TECTONICS, VOL. 10, NO. 6, PAGES1187-1203,DECEMBER1991
SEISMICITY
AND
INTERACTION,
JACINTO
FAULTS,
FAULT
SOUTHERN
SAN
FAULT ZONE AND ADJACENT
SOUTHERN
CALIFORNIA:
IMPLICATIONS
FOR
SEISMIC
HAZARD
earthquakes
were invertedsimultaneously
for source
mechanism,seismicmoment,rapturehistory,and
centroiddepth. The complicatedwaveformsof the
1968event(Mo=l.2 x 1019N m) areinterpreted
in
MarkD. Petersen,
1Leonardo
Seeber,
2 LynnR.
Sykes,1 JohnL. N•ib•lek,3 JohnG. Armbmster,
2
termsof two subevents;
the firstcausedby rightlateralstrike-slipmotionin themainshockalongthe
CoyoteCreekfault andthe secondby a rapture
locatedabout25 km away from the masterfatfit. Our
JavierPacheco,1 and KennethW. Hudnutn
waveform
inversion of the 1969 event indicates that
strike-slipmotionpredominated,
releasinga seismic
momentof 2.5 x 1017N m. Nevertheless,
therightAbstract.
The southern San Jacinto fault zone is
characterized
by high seismicityanda complexfault
patternthatoffersan excellentsettingfor
investigatinginteractions
betweendistinctfatfits.
This fault zoneis roughlyoutlinedby two subparallel
masterfault strands,theCoyoteCreekandClark-San
FelipeHills faults,thatarelocated2 to 10 km apart
andare intersectedby a seriesof secondary
cross
faults. Seismicityis intenseon bothmasterfaults
and secondarycrossfatfitsin the southernSan
Jacintofault zone. The seismicityon thetwo master
strandsoccursprimarilybelow 10 km; theupper10
km of the masterfatfitsarenow mostlyquiescentand
appearto rapturemainlyor solelyin large
earthquakes.Our resultsalsoindicatethat a
considerable
portionof recentbackground
activity
neartheApril 9, 1968,BorregoMountainrapture
zone(ML=6.4) is locatedon secondary
faultsoutside
the fault zone. We name and describe the Palm
Washfault,a very activesecondary
structure
located
about25 km northeastof BorregoMountainthatis
orientedsubparallel
to theSanJacintofault system,
dipsapproximately
70øto thenortheast,
and
accommodates
right-lateralshearmotion. The
VallecitoMountainclusteris anothersecondary
featuredelineatedby therecentseismicity
andis
characterized
by swarmingactivityprior to nearby
largeeventson the masterstrand.The 1968 Borrego
Mountainandthe April 28, 1969,CoyoteMountain
(ML=5.8) eventsareexamplesof earthquakes
with
aftershocks
and subevents
on thesesecondaryand
masterfaults. Mechanismsfrom thoseearthquakes
andrecentseismicdatafor theperiod1981to 1986
arenotsimplyrestricted
to strike-slipmotion;dipslipmotionis alsoindicated.Teleseismic
body
waves(long-periodP and SH) of the 1968 and 1969
1Department
of GeologicalSciences
andLamontDohertyGeologicalObservatory
of Columbia
University,Palisades,New York.
2Lamont-Doherty
Geological
Observatory
of
ColumbiaUniversity,Palisades,New York.
3College
of Oceanography,
OregonState
University, Corvallis.
4Seismological
Laboratory,
California
Institute
of
Technology,Pasadena.
Copyfight1991 by the AmericanGeophysical
Union.
Papernumber91TC01240
0278-7407/91/91TC-01240510.00
lateralnodalplaneof thefocalmechanism
is
significantly
misoriented
(20ø)with respectto the
masterfault,andhencethe eventis notlikely to be
associated
with a raptureon thatfault. From this and
otherexamplesin southernCalifornia,we conclude
thatcrossfaultsmay contributesignificantlyto
seismic hazard and that interaction between faults has
importantimplicationsfor earthquake
prediction.
INTRODUCTION
The southern California section of the Pacific and
Noah Americanplateboundaryis a complexshear
zone located in a transitional tectonic environment.
Extensional
tectonicsareassociated
with theopening
of the Gulf of California to the south and the Basin
andRangeprovinceto theeast,whilecompressional
tectonicselevatetheTransverse
Rangesto thenorth
andwest. Thusdeformationis complexin theSalton
Searegion.Displacement
is primarily
accommodated
by majorfaultsystems
thattrend
northwest:the San Andreas, San Jacinto,and
Elsinorefault zones(Figure1).
On a regionalscalethe SanJacintofault is a
classicalexampleof a faultzonecharacterized
by
steeplydippingdextralfaults[e.g.,Dibblee, 1954,
1984], intenseseismicity[e.g.,Hamilton, 1972],
andlonglineaments
observed
on satelliteimagesof
the area [e.g., Rockwell et al., 1990]. The zoneis
definedby two dextralmasterfaultsthattrend
noahwest,subparallelto theNorth American-Pacific
plateboundary(Figure 1). Thesemasterstrandsare
from 2 to 10 km apartanddefinethe widthof the
fault zone.
Shearingalongthe southernSanJacintofault zone
is accommodated
not only by fight-lateralmaster
faultsthattrendnorthwestbut alsoby shorter
secondary
fight-lateralfaultsthatare subparallel
to
themasterfaultsandby left-lateralcrossfatfitsthat
trend northeast[Nicholson et al., 1986; Seeberand
Nicholson, 1986]. Crossfaults are also observedas
prominentfeaturesin geologic[e.g.,Sharp,1972],
seismic[e.g., Nicholsonet al., 1986],
geomorphologic
[e.g., Rockwellet al., 1990], and
gravity data (e.g., P.A. Cowie and L. Seeber,
unpublished
manuscript,1990). Many crossfaults
areshortandterminateagainstthemasterfaults.
Surfacestructure[e.g.,Sharp,1972],reflectionand
refractiondata(e.g.,Bondet al., unpublished
manuscript,1990, and Severson[1987]), and
seismicity[e.g.,ThatcherandHamilton, 1973;
Hamilton, 1972] indicate that thrustand normal
faultshavebeenactiveasweredetachmentfatfits,
1188
Petersen
et al.:Seismicity
of theSanJacinto
FaultZone
can segmentthe masterfault by creatingbarriersto
therapturepropagation
andtherebycontrolthe
characteristic
lengthof rapture. They may also
triggereventson masterfaults,asdemonstrated
by
the 1987 Superstition
Hills events[Hudnutet al.,
1989a]. Thesesecondaryfaultsmay therefore
controlthetime-space
distributionof earthquakes
on
masterfaults. Patternsof seismicityon secondary
faultsmay indicatewhenrapturealonga masterfault
will occur.
1968
-117 ø
x'x
"•
-116o
Fig. 1. Index map of southernCaliforniamaster
faultsinclu•ng •e S• An•eas, S• Jac•m, •d
Eisaore
fault zones. Rel•at•
•tershock
zones
from S•ders et •. [1986] for thel•ge (M • 5.5)
events in 1937, 1942, 1954, 1968, 1969, 1980,
1987•e shownwith •agon• sffipedpattern.
Seis•c gapsidentified• thisstudy•e shownas
opendashedckcles. •fi•fions of the Anza seis•c
gap •om S•ders et •. [ 1986] •d •atcher et •.
[ 1975] •e •so indicated.Dist•ce sc•e is • upper
right comer.
i.e., near-horizontal
dippingfaultsat midcrustal
depths.
Masterfaultsslip morerapidlyandtendto be
longerthanindividualcrossfaults[e.g.,Hudnutet
al., 1989b]. Large (M > 5.5) earthquakes
in the
studyarearapturedmasterfaultsin 1987 (ML=6.6),
1968 (ML=6.4), 1954 (ML=6.2), andpossiblyin
1937 (ML=6.0; Figure 1). Althoughsecondary
faultsoftenrapturein burstsof microseismicity
[SandersandKanamori,1984],theymay also
rapturein moderate(4 < M < 5.5) to large(M > 5.5)
earthquakes.The first eventof the 1987 Superstition
Hills sequence
(ML=6.2) occurredon a left-lateral,
northeast-striking
crossfault andwasfollowedby a
largerright-lateralraptureon thenorthwest-striking
SuperstitionHills fault (ML=6.6) about12 hours
later. Othercrossfaultsmay haverapturedin 1969
(ML=5.8; this paper) and 1942 (ML=6.5 [Doser,
1990]; seeFigure 1).
This research,in conjunctionwith previouswork
[Nicholsonet al., 1986], suggests
that an
understanding
of secondary
faultingis importantfor
at leasttwo masons.First, significantstrainis being
accommodated
by slipon secondary
faults,in
particularcrossfaults. The contributionto the
regionalstrainby secondary
faultsis accompanied
by
a significantcontributionto earthquake
hazard.
Second,the stress-strain
fieldsof masterfaultsmay
be coupledwith thoseof crossfaults. Crossfaults
In the northemmost
portionof the April 9, 1968,
BorregoMountainrapturezone,whereseveralcross
faultshavebeenmapped,aftershocks
trendnortheast
[Hamilton, 1972]. Cross-faultactivityon the
InspirationPointfaultis thoughtto havecauseda
barrierto thenorthwardpropagation
of the 1968
rapture[SeeberandNicholson,1986]. The
waveformsof thisearthquake
wereinvestigated
extensively[WyssandHanks, 1972;Burdickand
Mellman, 1976;HeatonandHelmberger,1977;Ebel
andHelmberger,1982;Kikuchi andKanamori,
1985]. Ruptureduringtheearthquake
centered
primarilyalongtheCoyoteCreekfault,displacing
sediments
in thevicinityof theepicentera maximum
of 38 cm horizontallyin a fight-lateralsense[Clark,
1972]. Aftershocks,
however,occurrednotonlyon
the CoyoteCreekfault but alsoon otherstructures
in
a wide andcomplexzone [Hamilton, 1972].
The CoyoteMountainearthquake(ML 5.8)
occurredabouta yearlater(April 28, 1969)andjust
to the northof the 1968 surfacerapture(Figure1).
While theBonegoMountainaftershocks
extended
over mostof the upper10 km of the fault [Hamilton,
1972], the CoyoteMountainaftershocks
were
concentrated
at depthsbetween10 and 12 km
[Thatcher and Hamilton, 1973]. Reverseand strike-
slipmechanisms
werereportedfrom analysisof
aftershock focal mechanisms of the 1968 and 1969
events [Allen andNordquist,1972;Hamilton, 1972;
ThatcherandHamilton,1973]. Surfaceraptures
with left-lateraldisplacement
werealsoobserved
on
fracturesfollowingthe 1968rapture[Clark, 1972].
Many of thesefractureswereobservednearBorrego
Mountain; the fractureshad maximum left-lateral
offsetsof between30 and40 mm andmanytrended
northeasterly.In addition,paleomagnetic
data
[ScheuingandSeeber,1989]andgeodeticdata
[HudnutandSeeber,1987] give evidencefor
substantial
Plio-Quatemary
blockrotationnearthe
1969rapture,whichwouldimply largeleft-lateral
offsets on cross faults that bound the blocks. Our
resultsindicatethatthe 1969earthquake
mayhave
occurred on a cross fault. Thus the 1968 and 1969
earthquake
sequences
involvea complexseriesof
raptureson bothmasterand secondaryfaults.
The purposeof thispaperis to examinethe
relationship
betweensecondary
andmasterfaults
alongthe southernSanJacintofault zone. Our
researchincludesa studyof the spatialandtemporal
aspectsof thepresentseismicity,waveform
inversions for the 1968 and 1969 events, and an
analysisof focalmechanisms
producedfrom shortperiodseismicdata. We showa complexpatternof
Petersen
et al.: Seismicity
of theSanJacintoFaultZone
recentseismicactivitythatdiffersdrastically
fromthe
patternof aftershocks
followinglargereventsthat
rupturealongmasterfaults. Interseismicactivity
alongthemasterfaultstendsto occuraroundtherims
of majorrupturesin a patternthatresemblesthe
seismicityalongtheLomaPrietasegmentof the San
Andreasfault [PlafkerandGalloway, 1989].
Prominentseismicityoutsidethe SanJacintofault
zoneis associated
with a newly discovered
secondary
fault thatwe namethePalmWashfault
(located25 km eastof theepicenterof theBorrego
Mountainearthquake)andwith a clusterof activity
beneaththe Vallecito Mountains(located15 km west
of the 1968 epicenter).Secondaryfault activitymay
be an importantprecursorysignalthatmay be useful
for shortto intermediate
termpredictionof large
earthquakes
on masterfaults.
RESULTS OF THE 1968 BORREC•
MOUNTAIN
AND 1969 COYOTE MOUNTAIN
WAVEFORM
INVERSIONS
Several authors have studied the waveforms of the
1968 earthquake[Hamilton, 1972; Burdickand
Mellman, 1976;Ebel andHelmberger,1982;
1189
Wide Standardized
Seismological
Network
(WWSSN) werehanddigitizedandinterpolated
at
an interval of 0.5 s. An excellent azimuthal
distribution of stations located between distances of
30ø and 90 ø was availablefor both events;this is
essentialfor well-constrained
nodalplanes.
In our inversion,observedseismograms
are
comparedto syntheticseismograms
in a leastsquares
senseto estimatethesourcemechanism
(strike,dip,
rake),centroiddepth,scalarmoment,andshapeof
the source-time
function[N•ib•lek,1984]. Synthetic
bodywave seismograms
consistof directP andS
arrivals as well as reflections from the free surface.
Complicated
bodywavesaremodeledby longer
sourcedurationandadditionalsourceenergyandare
treatedas the superposition
of subeventwaveforms.
The source-time
functionis represented
by a seriesof
overlappingtriangles.The elementarytheoretical
seismograms
areproducedby convolvingthe source
functionwith geometrical
spreading,
anelastic
attenuation(t*=l for P waves,4 for SH waves),
instrumentresponse,
andsource-receiver
Green's
functions.We usearrivaltimesfrom short-period
seismograms
to constrainthe starttime of theP wave
inversion
window.
Kikuchi and Kanamori, 1985]. All of theseauthors
agreethathighmomentreleaseoccurredover6 s, in
what we refer to as the first subevent,with
orientation that is similar to the strike of the master
Resultsof WaveformInversionfor the 1968
Earthqt•ke
fault. Someauthorshave,however,alsosuggested
that seismic moment was released after the first
subeventin subsequent
events. Burdickand
Mellman [1976] suggested
thatthe momentwas
releasedin three subevents,from three distinct and
separate
regionsandnot simplyalongthemaster
fault. KikuchiandKanamori[1985] identifythe
momentreleaseafterthefirst subevent
but suggest
thatthismomentis insignificantin decreasing
the
residuals in the fits of the waveform data. Thus in
thispaperwe reanalyzethe 1968eventandcomplete
statistical tests to determine whether the 1968 event
releasedsignificantmomentin a secondsubeventon
a secondaryfault. We alsoanalyzetheresolutionof
eachresultantparameterfor the secondsubevent
sincewe areinterestedin understanding
secondary
faultsandtheirrelationshipto masterfaults.
The 1969earthquakehasbeenassociated
with a
ruptureon themainstrandCoyoteCreekfault. The
aftershockdistributionis dispersedin space,and
severalof the aftershocks
wereprobablynot located
on themasterfault. If the 1969earthquake
occurred
alonga secondary
faultinsteadof alongthemaster
fault,thelengthof theAnza seismicgapmaybe
considerably
longerthanwaspreviouslythought.
Thuswe analyzethe 1969earthqu•e to determine
if
the waveformsgive someindicationof whetherthe
ruptureoccurredalongthe masterfault or someother
secondaryfault.
Initially, a singlesource-time
function6 s longwas
usedin the inversionfor the 1968BorregoMountain
earthquakeandwassufficientto resolvemost
featuresin the observedseismograms.
The
mechanisms,waveforms, and source-timefunction
of the 1968 eventare shownin Figure2 andthe
parametersandtheiruncertainties
in Table 1. The
mechanism of the first subevent,obtained from this
inversion,represents
motionon the CoyoteCreek
fault andis consistent
with otherpublishedresults
and aftershockpatterns[Hamilton, 1972;Burdick
andMellman, 1976;Ebel andHelmberger,1982;
Kikuchi andKanamori,1985]. Our preferrednodal
planestrikesnearlynorthwest(311ø),consistent
with
theobservedorientationof surfacerupturesalongthe
masterfault [Clark, 1972] anddipssteeplynortheast
(78ø),in agreementwith the aftershockdistribution
[Hamilton, 1972]. The first subeventreleaseda
seismic moment of 9.2 x 1018 N m at 10 km centroid
depth,nearthebaseof the seismogenic
zoneinferred
fromthecurrentseismicityandaftershocks
[Doser
and Kanamori, 1986].
We introduced a second subevent 12 s after the
Data and Method
first subevent;syntheticseismograms
with two
subevents
yieldedlowerresidualsthanseismograms
with a single-eventsource.AlthoughKikuchiand
Kanamori[1985] concludedthatthepresence
of a
secondsubeventis not significant,a pairedt test
[Huanget al., 1986] indicatesthatthe existenceof a
secondsubeventis significantin loweringresiduals
Long-periodP and SH wavesof boththe 1968 and
1969earthquakes
thatwererecordedby theWorld-
did not lowerresidualswith respectto a point
source.Hencewe invertedfor the sourceparameters
at a 99% confidence level. A line source,however,
1190
Petersenet al.: Seismicityof theSanJacintoFaultZone
waveforminversion.The secondsubeventmay be
of thesecondpulseof energyasan independent
less resolved than the first subevent because of three
point source.
reasons. First, the second subevent comes in the
Parameter resolution of the second subevent
codaof the first large subevent.Second,the second
(Figure3) is moreuncertainthanfor thefirst
subevent.While strike,dip, andslipparameters
for
subevent has a low moment release rate and therefore
did not exciteasmuchhigh-frequency
energy.The
short-period
recordsindicatea secondsubevent,but
it is difficult to interprettheonsetof thatsubevent.
Third, the secondsubeventhaslow amplitudewith
respectto the first subevent.Thesethreefactors
causea low signal-to-noise
ratiofor the second
subeventthatcausesdifficultyin resolutionof source
the second event of the inversion exhibit narrow
well-def'med
minima,depth,distance,andazimuthof
thetwo centroids
arenotwell constrained
by this
BORREGO
MOUHTAIN
EARTHQUAKE
parameters.
Our inversionresultssuggesta momentreleasein
thesecond
subevent
of 2.6 x 10•8N m, nearlya
third of the moment released in the first subevent.
The mechanism for the second subevent indicates
:.-"•./••
•
•NAT
^
HNR
/
MAINSHOCK
•
SUBEVEnT
..• ß
slipon eithera shallowdippingtlmastor a steep
reversefault strikingnortheast(Figure2). The
inversionresultssuggesta locationfor the second
subeventwell awayfrom themasterfault (about25
km). The waveformsarebestfit usinga subevent
'- ¾"
•
'0 '2'0 '
• 0
sec
located either northeast or southwest of the first
1'0 •0
sec
KRK NUR
subeventlocatedon the CoyoteCreekfault. These
directionsand distancescorrespond
with two
prominentaftershockclustersdescribed
by Das and
Scholz [1981].
BurdickandMellman [1976] alsofoundenergy
releasesubsequent
to thefirst subevent
locatedon the
CoyoteCreekfault. They, however,separated
the
momentreleaseinto three subevents(insteadof two).
The second and third subevents from their inversion
were located south of the first subevent. We used
the source-time function of Burdick and Mellman
[1976] andappliedit to ourdatasetto testthe
direction of our second subevent from the first
subevent.We invertedfor only two subevents,
however, and not three subeventsas in their
inversion.Residualswere againminimizedwhen
moment in the subevents was released to either the
northeast or southwest rather than from the south.
Thus our inversion results indicate that a second
o 0• 102'03'0
S•C
subeventis locatedon a secondary
fault to the
•i•. •. •on•-pc•od wavcfo• •nvc•sionof the
•968 Bo•c•o •oum•n c•hqu•c. (Top) P waves
•d m•afion pa•c•. (Bosom)S• waves•d
•a•a6on pattc•. Bothobsc•cd wavcfo•s (so]•d
l•cs) •d symhcfics
(do•cd]•cs) • shown.•c
northeastor southwestof the first subevent,located
on the CoyoteCreekfault.
Resultsof WaveformInversionof the 1969
Earthq•ke
f•-•c]d
so•cc-t•c
func6on Js ccn•
•ncath P
wavcfo•s.
Wavcfo•
scale fo• P waves •s shown
The waveformsof the 1969CoyoteMountain
eventaremuchsimplerbut alsonoisierthan
waveformsof the BorregoMountainevent. We
foundthatan inversionusinga simplepoint source
givesan adequatefit to thedataanda result
consistent
with publishedshort-period
mechanisms
to •ht of sourcc-•mcfunction•d fo• S• wavesat
bottom•ht. •cch•isms of ma•nsh•k •d
subcvcm •
shown to ]owc• ]cft of P wavcfo•s
on
equ•-•a pmjccaonsof ]owc•hc•sphcm. Sold
quaYantsdenotecompress•on•
P wavemoilore
TABLE 1. Resultsof Inversionsof Long-PeriodData
Strike,
Dip,
Slip,
Depth,
Moment,
,,
Event
x 1017N m
Distance, Azimuth,
deg
deg
deg
km
km
deg
Bonegomainshock
Bonegosubevent
311 + 2
40 + 7
78 + 2
80 + 5
179 + 2
80 + 10
10 _+1
7+ 5
92.0 + 2.5
29.0 + 5.0
25 + 5
40 + 10
CoyoteMountain
295 + 5
69 + 4
169+ 4
12 + 1
2.5 + 0.7
-
-
a Standard
errorsof inversions
fromNabelek
[1984]arescaled
similarly
to thoseof McCaffrey
[1989].
Petersenet al.' Seismicityof theSanJacintoFault Zone
•
0.33
0.33
031
0.31
1191
0.33 --
0.31
'F-_.
0.29
0.29
0
30
60
90
120
50
70
0
90
30
60
033
0.33
031
0.31
0 29
gO
120
150
slip
dip
strike
0.33
0 29
0
10
20
30
40
50
0
distance
5
10
15
0
depth
100
200
300
400
azimuth
Fig. 3. Resolutionof the secondsubeventparametersfor the 1968 BorregoMountain
earthquake.While strike,dip, andslip shownarrowwell-definedminima,distancefrom
centroid,depth,andazimuthof the subeventarepoorlyconstrained.
[ThatcherandHamilton,1973]. The mechanism,
COYOTE MOUNTAIN
EARTHQUAKE
waveforms, and source-time function from the
CoyoteMountaininversionare shownin Figale 4
with parametersanduncertainties
outlinedin Table 1.
The centroiddepthis consistent
with the 11 km depth
WES
OGD
for the mainshock inferred from the aftershocks
[Thatcher and Hamilton, 1973].
SCP
A significantfinding,however,is thatthe
orientationof thenorthwesterly
strikingnodalplane
of theCoyoteMountaineventis inconsistent
with the
suSkeof the CoyoteCreekfault. The latterstrikes
about315ø nearthehypocenter,
whereasa nodal
planewith a strikeof 295øallowedthebestfit to the
waveform
data. We determined the resolution of the
E
strikeparameterin theinversionto interpretthe20ø
difference between the surface strike of the master
,
'
sec
1'0'
sec
fault andthe orientationof thenodalplane. Error
analysisof the secondsubevent
indicatesthatstrike
parameters
morethan10øfromthe295øoptimalvalue
giveresidualsthataresignificantly
higherat the95%
confidencelevel (Figare5). Unlessthe strikeof the
fault changessignificantlyat depth,whichwe have
no reasonto believe,the 1969 eventwasprobably
notcausedby ruptureof themasterfault. Regional
crossfaultsstrikeabout40ø, andourconjugatenodal
planestrikes29ø,anddips80ø southeast,
with a rake
of 21o. Hencethe strikeof theconjugatenodalplane
differsby about11ofrom thatof regionalcross
faults. This 11o discrepancy
is withintheerror,
consideringthat the inferredcrossfault is not
exposed.Aftershocksof the 1969eventspanacross
the San Jacinto fault zone, between the two master
faults,where severalcrossfaultshavebeenmapped.
In addition,seismicitybetween1981and 1986
suggests
that a crossfault is locatednearthe 1969
aftershocks(discussed
later). Thus a secondaryfault
rupture,probablya cross-faultrupture,may have
causedthe 1969eventratherthanslipon a
northwesterlystalkingmasterfault.
SEISMICITY
The southern San Jacinto fault zone is one of the
mostseismicallyactiveregionsof Califomia(Figares
0.18
0 sec
' 1'0'
Fig. 4. Long-periodwaveforminversionof the
1969 CoyoteMountainearthquake.(Top) P waves
andradiationpattem. (Bottom)SH wavesand
radiationpattern.Bothobservedwaveforms(solid
lines)andsynthetics
(dottedlines)areshown.The
far-field source-time function is shown centered
beneath the P waveforms. Waveform scale for :he P
wavesis shownto fight of source-time
functionand
for SH wavesat the bottomfight.
1192
Petersenet al.: Seismicityof the SanJacintoFault Zone
(a)
(b)
eachsideof the fault separatelyusingonly stations
located on the same side of the fault as the
earthquakes.For example,datafrom eventslocated
east
of the fault were invertedusingonly stationson
o
© 0.3s
the eastsideof the fault. Theseinversionsyielded
-,
/"
95% differentvelocitymodelsfor eachsideof thefault.
To testwhetherthesevelocitymodelswere
significantlybetterthanthe Hamilton [1970] model,
we relocatedseveralhundredeventsusingthe new
290
310
330
290
• 330 velocitymodels.While theresidualsobtainedusing
strike
strike310
CCF
the new velocitymodelswere lower thanthose
obtainedusingtheHamilton[ 1970]model,residuals
Fig. 5. Resolutionof s•kc of the 19•9 Coyote
weresignificantly
reducedonly at the 60%
•oumain c•qu•c.
(a) V•cc
exhibits a
confidencelevel. Resultsof many inversions
•nima for s•kc p•amctcr at 295% (b) A t test
indicatea velocitygradient,increasing
about10%
in•catcs s•kc v•ucs •at •c signific•dy •ffcrem
0.25' ' ' ' '
•-'0•
at 90% and 95% confidence Icyels. A]] values of
from southeast to northwest.
s•e
consistent
with mappedsurficialgeology,which
that have a t tests•fisdc aboveone ]eve], say
90%, haveresiduals•at •c higherth• them•a
at 90% confidence.•c s•c of •c CoyoteCreek
fauk •s about 315 ø nc• •c
19•9 cvcm.
6a and 6b) with earthquakes
locatedon bothmaster
and secondaryfaults. While it hasbeenlong
recognizedthatlargedeformations
areaccommodated
by masterfaults,our researchindicatesthat
significantstrainis alsoaccommodated
by cross
faults. In the next sectionwe investigatethis
hypothesis
from thepointof view of background
seismicity.We chooseto relocateearthquakes
in a
small area of the southern San Jacinto fault zone
usinga local velocitymodel. It is betterto usethese
locationsthansimplyusethe catalogdatathathas
beenlocatedwith a regionalvelocitymodel. These
relocatedearthquakes
allow usto studythe detailed
distributionandrelationships
of seismicityon both
masterand secondaryfaults.
Data and Relocation Procedure
This result is
indicates thick sediments on the east side of the
CoyoteCreekfault andcrystallinerockson the west
side[Rogers,1985], andheatflow datathatindicates
a northwestto southeast
heatflow gradient[Doser
andKanamori,1986]. The lithologymay therefore
be quiteheterogeneous,
and a three-dimensional
inversionmay be moreappropriate
thanthemethod
we applied.
Eventsrecordedby a minimumof 15 stationswere
relocatedusingtheHamilton[ 1970]velocitymodel.
The relocatedhypocenters
arecomparedwith the
sameeventsfrom the southernCaliforniacatalog
(Figure7). The stm½rures
arebetterdelineated
by
therelocatedhypocenters.Relocatedseismicity
allows features such as the Palm Wash fault to be
distinguished
from a simplecluster.Accurately
relocated data from 1981 to 1986 are shown in
Figure6b. Histogramsin Figure8 indicatethe
vertical and horizontal standard errors of these
relocatedevents. The standarderrorsof the depth
(+2 km) andhorizontal(_-_t-0.7
km) locationsare
small. Those uncertainties are smaller than the size
While cataloghypocenters
areusefulin examining
grosstectonicfeatures,they are inadequatefor local
detailedtectonicanalysis.Thereforewe investigated
recent fault activityby relocatingtraveltimedata
from the CaliforniaInstituteof Technology/ United
StatesGeologicalSurvey (CIT/USGS) network
of manyfeaturesdelineatedby the seismicity.
Thereforewe suggestthatthesefeaturesarenot
simplyartifactsof therelocationprocess;
theyreflect
fault structure.
SeismicityAlongthePalm WashFault
collected between 1981 and 1986. Locations were
obtainedwith HYPOINVERSE [Klein, 1978],
downweightingstationsgreaterthan50 km away
from the epicenter.We relocatedearthquakes
using
severalregionalvelocitymodels[Ha•ton, 1970;
Fuis et al., 1982; Doser and Kanamori, 1986] and
foundthathypocenters
usingtheHamilton[ 1970]
model weremoretightlyclusteredandyieldedthe
lowest travel time residuals. The Hamilton [1972]
velocitystructurewasobtainedfrom analysisof
explosiondatain the vicinityof theBorrego
Mountainearthquake
andappearsto be a good
approximation
of thelocalvelocities[H•lton,
1970].
We unsuccessfully
attemptedto refinetheHamilton
[ 1970] velocitymodelby invertingsimultaneously
for hypocenters
andvelocitiesusingthemethodof
erosson [1976]. We inverted for eventslocatedon
One of themostprominentfeaturesof therelocated
seismicityfrom 1981to 1986is a clusterof
earthquakes
situated25 km northeast
of theBorrego
Mountainepicenter(Figure6c). This clusteris
resolvedinto a narrowzoneandis interpretedasa
faultthathasnotbeenpreviouslyidentified.This
faultdips70øtotheeastandstrikes
subparallel
tothe
CoyoteCreekfault(Figure9a). Thestructure
is
namedherethePalm Wash fault for a nearby
topographic
feature
onthegeologic
mapof California
[Rogers,1985]. Ourbestfocalmechanisms
from
therelocated
phasedataindicate
thatright-lateral
strike-slip
faultingpredominates
onthePalmWash
fault.
Therelocatedseismicity
on thePalmWashfault
correlates
with a discontinuity
in seismicreflectors
observed
on a reprocessed
seismic
reflection
profile
Petersenet al.: Seismicityo[ the San JacintoFault Zone
1193
33.5 N
33 N
o
116.5W
116W
.
10Km
,
:
116.5W
116W
Fig. 6. (a) Index map of the southernSanJacinto
faultzoneandSaltonSea. Linesrepresent
faults
after Rogers[ 1985]. Masterfaultsof the southern
San Jacinto fault zone are shown as heavier lines and
116.5 W
116 W
(c)
33.5 N
•
'
'
' ';.•.
'
_
"%
I
aredesignated
asCCF (CoyoteCreekfault),CF
(Clark fault), SFHF (SanFelipeHills fault), SMF
(Superstition
Mountainfault),andSHF (Superstition
Hills fault). Otherimportant
faultsdesignated
by
thinnerlinesincludeSAF (SanAndreasfault), EF
(Elsinorefault),BRF (BuckRidgefault)PWF (Palm
Washfault), andcrossfaultsIPF (InspirationPoint
fault) andERF (ElmoreRanchfault). The
abbreviation
VM represents
VallecitoMountains.AA', B-B', andC-C' denoteendpointsof cross
sections
shownin Figures9 and12;lengthof dashed
line indicates
widthof crosssection.(b) Map of
relocated
earthquakes
(1981-1986)shownasopen
circles.Earthquake
magnitude
scaleis in theupper
fightcomer.(c) Map of locations
of the 1968
BorregoMountainmainshock(solidcircles)andits
aftershocks
(opencircles)afterHamilton [1972].
Earthquakemagnitudescaleis in lower left comer.
Thethickestportionof linerepresenting
theCoyote
Creekfaultindicates
theobserved
surface
raptureof
the 1968earthquake
[Clark,1972].(d) Map of
locations
of the 1969CoyoteMountainearthquake
(solidcircles)andits aftershocks
(opencircles)after
ThatcherandHamilton[ 1973]. Earthquake
magnitudescaleis locatedin lower left comer.
Faultssamein all subfigures.
_
(line C3) publishedby Severson[ 1987]. However,
we couldnot find anyresearchthatdocuments
the
surfacetraceof the fault. Two smallfault segments
that strikenorthwesterlywere mapped[Rogers,
1985]just to the northwestof the concentrated
patternof seismicityandmay representa
33 N
northwestward
f
116.5W
116W
continuation of the Palm Wash fault.
In addition,thisfault is alignedalongstrikeof the
Buck Ridgefault, alsolocatedto thenorthwest
(Figure 6a).
1194
Petersenet al.: Seismicityof the SanJacintoFault Zone
(a)
EF
t• [
VM
(a)
CCF
SFHF
I
I
PWF
t
EF
c,
CCF
SFHF
PWF
o
o
• o
IO
30
20
40
II !
50
t)
m
10
I• I .......ø
20
30
40
50
SFHF
PWF
EF
o I••
VM
o
o)
EF
o
2(}
CCF
('t
.
VM
io
t
t
c'
•øø
ø •o••oo•
øo o
o'
•
0
l
20
30
40
oO
50
Fig. 7. CrosssectionC-C'. End pointsare shown
in Figure6. Horizontaldistanceis in kilometers.(a)
Selectedrelocatedearthquakes
from 1981 to 1986
usingHamilton [1970] model. (b) Sameearthquakes
asin upperfigurefrom the CIT/USGS catalog.
Earthquakemagnitudescalefor bothcrosssectionsis
in lower fight comer.
(a)
sc•s•E
,
20
0
I0
,._
20
30
,
40
......
1
,,
/
50
Fig. 9. CrosssectionC-C'. End pointsare shown
in Figure 6. Horizontaldistanceis in kilometers.(a)
Relocatedearthquakes
from 1981 to 1986 and (b)
1968 BorregoMountainhypocenter(solidcircle)and
aftershocks(opencircles). Both sectionsinclude
events located within 5 km of cross section line in
Figure 6a. Intersectionsof the Elsinorefault (EF),
VallecitoMountains(VM), CoyoteCreekfault
(CCF), SanFelipe Hills fault (SFHF), andthe Palm
Wash fault (PWF) with crosssectionline are
indicatedasarrowsabove. Earthquakemagnitude
scalefor both crosssectionsis in lower fight comer.
m 80
•
'
0
,
1
2
•
Activity on the PalmWashfault tendsto be
partitionedin spaceandtime. During early 1981,
just afterthe ML=5.5 Westmorland
earthquake,
activitywas high andconcentrated
over mostof the
4
southern Palm Wash fault. Between late 1981 to
HORIZONTAL ERROR (Km)
100-
m 80-
O
O4O
;
2O
0
1
2
3
4
5
VERTICAL ERROR (Km)
Fig. 8. Histogramsshowingprojected(a) horizontal
and (b) vertical standarderrors(from
HYPOINVERSE [Klein, 1978] of selectedrelocated
data from 1981 to 1986 shownin Figures6, 9, and
12.
1986, however,the rateof activitywasmuchlower
with mostseismicitylocatednorthof the swarming
activityin 1981. The activityto the southappearsto
have beenlargely shutoff afterthe earthquakeswarm
in early 1981. The spatialdistributionof the Palm
Wash eventssuggests,
however,that seismicityis
confinedto a singlefault thatstrikesnorthwest.
We alsoobserveda correlationbetweenthe timing
of largeregionaleventsandtherate of seismicityon
thePalmWashandSanFelipeHills faults.
Earthquakeepicenters
locatedwithina 0.2øx 0.1ø
area betweenthePalmWashandSanFelipeHills
faults,were extractedfrom the CIT/USGS seismicity
catalog. A histogramof the seismicitynearthePalm
WashandSanFelipeHills faults(Figure10a) shows
the total number of events M > 2.2 recorded as a
functionof time. Histogramswereproducedfor
eventsM > 2.2, becausewe believethe catalogis
completeat thismagnitudelevelbetween1979 and
1988andis not biasedby the stationdistribution.
We foundthatsignificant
peaksof activitybeginin
Petersen
et al.: Seismicit3,
of theSanJacintoFaultZone
(a)
6.6
I
M 5.5
Westmorland(ML=5.5), andthe November24,
1987,ElmoreRanchandSuperstition
Hills events
(ML=6.2 and6.6). Becausethe two earthquakes
in
M 6.0, 6.2
M4.4 M4.3
1987 occurred so close in time and are related
30
1•0
1•2
1984
1•6
1908
(b)
M 4.4
1195
M 4.3
tectonically,
we consider
theseeventsasa single
sequence
for thepurpose
of correlation
withchanges
in the level of seismicity.
The rateof seismicity
recordednearthePalmWash
andSanFelipeHills faultscorrelates
well with the
occurrence
of large(M > 5.5) regionalevents(Figure
10a). Eachof the threeregionalearthquake
sequences
is followedwithintwo monthsby a
significant
peakof activity. Giventhatwe havea
107monthsample,theprobabilityof havingone
earthquake
occurrandomlytwomonthspriorto the
oneof thefive significant
peaksof activitywouldbe
10/107. Thereforethejoint probabilitythatthethree
regionalearthquakes
wouldrandomlyoccurpriorto
thefive significant
peaksis evenlower,lessthan
1%. Thus we conclude that this correlation between
1
largeregionalearthquakes
andanincreased
periodof
seismicity
nearthePalmWashandSanFelipeHills
200-
faultsis significant.
Two of the five significantpeaksin thelocal
zlSO-
seismicityare,however,notcorrelated
with large
regionalmomentrelease.Moreover,theM=4.4 and
0100-
M=4.3 eventsin Figure 10a alsodo not appearto
increasethe seismicity(M > 2.2) on thePalmWash
0
1980
1982
1984
1•6
1988
Fig. 10. Histogramsshowingnumberof
earthquakes
from the CIT/USGS catalogbetween
1979 and 1988 in an (a) 0.2ø x 0.1ø arealocatednear
thePalmWashandSanFelipeHill faultsand(b)
or SanFelipeHills faults.A peakof activityonthe
PalmWashandSanFelipeHills faultstypically
beginsseveraldaysto a monthfollowingeach
regionalevent(M > 5.5) andpersists
for upto five
months.Thuswe suggestthattheincreasedrateof
0.1 o x 0.1 o area located near the Vallecito Mountains.
activityonthePalmWashandSanFelipeHillsfaults
followingregionaleventsis notcoincidental
butis
reflectingeithertheinteraction
betweenfaultsor
regionalstresschanges.
Arrowsabovehistogramsindicatedatesof regional
earthquakes
with largearrowsrepresenting
eventsof
SeismicityBeneaththe VallecitoMountains
M5 to M6 and smaller arrows events M4 to M5.
Dashed lines indicate two and three standard
deviationsfrom the averagebackgroundseismicity
andindicatepeaksthataresignificantlydifferentthan
the averageseismicity.
November 1979, November 1980, June 1981,
September1982, andJanuary1988 (Figure10a).
Thesepeaksaresignificantlyhigher thanthe
background
seismicityby two standard
deviations
whenusingall thedataor threestandard
deviations
wheneliminatingthehighestpeakin 1981 (Figure
10a). We calculatean averagerate of 1.2 eventsper
monthoverthe 10 yeartime periodfor eventswith M
> 2.2.
Regionaleventsof M > 4 werealsoextracted
from
thecatalogwith thefollowingcriteria:4 < M < 5
events within a circle of 30 km radius, 5 < M < 6
events within a circle of 40 km, and M > 6 events
within a 60 km radius from the center of the Palm
Washfault. Threeregionalearthquakes
of M > 5 fit
the distance criteria described in the time interval
between 1979 and 1988' the October 15, 1979,
ImperialValley (ML=6.6), theApril 28, 1981,
An intenseburstof seismicity(Figure6b) occurred
in late 1983 beneaththe Vallecito Mountains(VM in
Figure6a). Relocatedhypocenters
of this swarm
definea zonethatis narrowhorizontallybut stretches
about10 km in depth,primarilybetween5 and 15
km (Figure9a). This seismicityis centerednearthe
intersection
of mappednortheasttrendingcrossfaults
andnorthwesttrendingfaultsthatareoriented
subparallelto the masterfaults(Figure6a). Focal
mechanisms
thatwe producedindicatea prevalence
of strike-slipmotionon planesparallelto eitherthe
crossfaultsor the northwesttrendingfaults. Stress
concentration at the intersection of the cross faults
andnorthwesttrendingfaultsmay accountfor this
fightclusterandfor its verticalelongation.
The VallecitoMountainclusterbecameactiveprior
to nearbylargeor moderateeventson the Coyote
Creek fault andmay thereforebe a useful short-or
intermediate-term
precursor
to largeearthquakes
in
thisarea.A histogram(Figure10b)indicatestherate
of seismicityasa functionof time in a 0.1øx 0.1ø
area centered over the Vallecito Mountain cluster.
About 200 events were recorded in one month
1196
Petersenet al.' Seismicityof theSanJacintoFaultZone
beneaththe Vallecito Mountains; none of thesehad M
1969 IPF
AI
> 4 (Figure 10b). Nearly a monthfollowingthe
VallecitoMountainearthquakeswarm,a ML=4.3
eventoccurrednearthe CoyoteCreekfault. This
eventwas the only M > 4 eventto occurnearthe
CoyoteCreekfault between1981and 1986 (Figure
11). The proximityandtimingof the swarmandthe
CoyoteCreekeventindicatethattheseeventsmay be
related. The ML=4.4 eventof 1982 in Figure 10b
was the largesteventin that smallerswarm. That
swarmdoesnot appearto be relatedto a M > 4 event
(a)
I
A'
Ooo
.
I
....
l
oø
ø vø
.
I0
I
. . _1
20
30
....
!
....
40
I ........
I__
50
t
60
outside of the swarm area.
A burstof earthquake
activitywasalsoreported
beneaththe VallecitoMountainspriorto the 1968
event(Figure 11) [SandersandKanamori,1984].
Thustwo periodsof increasedseismicitypreceded
moderateto largeeventson themasterCoyoteCreek
fault. This activitymayberelatedto crossfault
interactionwith themasterfault,or it may simplybe
relatedto a regionalchangein mechanical
conditions
suchas stressor fluid pressure.However,one must
be carefulaboutdrawingconclusions
aboutseismic
precursorsfrom seismicitythatspansonly a short
{)
2½)
1969
time.
IPF
SeismicityAlong the CoyoteCreekFault
Seismicityalongthemasterfault strandthat
rapturedin 1968is currentlylow above8 km depth
andindicatesthatmostof the fault is locked(Figure
12b). Only oneof ourrelocatedeventswith M > 4
r"
• .......
•
,......
ø øø
•0
,
.
20
_ •........
s0
•.......
40
,..'
.... •_•
50
60
Fig. 12. CrosssectionA-A' •d B-B'. End points
•e shown • Fi•e 6. Hodzont• •st•ce is in
hlometers.
Sections include events 5 •
section•ne. •ows
•om cross
desi•ate probable•tersecfion
of crosshults. (a) Cross sectionA-A'. 1968
mainshockandaftershocks
(openckcles)•d •e
1969 m•nshock and aftershock(sold ckcles). •ft
33.2N
•ow
SCALE
MI
M3
ß
•
33 N
115.3 W
•cates
l•afion
of infe•ed
cross fault active
in the 1969 event;right •ow showslocationof the
mappedInspkationPo•t fauk (•F) •m b•me
activefollowingthe 1968 event.(b) CrosssectionAA'. Relocatede•qu•es
from 1981 m 1986 ne•
the CoyoteCreekfault. •ows •e sine asin
Figure 12a. (c) CrosssectionB-B'. Rel•ated
e•qu•es
•om 1981 to 1986ne• •e Cl•k and
SanFe•pe Hills faults. E•hqu•e •g•mde sc•e
for all crosssectionsis in thelowerrightcomer.
115 W
Fig. 11. Mapof CoyoteCreekfault(CCF)and
VallecitoMountainarea(VM) showingthe
relationship
of theswarmin December
1983andthe
M=4.3 eventin February1984ontheCoyoteCreek
fault(solidcircle).Dashedlineindicates
locations
of
preshocks
tothe1968event[Sanders
andKanamori,
1984]. Insetshowsfirstmotionfocalmechanism
of
the1984event.Magnitudescaleis in upperleft
comerof map. FaultsaresameasFigure6.
wasmostlikely locatedon theCoyoteCreekfault
(ML=4.3 in February1984; seeFigure 11). Unlike
thePalmWashandSanFelipeHills faults,the
CoyoteCreekfaultdoesnotappearto be stimulated
by regionalstressrelease.The 1984 eventwas
located about 10 km southeast of the 1968
hypocenter
andwasprecededby activitybeneaththe
Vallecito Mountains(describedabove). The focal
mechanism
we obtainedfor theeventsuggests
rightlateralstrike-slipmotion,with a largecomponent
of
normalfaultingdownto theeaston theplaneparallel
to the CoyoteCreekfault (Figure11). This normal
faultingcomponent
of theearthquake
mechanism
Petersenet al.: Seismicityof theSanJacintoFault Zone
may be relatedto the nearbyextensional
jog in the
CoyoteCreek fault.
Althoughthe 1984 ML=4.3 eventoccurredat a
depthof only 6 km, background
activitynearthe
CoyoteCreekfault is primarilyrestrictedto depths
between8 and 10 km nearthe 1968epicenterand
below 10 km to the northwestof thatearthquake
(Figures12a and 12b). In fact, this background
seismicitybecomes2 to 4 km deeperfrom southto
north,plungingabout3øto thenorth. The few
eventsfrom 1981 to 1986 thatappearto be located
on themasterfault areclusteredanddo notclearly
delineate the fault as did aftershocks of the 1968
1197
stresses
thatwerenot releasedduringthe 1954 event
may be slowlyreleasedby largeregionalevents.
Seismicityon CrossFaults
Combinedand separateSPOT andLandsatimages
indicatethatnortheasterlystrikingcrossfaultsare a
moreprominentfeatureof thisregionthanhas
previouslybeenappreciated[Rockwell,et al., 1990].
Many of thesecrossfaultsare seismicallyactiveand
someareknownto haverapturedin largeevents.
Probablythe bestdocumented
exampleof cross-fault
activityin southernCaliforniais the 1987raptureon
event. Indeed,the seismicityfrom 1981 to 1986 is
the Elmore Ranch fault [Hudnut et al., 1989b].
not located where aftershocks of the 1968 event
Doser [ 1990] completeda bodywaveforminversion
of the 1942 earthquakeandits largestaftershockand
concludedthattheseeventsmay alsohaveoccurred
on noaheasttrendingcrossfaults(Figure 1).
The relocatedseismicityindicatesthatcrossfaults
areactive,especiallyin the southern
partof the San
occurredbut insteaddefinesa ring aboutthe
aftershockzone (Figures12a and 12b).
Abundantmicroseismicity
may indicateportionsof
the fault thatdo notrapturein largeearthquakes,
and
areasthatlack thisbackground
seismicitymay
indicateregionsthatarecapableof rapturein large
events. This patternwasobservedin the Loma
Prietasegmentof the SanAndreasfault [Plafkerand
Galloway, 1989] andalongthe Calaverasfault
[Oppenheimer,et al., 1990]. While Louie et al.
[1985] note,however,that the CoyoteCreekfault
may be creepingaseismically
at the surface,we agree
with HudnutandClark [ 1989] thatthiscreepis
continuedafterslipof the 1968eventandinvolves
only the uppermostkilometerof sediments.
Thereforethe seismicityrateandpatternindicatethat
the segmentof the CoyoteCreekfault thatraptured
in 1968 is currentlylockedandbuildingup stress.
Seismicity
AlongtheClark andSanFelipeHills
Faults
The relocatedseismicityis concentrated
alongthe
northwestern
portionof the SanJacintofault zone
from 1981 to 1986. This seismicityis scattered
betweenthe masterstrands(Figure 6b). The
seismicityoccursbelow 10 km (Figure12c)and
deepensto thenorth,similarto thatobservedalong
the CoyoteCreekfault (Figure12b). In fact,
seismicityabove10 km tendsto occuron cross
faults. A crosssectionB-B' (Figures12b and 12c)
indicates that two cross faults were active from 1981
to 1986 between the San Jacinto master faults. Thus
muchof theinterseismic
activitymay occuron either
cross faults located between the two master strands
or below 10 km on the master fault. The sections of
thesemasterfaultsthatlack seismicityabove10 km
probablyarelocked.
Seismicityon the southern
SanFelipeHills fault
appearsto correlatewith largeregionalmoment
release,similarto activityon theadjacentPalmWash
fault (describedabove). Eventsin 1979, 1981, and
1987inducedactivityon the sectionof the faultthat
rapturedin the 1954earthquake
asdefinedby
Sanderset al. [ 1986]. In thisportionof the San
FelipeHills fault (between30 and45 km in Figure
12c),naturallyinducedseismicityis locatedin a
clusterbetweendepthsof 5 and 13 km. Residual
Jacinto fault zone. Close observation of the relocated
seismicityin thiszonerevealsthatmanyeventsare
alignedalongnortheasttrends. CrosssectionsA-A'
(alongthe CoyoteCreekfault) andB-B' (alongthe
Clark andSanFelipeHills faults)indicate
intersections of two cross faults and the master faults
(Figures12band 12c). Arrowsabovecrosssections
in Figure12 indicatethelocationsof intersections
of
boththe crossfault thatmay havecausedthe 1969
raptureandtheInspirationPointfaultthatrapturedin
conjunction
with the 1968event. Seismicityabove
10 km depthis alignedalonga crossfault (seeleft
arrowmarked1969onFigure12) thatmayhave
causedthe 1969event. In additionthemapped
InspirationPointfault (seefight arrowmarkedIPF
on Figure 12) appearsto havebeenactivebetween
1981and1986,although
thisactivityis notasclearly
alignedalonga singlefault. Thusmuchof therecent
seismicityabove 10 km occurson crossfaults.
1968 and 1969 Aftershocks
The 1968 aftershockslocatedby Hamilton [ 1972]
andthe 1969 aftershocks
locatedby Thatcherand
Hamilton[1973] indicaterapturenotonly on master
faultsbut alsoon severalsecondaryfaults:the Palm
Washfault, the InspirationPointfault, andfaults
locatedneartheVallecitoMountains(Figure6a, 6c,
and6d). Das andScholz[ 1981] proposedthatthe
stress field associated with the occurrence of the
1968 event could induce the observed aftershocks on
the Palm Wash fault and near the Vallecito
Mountains.
Aftershocks of the 1968 event delineate the
structureof the CoyoteCreekfault. Theseevents
trendabout315ø,consistent
with surfacerapture
[Clark, 1972], anddip about85ø to the noaheast
(Figure9). Earthquakesextendedfrom the
mainshockat a depthof about10 km up to the
surface. Aftershocks
near the northern end of the
1968 rapturetrendnortheastand are locatedneara
mappedcrossfault, the InspirationPointfault
(Figures6a and 6c). The few earthquakes
located
1198
Petersenet al.: Seismicityof the SanJacintoFaultZone
northof thiscrossfault (crosssectionA-A', Figure
12a)weredeeperthaneventslocatedto the south.
The InspirationPointfault thatwasactiveat the
northernendof the 1968rapturezonemay have
created a barrier that controlled the extent of that
rapture[SeeberandNicholson,1986].
Our inversionresultsfor the 1969 earthquake
indicatethatthemasterfault is significantly
misorientedwith respectto thenoahwestnodalplane
of the focal mechanism.
Thus the event was
probablycausedinsteadby rapturealonga northeast
crossfault. The 1969 aftershocks
were significantly
deeperthanthe 1968 sequence[Thatcherand
Hamilton, 1973] andoccurredprimarilynorthof the
InspirationPointfault thatmay havecontrolledthe
rapturein 1968 (Figures6d and 12a). These
aftershocks were scattered between the two master
faults(Figure6d), similarto thepatternof recent
earthquakes
locatedin the samearea(Figure6b).
Thusthe 1969raptureappearsto haveinvolvedslip
on a crossfault, 13erha13s
as staticfatiguefollowing
the 1968 shockreducedthe strengthof its barrieror
asperity.
SEISMIC
HAZARD
Potentialof CrossFaults to GenerateSignificant
Earthquakes
Many crossfaultsaremappedwestof the Salton
Sea (Figure6a). Severalcrossfaultsare mapped
between the two master strands of the San Jacinto
fault zone; other crossfaults are known outsideof
thisfault zone. Seismicityhasbeenassociated
with
manyof thesecrossfaults. Crossfaultscould
rapturein largeearthquakes,
andthoseeventscould
triggerearthquakes
on adjacentmasterfaults,similar
to the 1987 sequence.Somelargeearthquakes
occurredon previouslyunrecognized
crossfaults.
The Elmore Ranchfault thatrapturedin 1987 (Figure
6a) andthe crossfault thatrapturedduringthe 1981
Westmorlandearthquakeare examplesof suchfaults.
Many otherasyet unmappedcrossfaultsmay be
capableof producingsignificantearthquakes.
Severalcrossfaultscouldrapturein M > 5
earthquakes.Hudnutet al. [1989a] suggestthatthe
Extra fault, a crossfault that is located between the
San Andreas and San Jacinto faults, could itself
rapturein a largeeventor couldtriggerthe
occurrence
of largeeventson oneof thenearby
master faults. Other cross faults with seismic
potentialarelocatednearthe VallecitoMountains,
just northof theElmoreRanchfault, andbetweenthe
masterfaults(Figure6a). Thesecrossfaultscan
rapturein moderateto largeeventsandshouldbe
considered in seismic hazard evaluations.
SeismicGapsAlongMasterFault Strands
Two typesof seismicgapsare situatedalongthe
southern San Jacinto fault zone. We observe
patternsof seismicityandquiescence
between1981
and1986thatallowdefinitionof theseseismicgaps.
The f'u:sttypeof gapis characterized
by fault
segments
thathavenotrapturedin historic
earthquakes
buthaveabundant
microseismicity
near
a depthof 10km andminimalseismicity
abovea
depthof 10 km. The secondtypeof gapincludes
segments
thathavelow microseismicity
throughout
theseismogenic
zone. Bothtypesof gapsarelocated
on or betweenmappedfaultsandhaveadjacent
segments
that haverapturedin historiclargeevents.
The firsttypeof gapoccursalongmasterfaults
locatednoahwestof the 1968rapturezone. The
CoyoteCreek,SanFelipeHills, andClark faults
(Figure6a) all exhibitthistypeof seismicbehavior.
Recentseismicitybetween1981 and 1986,
interseismic
activity,is abundantalongmasterfaults
(Figure 6b). Most of this seismicityis, however,
locatedat or belowabout10 km depth(Figures12b
and 12c);theupper10 km is primarilyquiescent.
Sincewe know thata largeearthquakeoccurredin
1968alongthe CoyoteCreekfault andraptured
mostlyabovea depthof 10 km, we caninfer that
largeearthquakes
mayrapturethefault abovethe
deepcharacteristic
interseismic
activity. Hencethose
portionsof themasterfaultthatarequiescent
during
interseismic
periodsmaybe capableof breakingin
largeearthquakes
eventhoughtheremay be abundant
seismicityat depth.Usingthisanalogywe identify
two segments
alongtheCoyoteCreekandClark
faultsthatmay be capableof rapturingin large
events. Thesegapsareshownasdashedlinesin
Figure 1.
We suggestthatthe 1969 eventwasdeepandmay
haveevenrupturedalonga crossfault. Thereforea
gapalongthe CoyoteCreekfault wouldextendfrom
thenorthernendof the 1968raptureto the
intersection
of the CoyoteCreekandClark faults
(Figure 1). This gapwould includemuchof the
southemportionof theAnza gapasoriginally
definedby Thatcheret al. [1975] (Figure1). The
gapalongthe Clarkfaultis not asclearlydefined.
The gapextendssouthto the 1954 rapturebutmay
extendas far northasthe 1980 (ML=5.5) raptureor
possiblyeventhroughtheAnza gap (Figure1)
[Sanderset al., 1986]. The sizeof this gapdepends
on whetheror not the 1937 and 1980 earthquakes
releasedstresson theBuck Ridge,Clark, or cross
faults. We haveshowna conservative
gapestimate
in Figure1 assuming
thatthe 1937eventoccurredon
the Clark fault.
Seismicquiescence
definesour secondtypeof
seismicgapandoccursalongmasterfaultslocated
primarilysouthof the 1968rapturezone. Although
northwesttrendingmasterfaultsare mapped,
minimalmicroseismicity
occursalongthese
segments.The Superstition
Hills, Superstition
Mountain faults, and the area betweenthe
Superstition
Hills andSanFelipeHills faultsare
examples
of segments
thatexhibitthistypeof
seismicbehavior.The BuckRidgefault (located
northof the 1968rupture)alsoexhibitslow
seismicity
between1981and1986. Thesequiescent
faultsare,however,knownto be capableof
producing
largeearthquakes
(e.g.,the 1987
Superstition
Hills event).Thussuchquiescent
Petersenet al.: Seismicityof the SanJacintoFault Zone
regionsthatarealignedalongnorthwesttrending
faultsor alongstrikeof thesefaultsmayhave
considerable
seismichazard. We identifyonegap
locatedbetweenthe 1954 and 1987rupturezones
thatmay be capableof generatinga largeearthquake
(Figure 1).
FOCAL
MECHANISMS
AND
TECTONICS
We computefocalmechanisms
for many
earthquakes
in the southernSanJacintofault zone
and adjacentfaults. Thesemechanismsallow us to
studythedetailedkinematicsof earthquakes
on the
masterand secondary
faults. The abundance
of
short-periodphasedatafrom the CIT/USGS network
allowsmanyfault planesolutionsto beproduced,
revealingdetailsof faulting. SeeberandArmbruster
[ 1988] developeda grid searchmethodfor
computingfn:st-motion
focalmechanisms.We
computedseveralhundredfocal mechanisms
for the
earthquakes
locatedwith theHamilton [ 1970]
velocitymodel. To obtainthe bestpossible
mechanisms,
only thoseeventsthatweredeeperthan
5 km andpossessed
greaterthan 10 weightedarrivals
were considered.Many of thesecomputed
mechanisms
wererejecteddueto inconsistencies
in
polaritieswithin focal quadrants.
While a few of the individualsolutionsmay be
poorlyconstrained,
themajorityof the solutions
allow only smallchangesin theorientationof focal
planes. Moreover,the largequantityof solutions
permitsa statisticalanalysisof the results.Not only
were earthquakephasedatafrom the relocatedevents
computedusingthe grid searchalgorithm,many
werealsocheckedvisually. The grid searchmethod
15'
P AXES
1199
gaveresultssimilarto analysisof individualevents.
The compressional
(P) and tensional(T) axesof
the solutionsare separately
plottedin Figure 13 to
showregionswhereeachof thedifferentfault styles
may be active. The regionwestof the SaltonSea
appearsto be structurallycomplex,with events
havingstrike-slip,thrust,andnormal-faulting
mechanisms.Most of themechanisms
suggestlarge
strike-slipcomponents
of motion;P andT axesare
nearlyhorizontal(Figure 13). Solutionsfor events
on the CoyoteCreek, SanFelipe Hills, Palm Wash,
and Elsinore faults as well as beneath the Vallecito
Mountainsall havelargestrike-slipcomponents.
Normal faultingis suggested
by mechanisms
of
eventsin a smallclusterjust 10 km southeast
of the
Vallecito Mountains(seesteeplydippingP axes
indicatedby short-linesegments
in Figure 13).
Many mechanisms
arepredonfinantly
strikeslipbut
involve sizablecomponents
of thrustor normal
faultingmotion. The SaltonTroughis an extensional
basin,andnormalmechanisms
would be expected.
In fact, a simpleextensional
tectonicframework
wouldpredictmorenormalfault mechanisms
than
were observedin the data. Compressional
and
tensionalaxesareplottedin Figure 14 andindicate
that more P axes than T axes are vertical.
This result
suggests
a predominance
of normalfaultingwith
respectto thrustfaulting. We alsoobtainonly a few
mechanisms
thatincludenodalplanesthat suggest
detachmentfaulting.
All compressional
andtensionalaxesareconsistent
with a singlestressfield orientedaboutnorth-south
(Figure 14). This resultis consistentwith northsouthregional stressindicators[Jones,1988;
Sanders,1990]. The hemisphereprojectioncan be
15'
116 ø
ß
'
33•
.
T AXES
116 ø
'x%
•
Fig. 13.Mapof southern
SanJacinto
faultzonewithfaults(dottedlines)afterRogers
[ 1985]. Compressional
axes(P axes,left figure)andtensional
axes(T axes,fightfigure)
arefrommechanisms
determined
in thisstudy.Lengthof lineis proportional
to cosineof
dip;i.e., horizontaldip hasa longerlength,andnearverticalhasa shorterlength.
I
33
ø
1200
Petersenet al.: Seismicityof theSanJaclntoFaultZone
Major burstsof earthquakes
wereobservedalong
secondary
faultsbothpriorto andaftermoderateto
largeregionalevents.Earthquakeswarmswere
observedbetween2 yearsanda few monthsprior to
the 1937, 1954, and 1968 events [Sandersand
Kanamori,1984; Sanderset al., 1986]. Activity
beneaththeVallecitoMountainsalsooccurredprior
to the 1968 and 1984 events. These small
earthquakes
may be actingasa kind of meter
signalinga rise in stresses,
similarto a persistent
sourceof swarmslocatedneartheAnza gap,the socalled Cahuilla swarm [Sandersand Kanamori,
1984]. Seismic moment releasehasbeen found to
'\T_$T:--
n:'
•-
:>
/
increaseover a wide areapriorto largeearthquakes
in northernCalifornia[SykesandJaum6,1990].
Henceforerunningactivityoccursnot only on one
fault, but overa largeregion. An earthquakeswarm
suchasthatneartheVallecitoMountainsmay simply
be respondingto a regionalstressincrease.
Mechanisms
Fig. 14. P andT axesof eventsin Figure 13
projectedon upperhemisphere
equal-areaprojection.
Dash mark next to each P axis indicates direction to T
axislocated90øawayandviseversa.
neatlyseparated
into fourquadrants
with P andT
axesdistributedthroughouteachof the quadrants.
Sincemuchof therelocatedseismicityoccurson
secondary
faults,we assumethatour mechanisms
arefrom earthquakes
locatedon a wideorientation
of
faultsandnot simplyfrom eventslocatedalongthe
San Jacinto master faults. Thus the P and T
orientations
on thisplot aretakento be indicativeof
an averageregionalstresswith maximum
compression
orientedaboutnorth-southandleast
compression
orientedabouteast-west.
Even thoughthe orientations
of P andT axesare
locallyconsistent,
compressional
axesof earthquakes
located in the Vallecito Mountain cluster differ from
axes of events situated on the Palm Wash fault
(Figure13). EventslocatedbeneaththeVallecito
Mountain have P axes with a mean trend of 7 ø
azimuthand 8øplunge,whereasP axesof events
situated on the Palm Wash fault have a mean azimuth
of 352ø anda plungeof 5ø. The null hypothesis
of
sameness
of the two populationscanbe rejectedat
the 98% level by Fisherstatistics[Fisher,1953;
McFadden and Lowes, 1981]. Thus axes from
events located on the Palm Wash fault are rotated
with respectto thosebeneaththeVallecito
Mountains.
The events in these two clusters must
haveoccurredon faultsthatdiffer significantlyin
orientation.Perhapsmanyof theeventsbeneaththe
VallecitoMountainsarerapturing'alongcrossfaults
thathavebeenmappedin the area. Alternatively,
unusualfluid pressures
may be presentat thatcluster
thatleadto differenteffective(intergranular)
stresses.
DISCUSSION
for the interaction that leads to
concentrations
of stressin bothspaceandtime may
involvecrossfaults,detachments,
or porefluid
diffusion.Largeearthquakes
mayinduceseismicity
on faultslocatedat considerable
distance(Figure
10a). The Palm Wash andSanFelipeHills faultsare
locatedabouttwo rapturelengthsawayfrom boththe
1979 and 1981 rapturezones. However we found a
temporalrelationshipbetweenthe 1979raptureand
activitylocatedabout60 km northon thePalmWash
andSanFelipe Hills faults.
Four mechanisms
havebeenproposedto explain
loadtransferthatmay induceseismicityon thePalm
Washand SanFelipeHills faults. One mechanism
suggests
that staticstrainor stresschangesfrom an
earthquakecaninduceseismicityon othernearby
faults (Das and Scholz, 1981). A secondmechanism
impliesthatstrainmaypropagate
in theviscoelastic
layerof the crestby spatialdiffusion[Knopoff,
1989]. In a third mechanism,detachmentfaultsmay
transmitstrainby elasticcreep(G.C. Bondet al.,
unpublishedmanuscript,1990). In a fourth
mechanism,increasedporefluid pressures
canlower
theeffectivestresson thefaultandpromotefailure
[Simpson,et al., 1988; Hudnutet al., 1989a].
Staticstrainchangesassociated
with regional
energyreleasearecapableof inducingseismicity
on
nearbyfaults. We testedwhetheror not staticstrain
changesfrom the 1979ImperialValley earthquake
wouldbe sufficientto induceactivitynearthePalm
Washor SanFelipeHills faultslocatedabout60 km
northof thatrapture.The 1979eventwasmodeled
with a 1 m dislocation12 km wide and33 km long
[Savage,1980]. The staticstrainchangesnearthe
PalmWashfaultresultingfromthisdislocation
areof
tidalorder(1 to 2 x 10-7).Thusthe1979earthquake
couldprobablynotinduceactivityin thedistance
rangeof thePalmWashfaultfrom simplestatic
strainchanges.It 'alsoseemsunlikelythatthese
strainscouldcausesignificantchangesin fluid
pressures
at thatdistanceandtherebychangethe
effective stress on faults. Thus mechanisms for
Faultactivityto thewestof theSaltonSeainvolves
complexdynamicsandcouplingbetweenfaults.
spatialdiffusionof porefluidsor staticstraincaused
by a largeregionaleventdo notappearto be viable
Petersenet al.: Seisnficityof the SanJacintoFault Zone
mechanisms
for inducingthe observedrisein
seismicityat thePalmWashor the SanFelipeHills
faults.
Strainrelaxationin a midcrustalviscouslayeris
anothermechanismthatmay induceseismicity.
Knopoff [ 1989] suggests
that stresses
beneatha
fractureareredistributed
by localrelaxationandby
spatialdiffusionin the subseismogenic
layer. This
mechanismhasthe advantageof explaininga time
delaybetweentheregionaleventandtheinduced
seismicity.We modeledthetimedelayobserved
from the seismicityby calculatingtherelaxationtime
for a Maxwell viscoelasticmaterial. Using upperand
lowerbounds
of 1018-1020
Pa s for viscosity
of the
uppermostasthenosphere
inferredfrom glacial
rebound[Walcott,1973]and4-7 x 10løPa for
Young'smodulusinferredfor characteristic
asthenosphere
lithology[Turcotteand Schubert,
1982] we obtain a minimum viscoelasticrelaxation
time of about 1 year. Thus,unlessthe viscosityis
significantlylowerthanthevaluethatwe used,the
relaxationtime is muchlongerthanthemonthtime
delaythatwe observe.Thereforewe wouldnot
expecta viscoelastic
diffusionmechanismto have
propagatedthe load over a 60 km distancein sucha
short time.
Anothermechanismthatcouldinduceregional
activityis slip alonga subhorizontal
detachment
fault. The secondsubeventof the 1968 earthquake
waveforminversionhada nodalplanethatwas
consistent with either a subvertical or detachment
fault. It releasednearlya thirdof themomentof the
first subeventon the CoyoteCreekfault. The
locationof this subeventis poorlyconstrained
and
thereforedifficult to associate
with a specificfault.
The nodalplanethatcorresponds
with thefault that
rapturedin the secondsubeventis notclearfrom the
waveform
inversion.
The sourcedurationandmagnitudedeterminations
suggest,however,thata detachment
fatfit may have
rapturedin the 1968 earthquake.The subevent
rapturedratherslowlyover a 12 s duration(see
sourcetime function,Figure2). Sucha slowrapture
maybemorecharacteristic
of detachmentthan
reversefaulting. We estimatea momentmagnitude,
includingboththe first andsecondsubevents,
of 6.6
from our body wave inversionof the 1968 event.
This magnitudeis consistent
with a bodywave
magnitude(Mb) of 6.6 but it is quitedifferentfrom
the surfacewave magnitude(Ms) of 7.0; boththese
valueswere reportedby Abe [1981]. Perhapslongperiodenergywasradiatedslowlyfrom a detachment
fault, causingthe long-periodsurfacewavesto be
relativelylargerthantheshort-period
bodywaves.
This slowradiation,rich in long-periodenergy,is
consistent
with rapturealonga detachment
fault.
Detachmentfaultsarenot observedto be seismically
active and are therefore often inferred to move
aseismically.
The Borregoareais characterized
by eastdipping
Miocenedetachments
anda metamorphic
core
complex[Blom et al., 1988;Engel and Schultejann,
1984]. G.C. Bond et al. (unpublishedmanuscript,
1201
1990) suggested
from geologicalandgeophysical
datathat the SaltonTroughbasinmay havebeen
formedby a simpleshear(detachment)
mechanism
ratherthanby pureshear,the morecommonly
assumed
process.If theyarecorrect,theinferred
southeast
dippingdetachment
fault systemprobably
extended into the Palm Wash area and could have
beenreactivatedduringthe 1968event. Detachments
arealsoan importantelementfor blockrotation,such
asthatinferredfrom paleomagnetic
datanearthe
InspirationPointfault [ScheuingandSeeber,1989].
Only a few mechanisms
thathada nodalplane
consistent
with detachmentfaultingwerecomputed
in our study. However,Nicholsonet al. (1986)
found focal mechanisms that were consistent with
detachmentfaultingto the northin the Transverse
Ranges. In addition,P.A. Cowie andL. Seeber
(unpublished
manuscript,1991) usegravitydatato
modela normalcomponent
of faultingon thecross
faults. From thisdata,they modela detachment
systemthatextendsthroughtheBorregoarea,
probablybelowthe seismogenic
zone. Thuswe
prefera detachment
mechanism
for generating
inducedactivityon thePalmWashandSanFelipe
Hills faultsat relativelygreatdistances
fromlarge
regionalevents.
CONCLUSIONS
Seismicitypatterns,waveforminversions,and
focalmechanisms
indicatethatmanyrecent
earthquakes
in andnearthe SanJacintofault zone
occurredon secondaryfaults. The waveform
inversion of the 1969 event and aftershocks of the
1968 and 1969 eventsindicatethatnortheaststriking
cross faults are active west of the Salton Sea. Two
crossfatfitswere recentlyactivein the northwestSan
Jacintofault zone:the InspirationPointfault thatwas
alsoactivein conjunctionwith the 1968eventandthe
otherthatwasmostlikely activeduringthe 1969
earthquake.Suchcrossfaultsmay controlthe
characteristic
lengthof theraptureon a masterfault
(e.g.,the 1968earthquake);
theymayalsotrigger
eventson a masterfault (e.g., the 1987 events).
Thuscrossfaultsmay be an importantcomponent
of
theraptureprocessof largeeventsalongmaster
faults.
Secondaryfaultsarealsocapableof producing
earthquakes
of largeseismicmoment.Our
waveformanalysisof the secondsubevent
of the
1968 eventindicatesthatslip alonga reverseor
detachment
fault radiatednearlya thirdof thetotal
momentof the BorregoMountainevent. The 1969
event(ML=5.8) mayhaveraptureda northeast
trendingleft-lateralcrossfaultratherthana northwest
strikingmasterfault. Our evidencefor this
hypothesis
is first,thestatistically
significant
mislocationof thenodalplanein ourwaveform
inversionwith respectto themasterfaultorientation.
Second,the aftershockpatternis scattered
andnot
simplylocatedon themasterfault. Third,recent
crossfault rapturesin theregion(e.g.,the 1987
1202
Petersenet al.: Seismicityof theSanJacintoFaultZone
earthquakes)
indicatethatonecannotassume(a
priori) thata rupturenearthemasterfaultis located
on thatfault. Secondaryfaultsareimportantin
seismichazardassessment
in thisregion.
We usedthe velocitymodelof Hamilton [1970] to
relocateseismicitybetween1981 and 1986 in the
studyarea. This modelappearsto be a good
approximation
of thevelocitystructure
nearBorrego
Mountain. The relocatedseismicityallowsfiner
resolutionof structures
thanis possibleusingcatalog
locations.
We named and characterized
seismogenetically
thePalm Washfaultthatstrikes
noahwestsubparallel
to themasterfaults,dips70øto
the east,andslipsfight laterally. Regionalmoderate
to largeeventswerecorrelatedwith significantpeaks
of activityon thePalm WashandSanFelipeHills
faults. We alsocharacterized
a clusterof earthquakes
beneath the Vallecito Mountains that occmxext in late
1983. The earthquakeswarmoccurredonemonth
prior to a moderatesizeeventon theCoyoteCreek
fault. This sameareawas alsoactivepriorto the
1968 earthquake.Thusthe swarmbeneaththe
VallecitoMountainsmay be a short-to intermediatetermprecursorto activityon the CoyoteCreekfault.
Recentseismicity(1981-1986)thatoccurson the
CoyoteCreekfault is distributedaroundthe
aftershocksof the 1968 event. Currentbackground
seismicityassociated
with theCoyoteCreek,Clark,
andSanFelipeHills faultsdeepensto thenoah and
is primarilylocateddee_per
than 10 km. This
characteristic
deepseismicitybelow10 km andlack
of seismicityat shallowerdepthsmostlikely indicate
thatthemasterfaultsarepresentlylockedabove10
km and are currentlybuildingu_pstress.Comparison
of the 1968aftershocksequence
andrecent
seismicityindicatethatactivitybeneath10 km depth
occursduringtheinterseismic
periodandthatlarge
earthquakes
rupturethe fault abovethatlevel. Using
thisanalogywith otherportionsof nearbymaster
faults,we find thatseismicgapsarelocatedalongthe
northwestCoyoteCreekfault aswell asalongtwo
segmentsof the Clark andSanFelipeHills faults.
Severalidentifiedcrossfaultsaswell asmany
unidentifiedfaultsmay alsorupturein large
earthquakes.
Compressional
andtensionalaxesfrom
mechanisms of events located west of the Salton Sea
indicatea predominance
of strike-slipmotionanda
north-south
orientation
of theaverageregional
maximumcompressive
stress.This motionis
consistent with transverse motion between the Pacific
andNorthAmericanplates. Althoughthelocationof
the studyareawithinthe SaltonTroughwouldalso
imply a largeamountof extension,only onecluster
of eventslocatedjust outsidethe SanJacintofault
zonehasmechanisms
thatsuggest
normalfaulting.
Moreover,althoughwe havereasonto believethat
detachment
faultsareactivein theBorregoarea,few
computedmechanisms
havea nodalplanethatis
consistent
with detachment
faulting.
The interactionof secondary
faultsandmaster
strandsappearsto be of primeimportancein
understanding
earthquake
nucleationprocesses
in
southernCalifornia.Earthquakeswarmson
secondary
faultsthatoccurpriorto largeeventson
masterfaultsmay be an importantprecursory
signal
in thisandothertectonically
complexregions.Other
swarmsthatoccurafterlargeregionalevents
probablyind/catecomplexcouplingandinteraction
of faults,possiblyon detachments.Thussecondary
faultsmaybe animportantaspectof earthquake
hazardassessment,
fault segmentation,
and
earthquake
prediction.
Acknowledgments.
This paperbenefittedgreatly
by discussions
with JohnTaber, JohnBeavan,Paul
Huang,Cliff Thurber,Micky Van FossenandKen
Howard. We alsothankDiane Doserfor herpreprint
andK. Nagaofor draftinga figure. Criticalreviews
by Diane Doser,Allison Bent, C.H. ScholzandG.
Bondimprovedthemanuscriptsubstantially.The
staffof the CaltechSeismologyLaboratoryand
USGS (Pasadena)
arealsothankedfor providingthe
microearthquake
data. This work wasfundedby
USGS grant14-08-001-G948.Lamont-Doherty
GeologicalObservatory
contribution4829.
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(ReceivedNovember28, 1990;
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