (Figure 2).
This focal volume has been the site of recurrent swarm activity since at least 1970 (Lee et al,1971).
To the north of this swarm area, epicenters concentrate within a 10 k=
broad zone lying between the Calaveras-Sunol and Concord faults. Along the Calaveras-Sunol fault and within the Livermore Valley a =arkedly lower level of seis=icity can be seen in this 11 year sample. South of the Livermore Valley, within the north end of the Diablo Range, earthquake epicenters cefine several tight clusters i= bedded in a diffuse background of activity.
10 The two faults in the region which are clearly defined by earthquake epicenters are the Hayward and Calaveras-Sunol faults (Figure 3).
The breadth of the epicentersi distribution measured transverse to the strike of these faults varies substantially with position along the fault.
Locally the width exceeds 3 km, or 6 timus the average relative epicentral error. We conclude that seismicity associated with these tw9 vertical strike slip faults has a resolvable breadth which locally
~
prevents unique association of the seismicity with a single fault trace.
To the east of these seismically well-defined faults, the distributed nature of the seis=icity complicates the association of specific earthquakes with known faults.
Selected focal mechanism solutions (Figure 4) illustrate the predominance of strike slip faulting within the regi:n. Fe:a1 mechanisms along the Hayward and Calaveras faults show that these faults form a continuous northwest-trending dextrel shear zone. To the east of these faults, the dextrel slip plane for most focal mechanism is retated clockwise from northwest to north. This pattern is similar to that observed in ti* Bear Valley region, southeast of Hollister, California along the San Andreas and Calaveras-Paicines faults (Ellsworth,1975).
Note also the presence of some thrust fault solutions, near Danville, Livermore, and Vallecitos Valley (Figure 4).
A summary diagram of P and T axes from available mechanism data (Figure 5) indicates that seismic strain energy release occurs predominantly on nearly vertical planes in response to a shear stress field oriented roughly parallel to the principal faults of the region.
_=. _
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a' These data are also in good agreement with the geodetically measured north-south compressional and east-west extensional strain field.
(Savage and Burferd,1973; Tnsteher,1975). Within the region northeast 4
of the Hayward and Oalaveras faults, focal mechanism solutions show a i
continuous progression fro: strike slip to thrust faulting. Normal faulting cannot be documented anywhere in the region on the basis of j
available first motion data.
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Classification cf Fault Activity Using Seis=olegical Evidence Documentation of the active or inactive state of a specific fault frc: a limited record of seis=icity data, such as is available for the Livermore region, is not a sy==etrically posed question. Wnile it is possible to demonstrate una:biquously the active state of a specific fault through the gec etric relationship of earthquake hypocenters and focal mechanis: solutions to t s surface expression of the fault, the inactive state (or non-existence) of candidate faults car. net be conclusively de=enstrated fro: these data. The absence of correlative seismicity by itself, is net evidence fer a specific fault being in an inactive state. For exa:ple, sections of the San Andreas fault which ruptured in great earthquakes in 1857 and 1906, are presently seis=ically quiet. The presence of a well documented regional stress field of known orientation requires that all faults whose movement could be cc patable with the stress field be considered as potentially active, unless specific geologic data indicate otherwise.
Quantitative evaluation of low levels of seismicity as indicators of
- otential activity of a fault is also not possible at present, given our li=ited understanding of the earthquake process. The specific exa:ple of the Greenville fault is particularly poignant in this regard.
Seismological evidence for the existence of the Greenville fault as an i=portant c0=ponent of the nectectonic fra:ework of the region is co:paratively waak on the basis of dcta shown in Figures 3 and 4.
Yet this fault produced one of the largest earthquakes to strike the San Francisco Bay Region in this century on January 24, 1980 (Figure 6; 1
Bonilla et al., 1380). Clearly, even a very low level of seismicity in
13 association with a specific fault is as important an indicator of fault activity as a dense alignment of epicentars along it.
In view of our incomplete knowledge of either the seismic record, the physical elements of the fault syster or the physics of earthquakes themselves, what then can be learned from a ec=parison of seismicity data with a suite of potentially active faults? At best, we can state that a fault is active or probably active, but we cannot quantify either the likelihood of occurence of its =aximu earthquake or the magnitude of The answer to these questions presently comes fro = the that event.
geologic record as aided by the historic record of large earthquakes, where available.
However, within these licitations, the syste:atic use cf seismicity data for classifiestier. cf f ault activity requires a consistent set Of rules. Table 3 lists the seismological criteria adopted here to define four broad classifications of fault activity. The specific names applied to the classifications are understood to derive meaning from these criteria alone. In applying these rules to the problem at hand it is also understood that the ter hypocenter encompasses not only the single best estimate of the earthquakes focus derived from the location procedure, but also the confidence region for that location.
14
~acle 3 Fau" t Activity Classification Criteria Fre: Seis=icity Data 1.
Seis=1callv Active Fault: Presence of earthquake hypocenters on the geologically defined or inferred fault plane that have well-constrained focal mechanir.3 solutions in agree =ent with ove:ent on the fault plane. The correlation between earthquake hypocenters and the fault plane solutions =ust exclude, at a high confidence level, the association of those events with other candidate faults.
2.
Probably Seistically Active Fault:
Earthquake hypocenters located on the geclogically inferred fault plane, the probable ec patability of focal =echanis: solutions with movement on the fault, and the existence of a regional stress field co:patible with the geclogic recced fault =cve=ent.
Tne correlation between earthquake hypocenters and the fault plane must be the
=cet probable association. The possible association of the events witn other faults =a-be permitted by the data.
3 Pessibly seis ically Active Fault: Earthquake hypocenters in pessible association with the fault plane and the existence of a regional stress field compatible with the geologic record fault movement. Available first-motion data must agree with movement on the fault. The association between earthquake hypocenters and the fault plane permits the interpretation that they are relatad but it lacks the precision to demonstrate the correlation with reasonable confidence.
4 Fault Unsuceerted by Seiscelegical Evidence: Absence of any seis:clogical evidence in direct support of the existence of a proposed fault. Regional stress field may or may not agree with movement on the fault.
15 Active Faults in the Livermore Region The fault classification criteria given in Table 3 are applied on a fault by fault basis to candidate active faults of the Livermore region.
The list of faults considered includes the principal recently active faults identified by Herd (Herd, 1977; Herd and Brabb, unpubl. admin.
report, 1930, Figure 40), the Livermore fault (California Report Water F.ecources,1963) and the Willia =s faults (Hall,1958). Except where noted, seismicity data used in the classification of these faults is restricted to the 1969-1979 sa=ple (Figures 3 and 4).
Calaveras-Sunel Fault: Seismically Active. This fault is clearly defined by a continuous, narrow zone of seismicity from the southern border of the study area to Calaveras Reservoir. Although the seismicity is syste=atically offset 0.5 km to the northeast of the mapped surface trace of the fault, the offset is smaller than the absolute uncertainty of the locations. We believe that it is an artifact of the location j
procedure caused by insufficiently detailed knowledge of crustal structure (Ellsworth and Moths, 1979). Well constrained focal mechanisms uniformly agree with right lateral movement on the fault. North of Calaveras reservoir, the Calaveras-Sunol fault is poorly defined by seismicity. There are a number of events in possible association with the fault plans, including several with strike slip focal mechanisms i
suitably oriented for right lateral slip on the fault. However, this correlation is not unique and may be fortuitous. The segment of this fault opposite Danville ruptured in the M 6 earthquake of 1861 (Toppo ada, et al._1979).
16 Concord Fault: Seisticallv Active. Seis:icity associated with this fault forms tne northeastern ecun:ary of a diffuse zone of epicenters lying between the Ocncord and Calaveras-Sun 01 faults.
Focal =echanis:
solutions ind :ste rignt-la eral ::vecen en the faul.
Gr.eenville Fault: Seiscieslly Active. The location of this fault on the periphery cf the network causes a degradation in the quality of epicentral deter:in2tiant. This cc ;11 cates the correlati n Of earthqua, ;s with the =apped fault trace.
It is c' ear, h: wever, that this area is characterized by a lower level cf activity than the Hayward, Calaveras-Sunc1 or Ocncord faults. Severa; :treng earthquakes locate near tne fault (in:luding the 21 ane 19-~ " '.
event at 37 33'N lat..
- 2*'
4 0 "a* long.) and have focal techanis: s:;utions 00 patatle with rig:',
slip on the fault. A careful re-examination of earthquake locations in the vicinity of the 24 January 1980 M 5.8 earthquake, shows that some earthquakes frc: the period 1969-1979 occurred within the aftershock volu e of the 1960 earthquake. This evert ruptured strands of the fault mapped by Herd (1977) and triggered seismic activity along a 30-km portion of the fault (Figure 6).
Had the January 1980 earthquake sequence not occurred, or if its relationship to the Greenville fault been unclear, the criteria of Table 3 would have led to classification of this fault as being probably active.
17 Hayward Fault: Seismically Active. This fault produced large earthquakes in 1836 and 1868 (Lawson and others,1905; Toppo:ada et al.,
1979). It is currently creeping at an average rate of about 1/2 cm/yr (Nasen, 1971) and is well-defined by its seismicity. Focal mechar. ism solutions clearly indicate right lateral strike slip motion. The continuous zone of seismicity connecting the Hayward fault to the Calaveras fault at Calaveras reservoir follows the trace of the Mission fault of Hill (1958).
Las P:sitas fault: Probably Seismically Active. This nortneast-southwest trending left lateral strike slip fault has a unique crientatien among active faults in the San Francisco Bay Region (Herd, 1979; Herd and Brabb, unpubl. admin. report, 1980). It is conjugate to the Creenville and Calveras-Sunal faults and thus responds to the same stress field known to be acting on them. Some earthquake epicenters in Figure 3 appear to be associated with this fault. First motion data for these events are consistent with left lateral slip on a vertical plane parallel to the strike of the Las Positas fault. However, they are not sufficiently well constrained to unambiguously demonstrate activity.
Livermore Fault: Possibly Seismically Active. Some of the many scattered epicenters within the Livermore Valley locate near the trace of the Livenmore fault, a northwest-trending fault inferred to exist near Livermore (Calif. Dept. of Water Resources, 1963). Focal mechanism solutions for these _ events are inconsistent with movement on this fault.
15 Strike slip focal mechanisms indicate movement on fault planes at an angle of about 45 to the Livermore fault. Thrust solutions corresp nd to events with foci that are too deep to be related to the fault.
However, other poorly constrained focal mechanism solutions might possibly agree with dextral slip on the fault.
South of the Las Positas fault, a single focal mechanism would suggest right lateral motion on a fault proposed by Earth Science Associates (1979) to be the southern extension cf this fault. However, this mechanism could equally reflect left-lateral movement en a fault that parallels the Las Positas fault at this point as =apped by Herd (1977). There is no seismological evidence which directly suggests a connection between the proposed pieces of the Livermore fault.
Furthermore, it is geometrically impossible for both the Las Positas and Livermore faults to exist as continuous features through their point of intersection. Geologic evidence strongly favor the interpretation of the Las Positas fault as the through-going fault (Herd, 1977).
Pleasanton Fault: Probably Seismically Active. Seismological evidence supports the identification of this fault as an active fault. However, few epicenters actually locate near and their association with the fault cannot be unambiguously demonstrated. The distribution of earthquake hypocenters and crientation of their focal mechanism to the north of the Pleasanton fault strongly suggest that this fault continues to the north along the east side of San Ramen Valley.
19 Verena Fault: Probable Saistically Active. Eartnquake locations and fccal cechanis=s su;;crt the interpretation of this feature as a northeast dipping thrust fault. Focal mechanis: sclut!on: for events in probatle association with the inferred downward con *,inuation of the fault plane (Herd and Brabb, unpubl. ad:in. report, 1980) all agree with north over south thrusting On the fault plane. Belatively weak fecal depth c0ntrol caused by inadequate seis : graphic coverage prevents the unambiguous classification of this fault as active by the criteria of Table 3 M ::
of the events with epicenters within 1 k: cf the surface trace of the Ver:na fault are pr:bably n:t associated with it, as they lie several k;10:sters bel:w tne inferred fault plane. We interpret these even;; 00 result frc: move:ent on as yet unidentified faults. However, some events located within 600 m of the fault have very shallow foci, and may be related to the Verona fault. This question is considered in greater detail in a following section of this report.
Northern Segment of the Willia s Fault: Fault Unsupported by Seiscological Evidence; Southern Segment of the Williams Fault: Persibly Seis=ically Active. Discussion of the Williams fault maybe logically divided between the segment of the fault said to exist north of the Las Positas and Verona faults (Earth Science Associates, 1979) and the segment south of them. Among the earthquakes in possible association 1
with the northern seg=ent of the Williams fault (Figure 3), all events with resolvable focal mechanism solutions can be demonstrated to be i
1 i
unrelated to it by virture of incompatable focal mechanis:s (Figure 8).
1
' 20 Two of these events have thrust solutions which place them in probable association with the Verona fault. The third event has a strike slip me hanism with nodal planes oriented at approximately 45 to the Williams fault. The existence of this segment of the fault is thus supported by seismological evidence.
The southern segment of the Williams fault has numerous small earthquakes located near its trace. Nodal planes of focal mechanism solutions for events located near this segment of the Williams fault do not agree with the orientation of this fault. Other poorly constrained mechanisms may possibly agree with dextral slip on the fault. Seismicity within this region is widely scattered and does not define any single fault but requires faulting to be distributed on numerous faults.
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J el Seis=ied-- Nea n= Verena Fau":_-
The classification er frJ.t: in :nt ".ver::re regi:n presented at:ye icentified the '.~erona fault as a pec:a:ly seis:ically active fault.
Be:ause this fault is a thrust, the relationship of earthquakes to the l
fault cannot be readily inferred fro: the maps in Figures 3 and 4.
Inferences atent the associati:n of Specific events with the fault must consider the three-dimensional relati:nsnip of the earthqua':es to tne fault plane at depth. The basic data relevant to this discussion are the earthquake hypocenter solutions, their estimated uncertainties and their fc0sl techanis: solution: (Table e, Figures 7 and c';.
in general, the location of ::st epicenters, relative to their i
neighbors,' are well constrained throughout the region. The relative uncertainty in their 10:ations (listed in Table 4) are censidered to be a good esticate of the relative epicentral precisi n.
Absciute errors of 1 k= are possible since the velocity structure of this area is not known in detail.
The precision of focal depth determinati na varies considerably throughout the region shown in Figure 7 because of the absence of nearby seismographic station coverage. The epicentral distance to the nearest station varies from just under 7 k= to over 15 k:.
As the presence of a station within a focal depth of the earthquake is critical to the accurate deter =ination of focal depth, most of the depths listed in Table 4 are known with less certainty than their corresponding e;icenters.
w
22 This is reflected, in part, in larger focal depth uncertainties. Events i
shallower than about 3 k= have artificially s=211 depth error estimates.
Their true uncertainty probably exceeds 2 km.
Fccal mechanis: solutions appearing in Figure 8 represent single event mechanisms for which the type of faulting could be clearly established. In detail, the strike and dip of any given nodal plane may be uncertain by 100 or more, owing to inadequate sampling of the radiation pattern and/or incomplete knewle:ge of crustal structure. The only exception to the statement applies te events with focal depths less than 2 k=.
Uncertainties in their depths permits interpretation of their mechanisms as either reverse faulting or strike slip faulting. The strike slip mechanisms appropriate to these events appear in Figure 8.
Reverse fault planes for these events dip at 450 to the northeast.
It is apparent from the distributien of hypocenters and focal mechanism solutions for earthquakes occurring near the Verona fault (Figures 7 and 8) that the seismicity is not restricted to a single fault plane. Focal mechanis: solutions show a continuous progression from strike slip to thrust faulting, all under the influence of a north-northeast-south-southwest oriented compressive stress field. In cross-section (a-a', Figure 7; Figure :) it can be seen that some of the events with thrust-type focal me:hanis=s locate near the inferred downward continuation of the Verona fault plane. The dip of the fault adopted in Figure 9 corresponds to the dip of the Verona fault where it is last seen in borehole EH-3 (Herd and Brabb, unpubl. ad=in. report, 1980, Figure 3). These events are in probable association with this fault. Although they are too few in nu=ber, and have sufficiently uncertain focal depths to prove this point, they demonstrate that thrust
23 faulting, at depth, extends into the center of the Livermore Valley.
Therefore, it is conceivable that the Verona fault extends to a depth of 6 k: cr more.
The occurrence of these and other thrust events (Figure S) supports the tectonic framework for the Livermore Valley proposed by Herd and Brabb (unpubl. admin. report, 1980, Figure 41). Left-slip movement on the Las Positas fault compresses the region immediately north of its projected intersection with the Calaveras-Sunol fault. Right-slip covement on the Calaveras-Sunol fault north of this imaginary point of intersection similarly compresses this zone. Reverse movement on faults within the zone, including the Verona, serves to relieve these stresses.
Not all of the earthquakes within this zone have reverse faulting 1
mechanisms. Several of the deeper events have strike slip fault plane solutions. The shear stress field of the greater region, inferred from regional focal mechanis:s (Figure 5), cast therefore be transmitted l
i through this zone as well.
The shallow event located within 1 k: of the surface trace of the This Verona fault (Figure 7) may also posess a strike slip mechanism.
event, which occurred on September 10, 1970, and the event of October 6, 1970 have unusually shallow focal depths, probably less than 2 km.
Uncertainties in ray take-off angles for these shallow events per=it the interpretation that they are thrust faulting events and that they locate on the Verona fault. However, their first motion patterns are more readily explained by strike slip faulting.
2k Although the precise relationship of these events to the Ver0na fault is unknown, their te0:enic significance is clear. Ineir presence at shallow depth near er witnin the Ver0na fault :One de:Ont; rate 0 that the te:tenic stress locally exceeds the failure strength of the rock. The P-axes of the focal mechanis: solutions for these events have a northeast-scuthwest orientation for either interpretation of their mechanisms. This orientation is favorable to the sense of displace =ent of the Verona fault reported by Herd and Brabb (unpubl. admin. report, 1953).
In su==ary, available seis=ological evidence demonstrates:
- 1) that the Verona fault has earthquakes in probable association with its fault plane that have focal =echanis:s in agree ent with north-over-south
- 2) that re5 onal stress field determined thrust =ove=ent en the fault, 1
frc: focal mechanis: solutions a6rees with the tectonic interpretation of the fault, and 3) that shear stresses locally exceed the strength of the crust within 2 k or less of the known surface trace of the Verona fault.
l
1 25 Acknowledgenents We are grateful to Allan Lindh and Darrell Herd fer their thorough technical review of this paper. Bob Morris, Bob Page, and Bob Wallace made several useful suggestions which we have incorporated in the text.
We also Thank, Barbara Moths, Linda Shijo, and Virgie Barbs for their assistance in preparing the report.
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27 Referen:es Aki, K. and Lee,'a'. H.
K.,
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Bonilla, M. G., Lienkae:;er, J. J., and Tinsley, J. C., 19S0, Surface faulting near Liver: ore, California associated with the January 1980 earthquakes:
U.S. Geological Survey Open-File Report 80-523, 32 p.
Bure, C. G., Lester, F. W. Meagher, K. L. and Wesson, R. L.1975, Catalog of earthquakes along the San Andreas fault syste in central California, April-June,1973:
U.S. Geological Survey Open-File Report 75-125, 44 p.
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California Department of Water Resources, 1964, Crustal Strain and Fault Move =ent Investigation, Faults and Earthquake Epicenters in California: Bulletin 116-2, 95 p.
Crosson, R.
S., Crustal Structure Modeling of Earthquake Data, 1.
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Earth Science Associates, 1979, Advisory Co: ittee on Reactor Safeguards, Meeting of GETE Sute:::ittee, Nove=ber 14, 1979, Presentatien by Earth Science Associates: Palo Alto, California. unpaginated.
23 Ellsworth, W.
L., Bear Valley, California, Earthquake Sequence of February-March 1972, Seismelogical Society of America Bulletin, v.
65, pp. 4S3-535.
Ellsworth, W.
L., and Meths, B. 1973 (i.e.,
1979), Seismic structure cf the San Andreas fault zone near Dry Lake, California (abs.):
Earthquake Notes, v. 49, no. 4, p. 99 Hall, C.
A., Jr., 1958, Geology and Palentclegy of the Pleasanten area, Alameda and Centra Costa counties, California: Univ. Califernia Pubs. Geol. Sci. Bull
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Herd, D. G.,
1977, Geologic Map of the Las Positas, Greenville, and Verona faults, Eastern Ala: eda County, California:
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3-12.
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29 Lee, W. H.
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the San Andreas fault system in central California, for the year 1974:
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4873-4880.
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~
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.Wesson, R.
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31 Wesson, R.
L.,
Bennett, R. E. and Lester, F. W. 1972b, Catalog of earthquakes along the San Andreas fault system in central Calife;*nia, April-June, 1972:
U.S. Geological Survey Open-File Report, 42 p.
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l 1
l i
1 i
1 I
l
32 Figure Caetions Figure 1.
Location map for the Liver: ore Valley, California study area.
Triangles denote seismograph stations used to locate earthquakes.
Star marks location of San Ra:cn shotpoint.
Figure 2.
Index cap for place na:es and fr.ults mentioned in text.
Figure 3 Seis:icity of the Livermore Valley, California region for the years 1359-1979 E a.dthquake epicenters (octagons) are scaled with
=agnitude. Candidate faults discussed in text which also appears in Figure 2 are shown.
Figure 4.
Representative lower hemisphere focal mechanism solutions for earthquakes in the study area. Compressional quadrant is shaded.
Figure 5.
P and T axes of focal mechanisms of earthquakes in the Liver-more Valley, California region.
Figure 6.
Well-located aftershocks of the January 24, 1980 Livermore earthquake (M 5.8).
t Figure 7.
Seismicity near the General Electrict Test Reactor (GETR) for the years 1969-1979. Earthquake symbols plotted are focal depth in kilometers (0-1 km:
0, 1-2 km:
.1, 10-11 km:
A, etc.).
Points 1
l l
labeled a and a' identify end points of cross section in Figure 9 1
1
Figure 6.
Lower henisphere fo:11 te:hanis: solutions for eartnquakes shown in Figure 8.
Co: pres:ional quad ar.: is snaded.
Figure 9 Longitudinal cross-section of seis icity along the dip direction of the Verona fault. Only events within 4 k: of section line are pictted. Sy:bols plotted are the same 1: tr.::e shown in Figure 7.
Dashed line represents position of the Verona fault for a 45 dip to the northeast.
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