ingly diverse collection of focal mechanism orientations observed in the aftershock sequence of the 1989 M s 7.1. Loma Prieta earthquake for the state of stress ...
GEOPHYSICAL RESEARC}{ LETTERS, VOL. 17, NO. 9, PAGES 1441-1444, AUGUST 1990
COSEISMICSTRESSCHANGESINDUCEDBY THE 1989LOMAPRIETA,CALIFORNIAEARTHQUAKE Andrew J. Michael
William L. Ellsworth
David H. Oppenheimer
U.S. GeologicalSurvey,Menlo Park, California Abstract. Earthquakefocal mechanismsfrom before and afterthe 1989 Loma Prieta, Californiaearthquakeare used to infer the coseismicstresschange. Before the main shock,mostearthquakes correspond to right lateralslip on
BACKGROUND 122ø
planessub-parallel to the SanAndreasfauk, andimply a generally N-S mostcompressional stressaxisanda vertical intermediate
stress axis.
Aftershocks
within
0>_
SEISMICITY 40'
37 ø
San And . .z•e
the main
o 10 o
shockrupture zone, however, display almost every style andorientationof faulting, implying an extremelyhetero-
10'
o 20 ø
^ .- o- a• • '.•• A' •ORTHI...... CENT'R AL--- SOUTH ....
/'
o 40 ø _
geneousstressfield. This suggeststhat the main shock
o 60 ø
relievedmost, if not all, of the shear stressacting on its fault plane. Aftershocksthat lie on the perimeter of the ruptureagreewith spatiallyuniform stressstates,but only whenconsideredin three groups:north, south, and above the main shock rupture area. In each of these areas the stressstatemay reflect stresstransfer by the main shock.
O 80 ø
O 100 o
¸
•
I20 ø
/
•ain Shock
0 140ø
0 160ø
AFTERSHOCKS ,,
,,/
,,,
Introduction
In this paperwe explorethe implicationsof the surprisingly diversecollectionof focal mechanismorientations observedin the aftershocksequenceof the 1989 M s 7.1 LomaPrietaearthquakefor the stateof stressacdngon the fault and for the stressdrop of the main shock. Unlike typical San Andreas fault system earthquake sequences wherethe aftershocksare very similar in both their locationsand focal mechanismsto the prior activity, the Loma Prieta aftershocks bear little resemblance to either the main
0
shockor backgroundactivity (Dietz and Ellsworth,1990; Olsonand Lindh, 1990; and Oppenheimer,1990). Beforethe main shockmostearthquakes are right lateral slipon the creepingsegmentof the San Andreasfault and the Sargentfault (Figure 1), but after both the locations and the mechanisms of the earthquakes changeddramatically. In particular,within the central part of the aftershock zone where the main shock rupture occurred almostevery type of focal mechanismcan be found (Figure4 in Oppenheimer, 1990),suggesting a veryhetero-
20
40 60 Distance (krn)
80
Fig. 1. Map and crosssectionsof backgroundseismicity and aftershockswith symbol size scaled to the O with respect to the stresstensor that perfectlyfits the main shock and minimizes O averaged over the background seismicity(shownon equal area stereonet,q•= 0.27). The O is the minimum!3for the two possiblefault planeswhere
[• is the angle betweenthe observedslip directionand shearstressimposedby a stresstensor.
geneousstressstate,in sharpcontrastto the relative simpli-
cityof thepriorseismicity. The apparentdissimilaritybetweenaftershockmechanismsis so strongthatit suggests a post-earthquake stress field with litfie resemblance to stressesreleased in the main
shock.We examinethispossibilityby usingbothforward modelingand a stresstensorinversion. While concentrating on the stress effects of the main shock, we acknowledgethat other mechanisms (e.g. pore fluid effectsor
rigid blockrotations)couldplay a role in creatingthe observed patterns.
Data
We relocatedthe backgroundseismicityand computed
faultplanesolutions suchthattheresults arecomparable to Oppenheimer's (1990) analysisof the aftershocks. After removingpoor solutions and eventsshallower than3km (due to poorly constrained take-off angles),the data set includes304 priorevents,Ma 0.9 to 5.2, fromJune1969 to September !989, and 350 aftershocks, M•t 0.9 to 4.5, from 30 minutes after the main shock on October 18, 1989
This paper is not subject to U.S. copyright. Published in 1990 by the American Geophysical Union.
to November30, 1989. On average36 stationsthat are well distributed on the focalspherewereusedto determine eachfault planesolution.The meanstandard deviation in strike,dip,andrake,asestimated fromthedata,is 130.
Paper number 90GLO!600
0094-8276 / 90 / 90GL- 01600503. O0 1441
1442
Michael et al.: CoseismicStressChangefrom Loma Prieta Forward
Method
and Results
We assumethat earthquakesrelax an unknownfraction of the shear stressacting on the fault plane. We also assumethat on averagethe slip vector is parallel to this appliedshearstressand at a minimumthe angiebetween the shear stressand the slip vector is 40 ø,wewillinterpret theinversion results to imply a spatial!yheterogeneous stateof stress.The simulations suggestthatwhenthe spatiallyaveraged stress vanishes • will be about65ø. Given the resultsof the forward modeling, we divide the
regioninto four volumes, basedon the inferredextentof the main shockrupture(Dietz andEllsworth,1990). The centralzone is presumedto containthe ruptureand the north and south zones flank it.
The central zone was
furtherdividedinto two depthintervalswith a boundary at 5 km, to correspond to the upper extent of faulting (Lisowskiet al., 1990). In each of thesevolumesthe
backgroundfocal mechanismsand aftershockfocal Inverse Method and Results
mechanismswere inverted separatelyto determinethe stressfield for that volume and time interval.
To determine the nature of the coseismic change we
solve a formal inverseproblemfor a stateof stressconsistentwith the datafor the background seismicityandaft-
,.,
ershocks separately andcompare the results.We usethe
BEFORE •
methodof Michael (1987a and 1987b) to determinethe
spatially uniformcomponent of thedeviatoric stress field. Tests with subsetsof our data show that the results are
robust,with respectto theconfidence limits,for datasets with 15 or more events. From the focal mechanismdata
we candetermine onlytherelativemagnitudes of thedeviatoricstresstensor,whichcan be represented by the orientationsof the three principalaxes and the quantity
,
o
171
q•= (S2 - S3)/(S1- S3),whereSl, S2,andS3arethethree principal stresses ordered frommostcompressional to most tensional (Ange!ier, 1979). No weighting by magnitude is used becausethere is no evidence that the stressfield
determinedis dependent on the magnitudes of the events used;indeed,Michael (1987b)demonstrated that the oppo-
•1• 13= 61
HETEROGENEOUS STRESS
site is true at Coalinga.
To applythismethodto faultplanesolutions we must selectonenodalplaneas the fault planeandestimateour confidence in this choice. While the seismicity is on a
varietyof structures, moststrikeNW-SE(DietzandEllsworth, 1990; Olsonand Lindh, 1990). Consequently, we
choose thenodalplanethatis closerto a NW-SEvertical planeas ourguessof thecorrectplane.We alsoassume an errorrateonthefaultplanepicksof 50%,thegreatest it can be if the true planesare orientedrandomlyto our
O-
60
120
180
MisfitAngle
assumption, whenassessing theconfidence limitson the
Fig. 2. Stressinversionresultsfor the deepcentra!region beforeandafterthe mainshock.The histograms showthe distribution of 13for thedatawithrespect to thestress tensor determinedby the stressinversion.The equalarea
inverse solution. Overestimating the error rate will increasethe size of the confidenceregionspreventingus
limits. Before the main shock (•=0.39
from overestimating the resolution.
stereonet shows the stress axes and their 95% confidence
confidencelimits of (0.!3, 0.48).
with 95%
Michaelet al.-Coseismic Stress Change fromLomaPrieta in the deep central zone the stressfield before the main
shock is relativelyuniform(13= 28ø) withS2 verticaland Si oriented N 8ø E (Figure2). Although thisstress tensor fitsthe main shockpoorly,with [3mainshoe k = 44ø, another within its 95% confidence limits fits the main shock to
within25% which is smallerthan [i
The most we can
conclude is thatthe stressinversion resultfrom theprior activityis not inconsistentwith the main shock. The aft-
ershocks have13= 61ø in the deepintervalanda large number of themhave13> 90ø (Figure2). As •' > 40ø we
The southzonecontainsmostof theprior seismicity; it showsa stressstatethat is relativelyuniform([3= 21ø) with S2 vertical and S1 orientedN-S. After the main shockSi trendsN 9ø E with [3= 14ø (Figure5). While the before and after stress states are distinct in a statistical
sense,we discountthe importanceof this modestrotation
(9ø),giventhedifficulties in constructing focalmechanisms in the southzone (Oppenheimer, 1990). BEF
do not show the inversion's solution because the true stress
tensor is unlikelyto be withinits 95% confidence regions; however,the distributionof [• is displayedbecausethere are no stresstensorsthat would better fit the data. Thus,
foraftershocks withintheinferredmainshockrupturearea the homogeneouscomponentof the stresstensoris smaller
1443
CO'
•cu
21 ø
u.i
thanits variability. Justabovethe main shockrupture, thereareinsufficientdatato determ/nethe pre-mainshock stressfield. The aftershocks in the shallow interval have
o0
[1= 21ø and a homogeneous stressfield with S3 vertical
60
o
andS 1 orientedN 11ø E (Figure3) fits the data.
' I :•0 ..... 11• 0 A
g•.
o
=21
= 14ø
ø
0
e0 MisfitAngle
o
0
60
MisfitAngle Fig. 3. Stress inversion results for the shallow central region. There are too few data to invert before the main
shock.Fortheaftershocks q)= 0.55 (0.16,0.73). SeeFig-
Fig. 5. Stressinversion resultsfor the southregion.For before q•= 0.39 (0.18, 0.42). For ½= 0.57 (0.37,0.85). SeeFigure2 for details.
after
We interpretthe differencesand similaritiesbetweenthe
ure 2 for details.
stress states before and after the main shock to be a cose-
In the north zone there are insufficient data to determine the stressfield before the main shock. The aftershocksfit
ismic temporalchangein stressdespitethe spatialchanges in the hypocentersfor two reasons. First, within the backgroundseismicity,we can find no evidencefor spatialvari-
a uniformstress ([•= 18ø),withS2 or S3 verticalandSt trendingN 29ø E, almostnormal to the San Andreasfault (Figure4).
ations in stress orientations. Second, the main shock is an
obvioussourcefor a temporalchange. Discussion
Before the Loma Prieta earthquakemostearthquakes in the region correspondto the releaseof shearstressin an environmentdominatedby north-southcompression.This same stress field also fits the aftershocks in the southern
part of the zone. Within the northernand shallow central
parts of the zone the aftershocksrelease compression orientedat a high angleto the SanAndreasfault. Many of the aftershocks have a steeply-dipping nodal plane that 0
60
120
MisfitAngle
parallelsthe fault, but releasealmostno dextra! shear. Too few eventsprecededthe main shockin thesetwo areasto permit a comparisonof the pre- and post-eventstress fields.
Fig.4. Stressinversion resultsfor thenorthregion.There are too few data to invert before the main shock. For the
aftershocks q•= 0.20 (0.00,0.44). SeeFigure2 for details.
Within the main shockruptureareano spatiallyinvariant stress tensor can satisfy the slip vectors of most aftershocks,and many have slip vectorsoriented>90ø from
1444
Michael et al.' CoseismicStressChangefrom Loma Prieta
the mean applied shear traction. The aftershockfocal mechanisms demanda highly heterogeneous stressfield. While this heterogeneity could includeboth spatialand temporal components, diverse and discordant focal mechanismsoccur throughoutthe first six weeks of the
in a spatially heterogeneous state. As most aftershocks near the rupturehave focal mechanisms that disagreewith the pre-stress,we suggestthat the main shockreleased most of the tractionactingon the fault planeresultingin an almosttotal stressdrop earthquake.
sequence. As few of the aftershockseither resemble the
main shockor correspond to releaseof the same stress field, we suggestthat the main shockreleasedmost,if not all, of the tractionactingon its fault plane. In other words, the main shock stressdrop appearsto be nearly
Acknowledgements. We thank Dave Hill, Art McGarr, and Paul Reasenbergfor critical reviews. References
total.
The pattern of stressreleasein the aftershocksequence qualitatively correspondswith the stresschangesexpected aroundan elastic dislocation,but only if the stresschange is a large fraction of the pre-stress. Dislocationstresses are largest near discontinuitiesin the dislocationpattern; these occur near the edges of the rupture and within the rupture if it contains variations in slip magnitude and orientation (Segall and Pollard, 1980). Thus we would expect the greatestheterogeneities in the stressfield to be found in the main shockrupturearea, as is observed. Outside of the rupture, a dislocationtransfersshearstressesof the same sense as it released to the continuation of its fault
Ange!ier,J., Determinationof the meanprincipaldirections of stressesfor a given fault population,Tectonophysics, 56, T17-T26, 1979. Dietz, L.D., and W.L. Ellsworth, The October 17, 1989 Loma Prieta, California, earthquakeand its aftershocks:
geometryof the sequence from high-resolution locations, Geophys.Res. Lett., in press,1990. Gephart,J.W., and D. W. Forsyth,An improvedmethod for determining the regional stresstensor using earthquakefocal mechanismdata:Applicationto the SanFernandoearthquakesequence,J. Geophys.Res., 89, 93059320, 1984.
plane. Geodetic models of the main shock (Lisowski et al., 1990) suggestpredominantlystrikeslip motionnearthe southernterminationof rupture, so we would expect it to
Jaeger,J.C., Elasticity,Fracture and Flow: With Engineering and GeologicalApplications,268 pp., Metheun and
transfer dextral shear to the fault farther to the south.
Lisowski, M., W.H. Prescott,J.C. Savage and M.J. Johnston, Geodetic estimateof coseismicslip during the 1989 Loma Prieta, California, earthquake,Geophys.Res. Lett., in press,1990. McGarr, A., Scalingof groundmotionparameters,stateof
As
N-S compressionwas the pre-main shockstressstate,the dislocationstresswould reinforce the pre-stress. To the north the same main shock model suggestsa larger thrust component.This might explainthe absenceof fight lateral shear on the San Andreas fault to the north and above the
ruptureduringthe aftershocksequence. While the complexpatternof focal mechanisms in the centralzonearguesfor nearlycompletestressreleasein the main shock, placing a numericalvalue on the stressis highly modeldependent.At one extremea uniformdislocation, as used in the geodeticmodeling,correspondsto a stressdrop of about 2.5 MPa, whereasa heterogeneous model consistentwith the main shockgroundmotionin the near field implies localized stressdrops of 100MPa or more (A. McGarr, personalcommunication, 1990;McGarr, 1984). A highly irregularslip distributionon the fault planecouldcreatesufficientvariationsin the stressfield to explainthe diverseaftershockmechanisms, althoughthis remains to be demonstrated.
Co. Ltd., 1971.
stressand focal depth,J. Geophys.Res., 89, 6969-6979, 1984
Michael, A.J., The use of focal mechanisms to determine
stress:A control study,J. Geophys.Res., 92, 357-368, 1987a.
Michael, A.J., Stress rotation during the Coalinga aftershock sequence,J. Geophys. Res., 92, 7963-7979, 1987b.
Olson, J. and A. Lindh, Seismicityprecedingthe Loma Prieta earthquake(abstract),EOS Trans AGU, 71, 289, 1990.
Oppenheimer, D.H., Aftershockslip behaviorof the 1989 Loma Prieta, Californiaearthquake,Geophys.Res.Lett.,, in press1990. Segall, P., and D.D. Pollard, Mechanicsof discontinuous faults, J. Geophys.Res., 85, 4337-4350, 1980.
Conclusions
Fault plane solutionsof the Loma Prieta sequenceprovide us with evidenceof how the earthquakealteredthe stateof stressat seismogenic depths. The post-earthquake
stressfield is complicated andwill requirefurtherstudyto understand it in detail. One basicpatternis clear, however, and we believe it will be unchangedby further study. The main shock left the stressfield within the rupture area
W.L. Ellsworth, A.J. Michael, and D.H. Oppenheimer, Branch of Seismology,U.S. GeologicalSurvey, MS/977, 345 Middlefield Road, Menlo Park, CA 94025 (Received March 6, 1990; revised July 3, 1900; acceptedJuly 3, 1900.)
