GEOPHYSICAL
RESEARCH LETTERS, VOL. 22, NO. 1, PAGES 13-16, JANUARY
1, 1995
Moment-tensoranalysisusingregionaldata: Applicationto the
25 March, 1993,ScottsMills, Oregon,earthquake
JohnNfib•lek andGanyuanXia
Collegeof Oceanic
andAtmospheric
Sciences,
OregonStateUniversity,
Corvallis
Abstract. In thispaperwe outlinea procedure
we usefor routine Becausethe eventwaswell recordedby theregionalshort-period
moment-tensoranalysisof regionaldatafrom broadbandseismic
network, as well as some teleseismic stations, we are able to
stationsin northwestern
North Americaandapplyit to the mo- comparetheresultsbasedon sparseregionalbroadband
datawith
mentmagnitude5.5, March, 1993, ScottsMills, Oregon,earth- those derived from other observations.
quake.The resultscomparefavorablywith thoseobtainedfrom
teleseismicdata. We found that the earthquakeoccurredat a
Regional Data and Inversion Procedure
depthof 13-15 km and had a mechanismwith approximately
equalamountsof reverseandright-lateralstrike-slipcomponents. Theregional
stations
usedin thisstudyareshown
in Figure1.
The estimatedstressdropof 40 bar is averageon a world-wide At the time of the ScottsMills event,therewere 8 broadbandstabasis,supportingthe view that the ratherlarge damagewas tionsoperating
withina radiusof 800km fromtheepicenter.
We
causedprimarilyby poor construction
andnot by exceptional useddatafrom7 of them:COR (operated
by theIncorporated
propertiesof the source.The ScottsMills earthquakeis most Institutions
for Seismology
andthe OregonStateUniversity),
likely relatedto the Mr. Angel Fault.This fault is a part of the ARC, WDC, MIN (Univ. of Californiaat Berkeley),andPGC
GalesCreek-Mr.Angelstructural
lineament
(GCMAL) extending and WALA (CanadianNational SeismographNetwork).
about150km acrossthe WillametteValley. At presentdataare AlthoughstationLON hasa broadbandsensor,its recordswere
not sufficientto estimatethelikelihoodof an earthquake
involv- saturated
dueto thelow-dynamicrangeof therecorder.Instead,
ing the entireGCMAL, but givenits lengthan earthquakeof we wereableto usedatafromthedigitallong-period
seismometer
magnitude7 is conceivable.The resultsof this study,together whichalsorecords
at thatsite.StationHA0 (U.S. Geological
with investigations
of other earthquakes,
suggestthat sparse Survey)is a digital strong-motion
accelerograph
locatedin
broadbandnetworkscan be used efficiently for determining Portland,
56 kmfromtheepicenter.
It recorded
surprisingly
good
sourceparameters
of earthquakes
of magnitudegreaterthan4.0 in low-frequency
datasuitable
for ouranalysis.
The inversion for the moment tensor and sourcetime function
regionswith infrequentseismicity.
involvesmodeling
of theentire3-component
seismograms.
The
sourceparametrization
andinversionprocedure
followclosely
thoseusedbyNdb•lek[1984]fortheanalysis
of teleseismic
body
waves.For a horizontallylayeredmedium,displacement
as a
Introduction
The ScottsMills earthquake
of March25 (M w= 5.5) wasthe
first of severalunusuallystrongearthquakes
thatrockedOregon function of time t observed at a station at a distance A and
in 1993.It occurredabout35 km eastof thestatecapital,Salem, azimuth
{ fromtheearthquake
epicenter
canbeexpressed
as
on the eastside of the Willamette Valley, in the foothillsof theCascademountains.For its magnitudeit causedan unusually
uPSV({,A,0
= {HPsv2
(A,h,t)
[1/2(myy+mxx)
large amountof damage[Madin et al., 1993]. The secondset of
-1/2 (myy-mxx)
cos2•+mxy
sin2•)]
M w = 6.0 earthquakes
occurrednearthe town of KlamathFallsin
+HPsvl
(A,h,t)
[my
zsin•)+mxz
cos•)]
+HPsvø
(A,h,t)mzz}*fl(t)
the southernpart of the statein September.
The analysisof the
KlamathFallsearthquakes
is presented
in a companion
paperby
Braunmiller et al. [1994b]. For its locationin a subductionzone,
12, ;'W
Oregonover the last hundredyearshashad anomalously
low
seismicity[Jacobsen,1986]. The recentearthquakes,
however,
indicatethat the earthquakepotentialin this regioncannotbe
ignored.
The ScottsMills earthquakeoccurredat the time whenseveral
newbroadband
digitalseismicstations
hadrecentlybegunoperating in the Pacific Northwest.Anticipatingthis technicaldevelopment,we hadbegunto deviseprocedures
for routineanalysis
of suchdataandwereableto applythemto thiseventas oneof
the first of applications.Using regionalbroadbandstationswe
now routinelyestimatethe sourcemomenttensor,depth,and
sourcetime function from low-frequencywaves for eventsof
magnitude4.0 and larger. Our efforts in the Pacific Northwest
parallel those in other parts of the world (e.g., California
[Romanowicz
et al., 1993; Ritsemaand Lay, 1993] or Europe
[Giardini et al., 1993;Braunmilleret al., 1994a]).
48'N
44'N
120'W
116'W
112'W
.co., ..
. .co...u.s
?,
ß -\
.
45' 12'N
.
45' O0'N
40'N
In this paperwe outlinethe techniquewe useto analyzethe
regionalwaveformsandapplyit to the ScottsMills earthquake.
-
,
122' 48•
1•'
36'W
Figure 1. (Background)Map of stationdistributionusedin the
regionalwaveanalysis.(Foreground)
Seismicity(mb>l.5, 19901993, WRSN data) in the area of the ScottsMills earthquake.
Mainshockis indicatedby the star.The mappedtraceof the Mr.
Angelfault is indicated.Topography,75-m contourinterval.
Copyright1995by the AmericanGeophysical
Union.
Paper number 94GL02760
0094-853 4/95/94GL-02760503.00
13
N*BgLEK AND XIA: MOMENT-TENSOR ANALYSIS USING REGIONAL DATA
14
uSH((D,A,t
) = œI-I
sH2(A,h,t)[1/2(myy-mxx)
sin2•+mxy
cos2q)]
+Hsm (A,h,t)[myz
cos(D-mxz
sin(Dl)
* •(t)
z
whereu?svsignifiesdisplacement
(verticalandradialcompo-
nents)resulting
fromtheP-SV coupled
seismic
waves,andusI•
signifiesdisplacement
(transverse
component)
dueto SH coupled
waves.mijarethesource
moment
tensor
components.
H's are
excitationfunctionsfor a sourceat a depthh with a unit-stepslip
history,and•'l(t) is the far-fieldsourcetime function.
FollowingNdb•ek [1984], we parameterizethe sourcetime
function
LON
-" V¾v -
-- 'v'"'vV-- - -
as
•'l(t) = Z akT2x(t-[k- 1Ix);
k = 1, 2..... n;
whereT2xis a seriesof n isosceles-triangle
functionsof a unit
area,duration2x, andoverlappedby x, andakarethe corresponding amplitudeweights(requiredto sumto 1). The resultingtime
functionhasamplitudesspecifiedat an equaltimeintervalx, with
the interveningsamplesinterpolatedby the trapezoidalrule. By
varyingx andn we cancontrolthe time resolutionandthe total
ARC- v"'"VV-
- W 'Vv,,v
duration of the source time function.
If we know the crustal structure, we can calculate the excita-
tion functionsand determinethe best fitting (we use the least-
squares
inversion)momenttensorandsourcetime function.The
depthcanbe determined
by observing
thefits for a suiteof trial
depths.We calculatethe excitationfunctionsusingthe method
discrete wavenumber summation of Bouchon [1982] for the
crustal model shown Table
1. This model was derived from a
refractionstudyin westernOregonandWashington[Tr•hu et aL,
1994], adjustingthe Poisson'sratio so that major phasesmatch
thearrivaltimesat distantstations.
Usingthesamecrustalmodel
for the entireregionlimits the frequencybandfor whichthe theoretical seismogramscan adequatelydescribethe observeddata.
We used 0.1-0.01 Hz frequencyband for the nearestthree stations, and 0.05-0.01 Hz band for the others.In thesefrequency
bandsseismograms
are dominatedby guidedwavesin the crust
(Pnl,Rayleigh,Love) andwide-anglereflections.
The momenttensorwas constrainedto be purelydeviatoric.In
order to give roughly equal weight to all stations,we usedthe
samenumberof samples(200) from eachseismogram
(the near
stationswere sampledat a rate of 2 spsandthe distantstationsat
1 sps)andthe amplitudesof the seismograms
were correctedto a
referencedistanceassumingcylindrical geometricalspreading.
Even after correctingfor the geometricalspreading,becauseof
the differencein pass-bands,
the power of the signalat the near
stationswas higherthan at the distantstations;we correctedfor
thisdiscrepancy
by down-weighting
thenearstations
by 40%.
In orderto counter-actsomeof the phasemisalignmentdue to
Time(s)
Time(s)
Time(s)
Max.amplitude:
676Izm(top3 rows);204Izm(bottom
5 rows)
Figure 2. Observed(solid lines) andbest-fittingsynthetic(dashed
lines) regionalseismograms
of the ScottsMills earthquake.Note
the differencein scalesbetweenthe top tracesfor stationsclosest
to the epicenterand the lower tracesfrom distantstations.
Resultsof the Regional Wave Analysis
Figure 2 showsthe observedand theoreticalseismograms
for
the best-fit model. The data showlarge variationsin the amplitudesof the three componentsfrom stationto stationreflecting
the radiationpatternof the sourcemechanism(momenttensor).
Table 2 liststhe results.Data are generallywell matched;the misfit is understandably
largerfor the distantstations.It appearsthat
someof the mismatch,in particularat WALA, can be attributed
to L/R anisotropy.
Figure 3 (top) showsthe variancereductionas a functionof
sourcedepth.The minimumis found for 12 and 13 km. If we
assume10% varianceincreasebeing significant,the uncertainty
extendsfrom about 11 to 15 km. The figure showsthat the esti-
deviations from the assumed crustal model and errors in the ori-
mated moment tensor varies as a function of the assumed source
gin time and epicentrallocation,we realignedthe observedand
syntheticseismograms
to maximize the correlation.The maximum shift was 3 s. Becausewe requiredall 3-componentsat a
given stationto shift together,this processdid not removemisalignmentscausedby Love/Rayleighwaveanisotropy.
depthfrom predominantlythrustmechanisms
at a shallowdepth
to predominantlystrike-slipmechanisms
for the lower of the calculateddepths.The northeast-dipping
planeof the focal mechanism is relatively stable (its dip increasesslightly with depth)
while the southeast-dipping
plane progressivelychangesthe dip
directionfrom southerlyto easterly.As the depthincreases,the
estimatedseismicmoment increases.For the best fitting depth,
the mechanismis mid-way betweenthrustand strike-slip,andthe
Table 1. Crustal layer parameters used for calculation
of synthetic seismograms
Thickness,
Vp,
vs,
Density,
km
kfia/s
km/s
1.0
5.0
3.0
3.0
5.0
5.0
4.0
9.0
6.0
4.00
5.50
6.35
6.47
6.67
6.75
6.93
7.16
6.50
2.25
3.09
3.56
3.63
3.74
3.79
3.89
4.02
3.65
2.14
2.56
2.80
2.83
2.89
2.91
2.96
3.02
2.84
8.10
4.55
3.29
half-space
seismic
moment
is 2.4xl0•7Nm (M• = 5.5).Nearthebest-fitting
depththe solutionis 90% double-couple.
g/cm•
Table 2. Source parameters of the ScottsMills
earthquake determined in this study
Data
Strike,
deg
Dip,
deg
Rake,
deg
Depth,
km
Mo,
Nm
Regional
302
51
145
12-13
2.5x10n
Teleseismic
297
48
138
14.3
2.4x 10•7
Duration:
2.0 s
Fault radius:3.0 km; Av. slip:22 cm; Av. stressdrop:40 bar
N•B•LEK AND XIA: MOMENT-TENSOR ANALYSIS USING REGIONAL DATA
15
0.5
ale
P
mbc
0.4
1.8,
92
0.3
..•
thj
yss
0.2
2.•,• •_
•.,•_•_.•. •
kip
2.4,
90•[Y
• 2.5,
89
0.1
,.
o.... .... .... .... io....
J
frb
, . .•
, .•
Imq
gac
,•
hrv
P:
214
nm
1 .••.goga
$H:
379
nm , , ,
o
102o3o
time (s)
Depth(km)
0.5
,
[]
Strike
"';:
-' ale
yss
ß i frb
.• 0.3
0.2
,..
Figure 4. Observed (solid lines) and best-fitting synthetic
(dashed)teleseismicbodywaves(displacement,
0.02-0.05 Hz) of
.... i'0"
the main shock.Focal sphereswith P and SH nodalsurfacesand
Deviation(deg)
projectionsof the departingray-pathsare alsoshown.Data are
Figure3. (Top) Variance(normalizedby datapower)vs. assumed from stationsof theFederationof theDigital SeismicNetwork.
depthof the source.The fault plane solutions(beachbahs), the
seismicmomentin unitsof 10]?Nm (firstnumber),andtheperclose-instationsusingseismograms
from suitableaftershocks
as
the empirical Green functionscalibratingthe propagationpaths
[e.g.,Mori and Frankel, 1990]. StationHA0 wouldbe a logical
(Bottom) Variance vs. deviation from the best-fit estimate of
choice for such analysis. Unfortunately, the only aftershock
strike,dip, andrakefor a sourceat a 13 km depth.
whichtriggeredthis stationshowedvery little coherencewith the
mainshockindicating a very different mechanismand source
Figure3 (bottom)showstheresolution
of thefocalmechanism depth,and thereforecouldnot be used.We usethe aftershockof
assuming
a northeast-dipping
faultplaneanda source
depthof 13 June8, 1993 (mb=3.7) whichproducedcoherentrecordsat CaR,
km. The bestresolvedparameteris the strikeof the fault plane, 74 km from the epicenter.BecauseCaR was nodal for vertical
while the rake of the slip angleis the leastresolved.For a 10% and radial components,the analysiswas carriedout only on the
varianceincrease,the boundsare -4ø to +4ø for strike,-7ø to +4ø transversecomponent.The estimatedsourcetime function(Fig.
due
for dip,and-7øto +9øfor rake,relativeto thebest-fitestimates. 6) hasa durationof 1.8 s. Accountingfor someunderestimate
centageof the double-couplecomponentof the momenttensor
(second number) estimated for each depth are also shown.
Usinglong-period
datathesource
timefunctioncannotbewell
resolved.All we cansayis thatthedurationis shorterthan3 s.
TeleseismicBodywave Analysis
to constrain
the mechanism
was probably about 2.0 s. Assuming a circular crack model
[Madariaga, 1976] with equalruptureandhealingvelocitiesof 3
kngs(0.8vs)we canestimatethe fault radiusto be about3.0 km.
Togetherwith theseismicmomentof 2.5x10•7Nm, thisimplies
The resultsof the regionalwave analysisare checkedagainst
those obtained from teleseismicbody waves. Typically such
analysisis performedusingbroadband
P andSH wavesin the3090øepicentraldistancerange[e.g.,Ntib•lek,1984].For theScotts
Mills earthquake
onlya few goodqualitybroadband
bodywaves
are available, nevertheless,after bandpassfiltering in the 0.020.05 Hz band (notchin the Earth'sambientnoisespectrum)we
were able to obtain good azimuthal distributionof P and SH
waveforms
to the finite duration of the aftershock, the mainshock duration
and seismic moment
(Figure4, Table 2). The resultsarein very goodagreementwith
the regionalwave analysis.The low-frequencybodywavescould
not resolvethe sourcedepth,but fortunatelytwo stationsin Asia
(TLY and YSS) producedgoodqualitybroadbandP waveswith
distinctP and sP arrivals(pP is nodal).Thesecouldbe usedto
determinethe sourcedepthprecisely(Figure 5), in good agreementwith the depthestimatefrom theregionalwaves.The pulse
an averageslip on the fault of 22 cm anda stressdropof 40 bar.
Summary and Discussion
The results of the regional and teleseismicanalysesof the
Scotts Mills earthquakeare remarkably consistent(Table 2).
They indicatea mechanismwith approximately
equalamountsof
reverseand fight-lateralstrike-slipcomponents
on a fault with a
strikeof about300ø anda dip of 50ø.The centroiddepthis 13-15
0.9
tly
yss
0.6
widths at these two stations indicate a source duration of ~2 s.
time (s)
0.5
Depth(km)
SourceDuration From Empirical Green Function
and StressDrop
Figure 5. Broadbanddisplacementseismogramsof P waves
recordedat TLY andYSS (solidlines) showingcleardirectP and
The teleseismic estimate of source duration for a medium
sP arrivals tightly constrainingthe depth of the mainshock
(dashed
magnitudeearthquakeis significantlyaffectedby the uncertainty (varianceplot). The best-fittingsyntheticseismograms
in the Earth attenuation. A better estimate can be obtained from
lines) and the estimatedsourcetime functionare also shown.
16
N/•B•LEK AND XIA: MOMENT-TENSORANALYSISUSING REGIONALDATA
routinemoment-tensor
determinations
for this regionare now
includedin the QuarterlyNetworkReportsof the WRSN compiledby theUniv. of Washington.
In specialcircumstances,
when
the earthquakeoccurswithin 100-150 km from severalstations,
higherfrequencydatacanbe includedin the analysisandthe
magnitudethresholdis lower. For example,Braunmilleret al.
[1994b] were able to applythis techniqueto aftershocks
of the
KlamathFalls earthquakethat had a magnitudeas low as 3.5.
Furtherimprovements
in theprecision
of theparameter
estimates
Time (s)
and applicationto smallereventswouldrequirea denserstation
coverage
andbettercalibration
of thewavepropagation
paths.
Figure6. Broadband
displacement
seismogram
(trans.comp.,20
We thankB. Romanowicz,
J. Cassidy,
H. Benz,and
sps,solidline) for the mainshock
recordedat COR. The source Acknowledgments.
time function(below) wasfoundby deconvolving
the mainshock
waveform with a waveform from an aftershock(dashedline).
A. Frankelwhoprovided
datausedforthisstudy.J.Cassidy,
R. Crosson,
andJ.Deweyprovided
helpfulreviews.
Thisresearch
wassupported
by
theU.S. Geological
Surveygrant1434-93-G-2326.
References
km and the moment magnitude5.5. Except for the somewhat
smaller dip, the parametersfound here are also in excellent Beeson,M.H., K.R. Fecht,S.P.Reedier,andT.L. Toluene,Regional
agreementwith thosereportedby Thomaset al. [1993] basedon
correlations
withintheFrenchman
Springsmemberof theColumbia
Riverbasaltgroup:New insightsintothemiddleMiocenetectonics
of
the first-motion analysisof regional short-periodseismograms
northwestern
Oregon,OregonGeology,
47, 87-96,1985.
from the WashingtonRegionalSeismicNetwork(WRSN) (strike
= 304ø, dip = 58ø, rake = 138ø;focal depth= 15 km). Usingdata Bonneville,D., A. Del Pino,andD. Kelly, The March25, 1993,Scotts
Mills earthquake
- effectson structures,
EERI SpecialEarthquake
from the WRSN and temporary local stationsthey found that
aftershocks
extended
from about 8 to 13 km. A subset of these
eventsapparentlylies on a WNW strikingplanewith a dip of 55ø
to 60ø to the northeast. A preliminary examination of data
recorded by temporary broadband stations deployed in the
epicentral area after the mainshock by the Oregon State
University (OSU) showsa wide variationof mechanismsof the
aftershocks,indicatingthat the aftershockscannotbe interpreted
in termsof a singlefault plane.
The estimatedstressdrop of 40 bar for the mainshockis average on a world-wide basis, supportingthe view that the rather
large damagewas causedprimarily by poor constructionof the
pre-1950 structures[Bonnevilleet al., 1993;Deweyet al., 1994],
andnot by exceptionalpropertiesof the source.
The ScottsMills earthquakeis mostlikely relatedto the Mt.
Angel Fault (MAF), whichhasbeenmappedat the surfacewest
of the epicenter(Figure1). The strikeof the MAF at its southern
mappedendis identicalto thatof themainshock
mechanism.
The
exactrelationship,however,is not certain.Given the estimated
dip, centroiddepthandepicenterof themainshock,
theprojected
fault plane would reachthe surfacesouthwestof the mapped
fault, thusif the MAF were the causativefault for the earthquake
it wouldhaveto curveupwardasit reachedthe surface.Another
alternativeis that the MAF is in fact a zone consistingof several
parallelfaultsandthat the earthquake
occurredon a stranddis-
placeda few km to thesouthfromthemappedfaultplane.
The Mt. AngelFaultis a partof the GalesCreek- Mt. Angel
structurallineament(GCMAL) extendingacrossthe Willamette
Report, 5-8, 1993.
Bouchon,M., The completesynthesis
of seismiccrustalphasesat
regionaldistances,
J. Geophys.
Res.,87, 1735-1741,1982.
Braunmiller,J., T. Dahm,andK.-P. Bonjer,Sourcemechanism
of the
1992Roermond
earthquake
fromsurface
waveinversion
of regional
data,Geophys.
J. Int., 116, 663-672,1994a.
Braunmiller,
J., J. Nfib•lek,B. Leitner,andT. Qamar,The 1993Klamath
Falls, Oregon, earthquakesequence:Sourcemechanismsfrom
regionaldata,Geophys.
Res.Lett.,in press,1994b.
Dewey,
J.W.,B.G.Reagor,
D.Johnson,
G.L.Choy,
andF.Baldwin,
The
ScottsMills, Oregon,earthquakeof March 25, 1993: Intensities,
strong-motion
data,and teleseismicdata,U.S. GeologicalSurvey,
Open-FileReport94-163, 1994.
Giardini, D., E. Boschi,and B. Palombo,Moment tensorinversionfrom
Mednetdata(2) Regional
earthquakes
of theMediterranean,
Geophys.
Res. Lett., 20, 273-276, 1993.
Jacobsen,
R.S.,Map of Oregonseismicity,
1841-1986,OregonDept.of
Geol.andMin. Ind. GeologicalMap SeriesGMS-49,1986.
Madariaga,R., Dynamicsof anexpanding
circularfault,Bull.Seis.Soc.
Am., 66, 639-666, 1976.
Madin, I.P., G.R. Priest,M.A. Mabey, S. Malone, T.S. Yelin, andD.
Meier, March25, 1993 ScottsMills earthquake
- westernOregon's
wake-upcall, OregonGeology,55, 51-57, 1993.
Mori, J., and A. Frankel, Sourceparametersof small eventsassociated
with the 1986 NorthPalmSprings,CA, earthquake
determined
from
empiricalGreenfunctions,Bull. Seis.Soc.Am., 80, 278-295, 1990.
Nfib•lek, J., Determinationof earthquakesourceparametersfrom
inversionof bodywaves,Ph.D.thesis,Mass.Inst.of Tech.,1984.
Ritsema,J., andT. Lay, Rapidsourcemechanism
determination
of large
(Mw>5)earthquakes
in thewesternUnitedStates,Geophys.
Res.Lett.,
20, 1611-1614, 1993.
Valley [Beesonet al., 1985]. The MAF structurehas been
Romanowicz,
B., D. Dreger,M. Pasyanos,
R. Urhammer,
Monitoringof
mappedusingwaterwell andindustryseismicreflectiondataby
strainreleasein centralandnorthernCaliforniausingbroadband
data,
Werner et al. [1992]. The offsetsof sedimentaryhorizonsindi-
Geophys.Res.Lett., 20, 1643-1646, 1993.
catea postMioceneage.In 1990 a seriesof smallearthquakes Thomas,G.C., R.S. Crosson,S. Dewberry,et al., The 25 March 1993
(mb< 2.5) occurredsouthof Woodbumat thenorthwest
endof
ScottsMills, OregonEarthquake:
Aftershockanalysisfromcombined
permanentand temporary digital stations(abstract),EOS, Trans.
the mappedfault (Figure 1). The eventsoccurredat a depthof
Arner.Geophys.Un., 74, 201, 1993.
about15 km andhad a strike-slipmechanismconsistent
with the
orientationof the MAF there,indicatingactivedeformationand
seismic
potentialof thefault[Werneret al., 1992].At present
we
do not have sufficient data to estimate the likelihood of an earth-
quakeinvolvingthe entireGCMAL, but giventhelengthof the
Trthu, A., I. Asudeh,T. Brocker,J. Luetgert,W. Mooney,J. Nfib•lek,
and Y. Nakamura, Crustal architectureof the Cascadiaforearc,
Science,in press,1994.
Werner,K., J. Nfib•lek,R. Yeats,andS. Malone,TheMountAngelfault:
Implicationsof seismic-reflection
data andthe Woodbum,Oregon,
earthquake
sequence
of Aug. 1990,OregonGeol.,54, 112-117,1992.
structure(about 150 km) and the seismogenicthicknessof the
crust(15 km) an earthquake
of magnitude7 is conceivable.
In conclusion,
ourstudyshowsthata relativelysimpleanalysis
JohnN•b•lek, Collegeof Oceanic
andAtmospheric
Sciences,
Oregon
of datafrom sparseregionalbroadband
stationscanyield quite StateUniversity,Ocean.Admin.Bldg.104,Corvallis,OR 97331-5503.
preciseestimatesof sourceparameters.In the northwestern GanyuanXia, now at the Dept. of GeologicalSciencesandInstitute
United Statesand western Canada, where there are 30 broadband for Geophysics,Universityof Texasat Austin,Austin,TX 78713.
seismicstationsoperatingat thistime,thistechnique
canbe routinelyappliedto eventsof magnitude
greaterthan4. The OSU (ReceivedJuly14, 1994;acceptedSeptember19, 1994.)