GEOPHYSICAL RESEARCH LETTERS, VOL. 22, NO. 2, PAGES 105-108, JANUARY 15, 1995
The 1993 Klamath Falls, Oregon, earthquake sequence:
Source mechanismsfrom regional data
JochenBraunmiller, John Nfib•lek, and Beate Leitner
Collegeof OceanicandAtmospheric
Sciences,
OregonStateUniversity,Corvallis
AnthonyQamar
Geophysics
Program,Universityof Washington,
Seattle
Distribution
Abstract. We use regionalbroadbandseismograms
to obtain Aftershock
seismicmoment-tensorsolutionsof the two September20, 1993,
Becausethe nearestseismicstationswere approximately70
Mw=6, KlamathFalls, Oregonearthquakes,
their foreshockand km fromtheearthquakes'
epicenters,
initiallocationswerebiased
largestaftershocks
(My>3.5). Severalsub-groups
with internally 5-10 km to the northwest [Qarnar and Meagher, 1993].
consistentsolutionsindicate activity on severalfault segments Deploymentof portablestationsby the U.S. GeologicalSurvey
andfaults.From the estimatedmoment-tensors
anddepthsof the andOregonStateUniversity(OSU) andinstallation
of permanent
main shocks and from the aftershock distribution we deduce that
stations(Figure2) in the epicentralregionconsiderably
improved
both main shocksoccurredon an east-dipping
normalfault, pos- later locations and allowed "master event" relocations of earlier
siblyrelatedto the Lake of the Woodsfault system.Rotationof events.Quality locationsfor eventswith M>0.5 are available
T-axes between the two main shocks is consistent with the two
from Oct., 1993, on. More preciselocationsfor the sequence's
dominanttrendsof the aftershocksand mappedfaults.We pro- early stageswill be possiblewhendatafrom all portablestations
posethata changein fault strikeactedastemporarybarriersepa- are analyzed.
ratingtheruptureof themainshocks.
EmpiricalGreen'sfunction
Figure2a showsthe aftershockactivitybetweenOct. 6 and
analysisshowsthat the first maineventhada longerrupturedu- Dec. 3, 1993 (primaryaftershockzone).Most activityoccursin
ration(half-duration
1.7s)thanthesecond
(1.2s).In December, the 4-12 km depthrange. Aftershockssouthof the first main
vigorousshallowactivitycommenced
nearKlamathLake'swest- shock(event2) alignin a northwest-trending
patternwhileafterern shore,5-10 km eastof the primaryaftershockzone. It ap- shocksto the north show a north-southtrend. Activity outside
pears a Mw=5.5 aftershockoccurringthe day before, though thesetwo segmentsis limited. Cross-sections
throughvarious
withintheprimaryaftershock
zone,triggeredtheactivity.
partsof the aftershockdistributiondo not reveala fault plane.
The dispersedseismicitymay be a real phenomenon,
but it is
Introduction
partiallytheresultof inadequate
depthestimate
precision.
Installation of an additional station (HOG) in Jan., 1994, im-
Two strong(Mw=6) earthquakes
struckthe KlamathFalls reprovedlocationquality.Figure2b showstheaftershock
activity
gionof southern
Oregonon September
20, 1993,at 8:28 pm and between Dec. 4, 1993, and Mar. 10, 1994. Aftershockscontinue
10:45pm local time (Figure1). The epicenters
(Table 1, Figure
2) were located25 km northwestof KlamathFalls. Strongshak-122 ø
-120'
-126'
-124'
-118'
ing producedground crackingand landslides,but no surface
•
'
faulting was found [Wiley et al., 1993]. A foreshock(Mw=4.2)
precededthe first maineventby 12 minutes.The aftershock
sei•'• SSW ß
quenceis vigorouswith morethan3000 eventsrecordedduring
the followingfive months.
KlamathFalls is locatedat the northwestern-most
edgeof the
BasinandRangeprovince.The regionis morphologically
dominatedby north-to northwest-trending
normal-faultescarpments,
• COR
someof Holoceneage [Hawkinset al., 1989].Prior seismicityin
the area has been, however, low. Only six earthquakeslarge
44'
44'
enoughto be felt occurredin the KlamathFalls vicinity during
the 50 yearsprecedingthe 1993-94 sequence
[Shetrod,1993].
IDBO ß
Installationof a sparseseismicnetworkof three-component
digitalbroadbandstationsin California,OregonandWashington
• KLAMATH
FALLS
(Figure1) hasmadepossiblesourcemechanism
retrievalof mod42'
vp(kin/s) 42'
erateearthquakes
(M>4) usingregional(A<1000kin) waveforms
4
6
8
YBH
[Ndb•lek and Xia, 1994]. This paper presentsmoment-tensor
analysesof thesedata for 21 of the KlamathFalls earthquakes.
We alsoshowaftershockdistributionlocatedby the Washington
WDC ß
RegionalSeismicNetwork's(WRSN) localandregionalstations;
MIN
40'
40'
describeempiricalGreen'sfunctionestimates
of sourcedurations
ORV ß
for the two main shocks;and concludewith a descriptionof the
-120'
-118'
-124 ø
space-timeevolutionof the entiresequence.
•©
LTY
46'
146'
.,. . .
o
Copyright1995 by the AmericanGeophysicalUnion.
Papernumber94GL02844
Figure 1. Map of epicentralarea(star)andbroadband
seismic
stations(circlesandsquares)usedin thisstudy.Squares:OSU
temporary
arrayin Oregon.DBO (Univ.of Oregon)wasinstalled
in thespringof 1994.Not shownarestations
MHC, CMB, BKS,
and STAN located further south. Inset shows the P-wave veloc-
0094-853 4/95/9 4GL-02 844 $03.00
ity/depthmodelusedin thisstudy.
105
106
BRAUNMILLER ET AL.: THE 1993 KLAMATH FALLS EARTHQUAKES
Table 1. Epicentral and Source Parameters for
Earthquakes Investigated in this Study.
Ev.
OT
#
date
h:m
Lat.
Lon.
S/D/R
(øN)
(øW)
(ø)
M0
M w DC C
(km) (Nm)
CD
(%) #
1
930921
03:16
42.32
122.02 331/45/246
6
2.6
15
4.2
76
36
2
930921
03:28
42.32
122.03
331/47/255
9
1.1
18
6.0
96
39
3
930921
04:16
42.27
122.01 339/23/263
6
7.1
15
4.5
84
16
4
930921
04:34
42.30
122.03 320/50/243
6
3.6
15
4.3
88
18
5
930921
05:45
42.36
122.06 351/42/269
9
1.1
18
6.0
89 39
6
930921
06:14
42.39
122.05 355/52/259
5
2.1
16
4.9
83 21
7
930921
07:28
42.40
122.09
23/36/285
6
1.6
15
4.1
64
15
8
930923
06:21
42.30
122.04 353/46/266
6
7.4
15
4.5
46
36
9
930924
01:57
42.34
122.06
10/55/283
6
5.8
14
3.8
85
29
10
930924
16:53
42.37
121.94 330/38/256
3
1.2
15
4.0
93 28
shore,5-10 km east of the primary aftershockzone. It appears
the largestaftershock(event13) occurringthe day beforethe onset of this activity, thoughwithin the primary aftershockzone,
triggeredtheseevents.
The aftershockdistributionshownin Figures2a and 2b, indicates activity on severalfault segmentsand faults during the
KlamathFalls sequence.
Regional-Waveform Inversion
11
930924
17:25
42.29
122.00
6
4.1
14
3.7
89
12
12
931130
03:30
42.28
121.99 304/56/247
6
2.5
14
3.6
84
12
13
931204
22:15
42.29
122.01 331/48/264
6-9
2.0
17
5.5
82
36
14
931204
23:50
42.23
121.96
7/26/25
6
1.2
15 4.0
97
15
We inverted seismogramsrecordedby regional broadband
digital stations(BerkeleyDigital SeismicNetwork, WRSN, and
IRIS/OSU stationCOR; Figure 1) to obtainthe seismicmomenttensor,sourcetime historyand centroiddepth.For the smaller
shockswe alsouseddatafrom a temporarybroadband
arrayoperatedby OSU in western-centralOregon(Figure 1). The inversionprocedureis describedin Ndb•lekandXia [ 1994].
We filtered completethree-component
seismograms
to a frequencybandwith goodsignal-to-noise
ratio.For themainshocks
and the largestaftershockwe usedfrequenciesbetween0.01 to
15
931225
12:33
42.28
121.95
9/43/301
6
1.3
15
4.1
85
30
0.05 Hz, for smaller events between 0.03 to 0.1 Hz. In these fre-
16
931231
18:08
42.29
121.94
8/25/290
3
1.8
15
4.1
79
24
17
940107
09:39
42.27
121.92
46/70/338
6
1.8 15 4.1
18
940108
02:55
42.25
121.92
55/62/338
3-9
6.1
14
3.8
97
30
19
940109
19:03
42.26
121.92
42/70/339
6-9
2.6
15
4.2
63
30
20
940119
22:27
42.29
121.95 346/34/269
6
1.7
15
4.1
55 24
21
940413
20:41
42.24
121.97 342/76/283
6
2.9
14
3.6
98
quencybandsthe simplecrustalmodel shownin Figure 1 providedan adequatematchto the data.A 0.29 Poisson'sratio was
chosenby matchingwaveformsat distantstations.All seismogram amplitudeswere normalizedto 100 km epicentraldistance
assumingcylindricalgeometricalspreading.
Our analysisincludesall eventswith MD>3.5'exceptoneevent
whosesignalwas contaminatedby codaof event 13. Epicenters
andsourceparameters
for these21 earthquakes
arelistedin Table
1. Fault planesolutionsare shownin Figure3. Within the source
time function'sresolution(-2 s), all investigatedeventshad simple rupturesexceptevent 11, whichhadtwo episodesof moment
releaseseparatedby 10 s. The solutionsdo showdefinite variability, but they are consistentwithin differentsub-groups,
indicatingactivity on variousfaults.
As an examplewe showthe matchbetweenobserved(solid)
and synthetic(dashed)seismogramsfor the secondmain shock
(Figure4). We observelarge azimuthally-dependent
amplitude
variationstightly constraining
the sourcemechanism.The ampli-
4/39/272
53 24
10
Ev.: eventnumber.OT: origin-time.S/D/R: Strike,Dip, Rake.
CD: centroiddepth. Mo: seismicmoment,integer number is
exponent.DC: double-couple
part of momenttensor.C: number
of seismograms
usedfor inversion.
to occurvigorouslyalong the two segmentsactiveearlier with
activity extendingfurther southeastthan before.Deeperevents
(blue) plot consistently
eastof shalloweraftershocks
(red) indicating an eastwardfault dip for both main shocks.The most
striking differencebetweenFigures2b and 2a is the vigorous,
mainly shallow(0-8 km) activitynearKlamathLake's western
42.4'
42.4'
.
VRC
'
LAB X!\
HOG
42.2'
-122.2'
-122.0'
-121.8'
Figure2. Aftershocklocations.
Deepereventsplottedontopof shallower
ones.Topographic
contours
in 200 m
stepsfrom 1000 to 2400 m (higherelevationsdarker).Faulttracesarefrom Walkerand MacLeod[ 1991]; LWFZ:
Lake of the Woodsfault zone.KF: KlamathFalls.Blacksquares
are short-period
stationsof theWashington
Regional Seismic Network installed in Oct., 1993; HOG installed in Jan., 1994. Stars mark locations of first
(event2) and second(5) main shockand of largestaftershock(13). a) Oct. 6 - Dec. 3, 1993. b) Dec. 4, 1993 Mar. 10, 1994.
BRAUNMILLERET AL.' THE 1993KLAMATHFALLSEARTHQUAKES
Z
107
R
T
Time (s)
Time (s)
LON
42.4'
I'•A
MHC
___• _..•.%a__
42.2'
-122.2'
-122.0'
-121.8'
Figure3. Locationsof eventsstudied(circles).Faultplanesolutions (lower-hemisphere
projections)with size proportionalto
Mw; numbersinsidecorrespond
to eventnumbersin Table 1.
Bold fault tracesof the LWFZ (marked A, B, C) are from highresolutionmapof Hawkinset al. [1989].
tudesof the observedseismograms
arewell matchedand,except
for nodal arrivals (transversecomponentat ORV and MIN), the
waveformfit is alsogood.
The sourceparametersof the investigatedearthquakesare
generallywell resolved.Figure 5a showsthe varianceas a function of centroiddepthfor the threelargestevents(2, 5, and 13).
The centroiddepthsof the two main shocksare well resolvedat 9
km, while the centroiddepthof the largestaftershockis between
6 to 9 km. The fault plane solutionof the secondmain shockis
stableover a wide depth-range(Figure5a). Figure5b showsthe
resolutionof the event'sstrike,dip andrake. Assumingan eastward dippingfault and a 9 km centroiddepth,we find strikeand
rake are moretightly constrained
thandip. Relativeto thebest-fit
the bounds for a 5% increasein variance are +1 ø for strike, +2 ø
for rake and +5 ø for dip. For the aftershocks,boundsfor a 5%
varianceincreaseaverage+10øfor strike,rake,anddip.
Empirical Green's Function Analysis
For the two main shocks we obtained source time function es-
timatesby simultaneous
aleconvolution
of three-component
displacementseismograms.Aftershocks(events1, 6, 8, and 9) lo-
Time (s)
Max. amplitude: 842 gm
Figure4. Observed(solid)andsynthetic(dashed)seismograms
for the secondmain shock (event 5). Z, R, and T are vertical, radial, andtransversecomponents.
lier aftershocks(Figure 2a) is consistentwith these rupture
dimensions.
Discussion
and Conclusion
The KlamathFalls earthquakesequence
is markedby a complexrupturehistorywith severalfaultsandfault segments
being
activatedduringdifferentstages.Faultplanesolutions(Figure3)
aredominatedby normalfaultingconsistent
with theregion'sextensionaltectonics[Hawkins et al., 1989; Sherrod, 1993]. The
aftershockpattern(Figure2) showsthreegeneralareasof activity: the northwest-trendingsouthwesternzone where the foreshock,the first main eventandthe largestaftershockare located;
the northern zone trending northward where the secondmain
cated close to and with similar mechanisms as the main shocks
were chosenas empiricalGreen'sfunctions(EGF). The analysis
was carriedout individuallyfor eachstation.In orderto decrease
the sourcedurationestimatessensitivityto poorly resolvedtails
of low moment release, we describe resultsin terms of half-dura-
tions (tu2), the durationaroundthe peakof the sourcetime function duringwhich 50% of the seismicmomentwasreleased.
Figure 6 showstheseresults.First main shockhalf-durations
for stationstowardssouth-southwest
(WDC andYBH) arelonger
thantowardsnorth(COR) indicatingnorthwardrupturepropagation. No consistentrupturedirectivitywas foundfor the second
main event; and the results dependedstrongly on the chosen
EGF. The first event's averagetl/2 is 1.7 s, while the secondmain
shockis shorter(averagetl/2 1.2 s). Assuminga circularcrack
model[Madariaga, 1976] with equalruptureandhealingvelocitiesof 2.0 krn/s,we obtaina roughestimateof ruptureradius,average stressdrop and averagedisplacementof 3.4 km, 125 bar,
and 85 cm for the first, and 2.4 km, 360 bar, and 105 cm for the
second main shock. The aftershock distribution's
extent for ear-
m 0.3
1
,
1.1, 89
ß' ' • ' '
ß
ß
i
10
....
Depth(km)
,
15
.
Deviation(deg)
Figure5. a) Variancevs. centroiddepth.For event5 estimates
of
thefaultplanesolution,seismicmoment(in 10•8Nm, firstnumber) anddouble-couplecomponentof momenttensor(in %, second number)are shownfor eachdepth.Circles: event2; triangles: 13. For plotting purposeswe added0.05 to variancesof
events2 and 13. b) Variancevs. deviationfrombest-fittingdouble-couplemechanism
for event5 for 9 km centroiddepth.
108
BRAUNMILLER ET AL.: THE 1993KLAMATH FALLS EARTHQUAKES
EVENT
2
EVENT
themainruptureepisodes.
High-resolution
mapof Hawkinset al.
[ 1989] showsthattheLWFZ formsthreedistinctsegments
(A, B,
C; Figure3) in the epicentralregion.The relativelocationsand
5
WDC
,',x",v
192'
'• -I
v
•'7•
•
mechanismsof the two main shocksand the largest aftershock
- •
ß 19s
f•
arebroadlyconsistent
with thefollowingscenario:thefirstmain
shock(event2) initiatednearthe step-overseparating
segments
C
and B, propagatednorthwestward
alongsegmentB (Figure3),
and stoppednear the fault bendseparatingsegmentsB andA.
A 1.5s
.
YBH
218'
•
11A A
.
[
The secondmain shock(event5) occurredalongthe north-trend-
•
A
--•
COR
339'
•...i....i....i....i....i....i....i i....i....i....i....i....i....i....i
0
•
2
3
4
5
6
70
•
2
3
SOC
4
5
6
7
SOC
Figure6. Far-fieldsourcetime functions(normalized)
fromsimultaneousdeconvolutionof three-component
EGFs. First main
shock(2): solid line, event 1 as EGF; dashedline, event 8 as
EGF. Secondmain shock(5): solid line, event6 as EGF; dashed
line, event 9 as EGF. Half-duration estimates' filled triangles
correspond
to solidlines,openinvertedtrianglesto dashedlines.
Number beneath station code is station azimuth.
shock is located; and the eastern zone near Klamath Lake, acti-
ing segmentA. The largestaftershock
(event13) appears
to have
occurredon segmentC. Initiationandterminationof earthquake
rupturesat geometrical
barriers(e.g.faultbendsandstep-overs)
hasbeenobservedfor manyearthquakes
[e.g.King andNdb•lek,
1985]. Scholz [1990] presentsa model of "subcriticalcrack
growth"to explainre-initiationof temporarilyarrestedruptures
acrossa barrierwhichcouldexplainthe delayednucleationof the
secondmain shockandpossiblymostof the aftershocksequence.
However,the shiftof activityto the easternfault zonefollowing
the largestaftershock
requiresa differentmechanism,
probably
involving stressrelaxation within a complex fault zone as
responseto the main events. Interestingly, many recent
continentalnormalfaultingearthquakes
occurredas sequences
of
two or morelargeshockswith comparable
magnitude(e.g.Dixie
Valley-Fairview Peak/Nevada, 1954; Corinth/Greece,1981;
Ethiopia, 1989), possiblyreflectingintrinsicsegmentation
of
normalfault systemsaspostulated
by Jacksonand White[1989].
vated on Dec. 4, 1993, trendingnorthwestand dominatedby
shallowactivity. The distributionof later aftershocks
suggests
thatthe two mainshocksoccurredon east-dipping
faultplanes.
The sequencestartedwith a foreshock(event 1) followed 12
min. later by the first main shock (2). The epicentersof both
eventsareadjacent.Largeraftershocks
(3, 4) aredisplaced
to the
Acknowledgements.
Comments
by J. Ritsemaandtwo anonymous
reviewershelpedto clarifythe text. This research
wassupported
by the
U.S. GeologicalSurveygrant1434-93-G-2326.Operationof broadband
stationsin Oregonwassupported
in part by grantsfrom the Oregon
south-southeast.The secondmain shock (5) occurredtwo hours
after and north of the first. In the following three days, after-
References
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Dreger,D., J. Ritsema,andM. Pasyanos,
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September,1993, KlamathFallsEarthquakeSequence,
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edge.
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trendingto
application
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and Faulting,439 pp.,
difficult at this time. For the two main shocks the causative fault
possibly
wastheeastward-dipping
Lakeof theWoodsfaultzone
(LWFZ, Figures2 and 3) [Hawkinset al., 1989]. The LWFZ
CambridgeUniversityPress,1990.
Sherrod,D.R., Historicandprehistoric
earthquakes
nearKlamathFalls,
Oregon,Earthquakesand Volcanoes,24, 106-120, 1993.
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000, U.S. GeologicalSurvey,1991.
shockepicenters
andundergoes
a changein trendconsistent
with Wiley, T.J., D.R. Sherrod,D.K. Keefer,A. Qamar,et al., KlamathFalls
the momenttensorresults.Surfaceprojectionsof 45ø eastward
earthquakes,
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20, 1993- includingthestrongest
quakeever
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dippingfault planespassingthroughthemainshocks'
hypocenters (10 km, WRSN) are close to the LWFZ's surface trace.
JochenBraunmiller,JohnNfil•lek, and BeateLeitner,Collegeof
However,no shallowseismicactivityhasbeenrecordedcloseto
Oceanicand AtmosphericSciences,OregonStateUniversity,Ocean
the trace. Shallow aftershocks are located several kilometers
eastward,
implyingaftershock
activityis confinedto thehanging AdminBldg 104,Corvallis,OR 97331-5503.(jbraunmi@oce.orst.edu)
AnthonyQamar,Geophysics
Program,Universityof Washington,
wall.
Seattle,WA 98195.(tony@geophys.
washington.edu)
An interestingquestionis why the main ruptureoccurredas
two M=6 earthquakes
closein spaceand time insteadof one
August27, 1994.)
M=6.2 earthquake.
We propose
thatfaultsegmentation
controlled (ReceivedJuly20, 1994;accepted