GEOPHYSICAL
RESEARCH
LETTERS,
VOL.
17,
NO.
7,
PAGES
1017-1020,
JUNE
1990
RUPTURE PROCESSOF THE MACQUARIE RIDGE EARTHQUAKE OF MAY 23, 1989
Joehen Braunmiller and John N•b[lek
Collegeof Oceanography,
OregonStateUniversity,
Corvallis,
OR 97331
Australia
plateboundary,
in agreement
withmotionpredicted
IncorporatedResearchInstitutefor Seismology(IRIS) and
GEOSCOPE are used to study the ruptureprocessof the
Macquarie
Ridgeearthquake.
Firstwedetermine
thecentroidal
parameters:
mechanism,
depth,seismic
momentandthesource
time historyof therupture.Thenwe examineif thedatashow
rupturespreading
in a unilateralor bilateral
fashionawayfrom
its nucleationpoint and estimatethe fault lengthand stress
byplatetectonic
models.
The totalseismicmomentis
drop.
Abstract.Broadband
bodywavesrecordedat 15 digital
seismic
stationsworldwideare usedto studythe rupture
pr•ess
oftheMay23,1989Macquarie
Ridgeearthquake.
Thecentroidal
solution(strike211o,dip 86%rake 180ø, and
depth
of10kmbelowtheseafloor)
indicates
shallow
rupture
•ithpureright-lateral
strike-slip
motionalongthePacific13.4x1020
Nm, 80% of whichwasreleasedin thefirst 24 s of
fierupture
process.
Modeling
favorsa bilaterally
propagating
rupture
withslightly
different
dipandrakeforthenorthward
158øE
159'
i 60 ø
161ø
162'
164"'
163ø
andsouthward
fault segmentsand similar momentrelease
along
bothdirections.
Theestimated
faultlengthis quiteshort,
50øS
.:
about
90 km, andthe derivedstressdropof 180 bar andaver-
...
•-.
age
displacement
of 17m areunusually
high.Thebathymetry
intheepicentral
regionshows
topographic
segmentation
ofthe
ridge,
possibly
indicating
faultsegmentation
whichconfines
ruptures
to shortsegments.
51'
Introduction
,::.:-.
...::..,
TheMacquarie
Ridgeearthquake
of May 23, 1989with a
..
o
surfacewave magnitude(Ms) of 8.3 is the largestevent
,oddwidein more than a decade. The focal parameters
according
to the PreliminaryDeterminationof Epicenters
,PDE)catalogueare: latitude 52.3ø S, longitude160.6ø E,
•,epth
10 km and origin time 10h54m46.3s.The-earthquake
occurred
in thecentralpart of the MacquarieRidge Complex
, 5/B3/89
/
t
ßO,T'•
i..:i
•Figure1), which formsthe Pacific-Australiaplate boundary.
andrunsapproximately
north-south
for 2000 km betweenthe
South Island of New
Zealand
and the Pacific-Antarctic-
)7
Australia
triplejunction.Globalplatetectonicmodelsplacethe
Pacific-Australia
poleof rotationcloseto theMacquarieRidge
'%..,..
....
•l• P•c,rlc
{MinsterandJordan,1978; DeMets et al., 1990), thusrelative
platemotionsalongthe plate boundarycan changerapidly,
•hich is reflected in its evolution. The region originally
formed
in an ocean-spreading
environment(Molnar et al.,
1975)
withtheyoungest
radiometrically
datedoceanicbasalts
fromMacquarieIslandof about10 Ma age (Duncanand
Va:ne,1988).Due to thesouthward
migrationof therotation
pole( Molnaret al., 1975; StockandMolnar, 1982), theridge
subsequently
servedasa strike-slipplateboundarywith the
northernmost
part(northof 50ø S) developing
intoanoblique
convergent
boundary(Ruff et al., 1989)wheretheAustralian
plate
isbeingsubducted
beneath
thePacificplate.
Bodywavesrecordedat 15 stationsfrom theGlobalDigital
Seismic
Network(GDSN)andthedigitalnetworksof the
locations and source mechanisms of the 1989 earthquake
(largestar)andtwothrustevents25 May 1966,M=6.5and23
May 1984,M=5.9 areshown.Thefirstweekof aftershocks
is
indicatedby solidcirclesandthoserecorded
sincethenuntil
December1989by opencircles(onlyaftershocks
recorded
by
20 or more stationsare depicted;PDE data).The regionin
which 80% of the momentreleaseoccurredduring the 1989
eventis dashed.Bathymetryis shownby 500 m contours.
Dark shadingindicatesareasdeeperthan 4500 m, light
shading2000-4500m, andwhite areasare shallowerthan
2000 m. Topographicsegmentation
of the ridge in the
epicentralarea is apparent.The insertshowsa tectonic
overview. The arrow indicates the relative plate motion
Cop)a'ight
1990bytheAmerican
Geophysical
Union.
betweenthe Pacific and Australianplatesaccordingto the
NUVEL-1 model (DeMets et al., 1990). 1800 m depth
contours
aredashed,
depthsgreaterthan4800m areshaded.
Paper number 90GL01162
0094-8276/ 90 / 90GL-0116250 3. O0
1017
1018
BraunmillerandN•ib•lek: The 1989MacquarieRidgeEarthquake
Data and Methods
TABLE 1. SourceModelsDerivedby BodyWaveInversion
The analysisfollowsthe procedures
describedby N•ib•lek
(!984). P andSH bodywavesrecordedby GDSN, IRIS and
GEeSCOPE digitalstationsin the epicentraldistancerange
Moment Depth Strike Dip Rake Duration cr
1020Nm km
13.4
deg deg deg
s
10
211
52
86
180
between 30 ø and 100 ø for P waves and 30 ø and 90 ø for SH
PS
wavesareused.In thisdistancerange,the waveformsarenot
contaminated
by stronguppermanfieor corephases.
The large
magnitudeof the earthquakecausedsomeseismograms
from
stationsin the abovedistancerangeto behavenon-linearly;
thesedatawereexcludedfrom the analysis.We deconvolved
the instrumentresponseto obtainbroadbanddisplacement
seismograms;dependingon the data quality and the
seismo•aph type, the data are broadbandfrom the Nyquist
periodof 0.6 s to a periodof 100 s or 200 s.For stations
BJI
andMDJ, whereonlylong-perioddatawereusedto obtainthe
broadband
seismograms,
thesheri-period
cutoffis 4 s. Dueto
a highnoiselevelat stationZOBO thelong-period
cutoffis 50
PSs
9.7
10'
211'
86* 180'
26
0.30
USS
9.3
10'
211'
86* 180'
26
0.34
USN
10.2
10'
211'
86* 180'
26
0.28
4.8
5.2
10'
10'
211'
21!*
86*
86*
180'
180'
16
28
4.7
5.5
10'
10'
210
212
95
82
174
187
16
30
S.
The crustalstructurein the sourceregionis assumed
to be a
half-spaceoverlain by a 3.8 km thick water layer. At the
receiverswe chosea crustalhalf-space.The P- and S-wave
velocitiesof the half spaceare: 6.0 km/s,3.46 kin/s,with a
BS1
0.26
south
north
BS2
0.24
south
north
The formalunceaaimies
(2c0 are:depth0.5 km, orientation
2ø,andmerrett
20%. PS - pointsource,PSs- pointsourcewith durationrestricted
to •mah
moment release, USS - unilateral rupture towards southwest,USN
unilateralrupturetowardsnortheast,BS1 - bilateralrupturewith soutin
mechanismfixed, BS2 - bilateralrupturewith sourcemechanismfree.
normalized variance of residuals. "*" - fixed.
rockdensity
of 2.8 g/cm
3. Theseismograms
aresimultaneouslyinvertedin a least-squares
sensefor the sourcemodel
parameters.A point source is used to obtain a average
(centroidal)
description
of thesource.
To examinecomplexities
aliasingstructuraleffectsinto the estimatedsourcetimehistory. Becauseabout80% of the seismicmomentis released
in
dueto sourcefiniteness
andrupturepropagation,
we useda
propagatingline sourceparameterization
as describedby
N•ib•lek(1984)andBoydandN•ib•lek(1988).To investigate
suchdetailsof theruptureprocess,
it is necessary
to precisely
determine
thearrivaltimes.The onsetof manyP wavesfor the
MacquarieRidgeearthquake
is quiteemergent;
we determined
the f•rstmotionusingbothbroadband
andhigh-pass
filtered
waveformsandby checkingfor consistency
with stationsat
on thisdominantpartof theruptureprocess.
For an eventof thissize,thepointsourcemodelprovides
a
surprisinglygoodfit to the data,indicatinga shortrupture
length,although,it doesnot adequately
matchtheamplitudes
for stationsfrom azimuthsalongthe strikeof thefault (Figure
4). Moreover,it failsto matchthefirstmotionpolarities
atKIP'
andAFI. Matchingthe first motionpolaritiesfor all stations
simultaneously
wasnotpossible.Slightchanges
in themechanismas the rupturepropagates,as well as deviationsfrom
greatcirclepathsof the rays,can affectthe first motionsfor
similar azimuths.
Inversion Results
The resultsof our analysisaresummarized
in Table 1. The
centroidalsolutionindicatesa purestrike-slipmechanism:
strike211o, dip 86ø andrake180ø.Thevariationof themismatchbetweenthe observedandsyntheticseismograms
asa
functionof depth(Figure2) indicates
thatthecentreidis less
than20 km belowthe seafloor.Themechanism
in thisdepth
rangeremainsvery stable,whiletheseismicmomentvariesby
about10% (Figure2). The leastmismatchis obtainedfor a
pointsourceat 10 km belowthe seafloorwith a seismicmo-
thefirst24 s,we concentrate
in thesubsequent
analysis
only
stations close to the P-wave nodes.
To determinetheprimarydirectionof therupturepropagationwe introduce
kinematicmodelswithruptures
propagating
unilaterallyandbilaterallyalongthestrikeof thefault.A rapture velocity of 2.0 km/s is usedand the depthand source
mechanism
arefixedto thepointsource
estimate;
in eachca
we invert for the momentreleasealongthe fault. Sincewe
havefour stations
alongthestrikeof thefault (KIP andAFim
0.60 .............
mentof !3.4x1020Nm, whichcorresponds
to a moment
magnitude(Mw) of 8.02. Followinganemergentbeginning,
about80% of the momentis releasedin a 22-second-long
pulsefollowedby 3 smallerpulses(Figure3a). The fit to the
17
0.55
=
0.50
;•
0.45
._•
0.40
E
0.35
dataatrepresentative
azimuthally
distributed
stations
for•the
pointsourcemodelis shownin Figure4.
The P waveforms,especiallythosenear the nodesof the
radiationpattern(KIP, AFI, RPN, CRZ andPAF),appearto
bestrongly
affected
by waterreverberations.
Thebathymetry
playsanimportant
rolein thecharacter
of thesewaves(Wiens,
19.89)andourhorizontally
layeredmodelis probably
inadequatein explainingthe P-waveseismograms
nearthe nodes.
The later partof the sourcetimefunctionshowsperiodic
pulseswith theperiodof thewaterreverberations,
probably
0.30
0.25
0
.
'•
10
ß
, "' ß
20
3;
ß
,
40
ß
11
50
depth (km below seafloor)
Fig. 2. Residualvariance(filled squares)
andseismicmo_•:t
(opensquares)
asa functionof centmiddepth.
BraunmillerandNfib•lek: The 1989MacquarieRidgeEarthquake
1.2
the waveformsand amplitudesbetterin thefirst 30-40 s than
the othermodels.Laterpartsof the P waveforms,
probably
contaminated
by waterreverberations
(e.g.AFI P wave),are
not modelledadequatelyby any of our models.Our final
1.0
0.8
0.6
model(Figure5 andTable1) hasthefollowing
parameters:
the
0.4
northern
segment
(Mo= 5.5x1020
Nm)hasa strikeof 212ø,
0.2
dip 82ø and rake 187ø, and the rupturelastsabout30 s,
0.0
whereas
thesouthward
segment
(Mo= 4.7x1020
Nm)hasa
0
10
20
30
40
60
50
time (s)
0.3
0.2
0.!
0.0
1,
s{
1019
,
i
,
-20
0
•uthwest
ß
ß
i
0
......
i
20
distance (kin)
ß
ß
40
ß
60
strikeof 210ø, dip95ø, rake174%andrupturelastingabout16
s. The kinematicbilateralmodel indicatesonly very small
momentreleasein thevicinityof theepicenter.
Thepossibility
of additionalslip at the epicenterduringlater stagesof the
ruptureis investigated
by allowingtheslipat theepicenterto
occurat arbitrarytime in a 25 s time frameandwe find that
additionalslipmighthaveoccurredthere.Theestimated
spatial
distribution
of theseismic
momentfor a bilateral
rupturemodel
with and withoutreslip in the epicentralarea is shownin
Figure 3b.
northeast
Fig.3. a) Seismic
moment
release
rateasa functionof time
(sometimefunction)for bestpointsourcePS (Table!). b)
Seismicmoment release along the fault length for the
bilaterally
propagating
source
BS2(solidline) (Table1) anda
bilateral
rapturemodelwith additionalslip in the epicentral
region
duringlaterstagesof the rupture(dashedline).Both
models
fit thedataequallywell.
thenorthandCRZ andPAF to the south),rupturepropagation
effectsshouldbe detectable.A southwestwardpropagating
rupture
fits the datatheleast,whereasa rupturepropagating
towards
northeast
slightlyimproves
themisfitcompared
tothe
pointsourcesolution.The bestfit is obtainedby a bilateral
rapture,
in whichslightlymoremomentis releasedalongthe
northward
than the southwardsegment(Table 1). Figure 4
shows
thematches
to thedataatrepresentative
stations.
$1•2:•5o
pm
....
SH
Becausethe P waves near the nodes continue to show sub-
stantial
mismatch,
weinvestigate
theimprovement
thatcanbe
achieved
by allowingvariationin the mechanisms
of the
northward
andsouthward
segments.
A smalldifferencein dip.
aMrakeappears
to considerably
improvethemismatch.
The
bilateral
modelwitha smallvariationin dipandrakematches
AFI.P 41'
PPT,P 68ø
PAF,P 235'
BJI,P 327"
AF!.$H 4I ø
CRZ,$H 227'
Fig. 5. Observed(solid) and theoretical(dashed)P and SH
wavesfor the modelBS2 in Table 1. Faultplanesolutionfor
the sum of the two subevents is shown at the center of the
figure.Seismograrn
amplitudes
arenormalizedto an epicentral
distance of 40 ø.
SummaryandDiscussion
The centroidalsolutionindicatesalmostpureright-lateral
strike-slipwith a strikeof 31o, whichis in goodagreement
with thelocaltrendof theridgein theepicentral
regionwhere
it hasa strikeof 33ø. Globalplatetectonic
modelspredicta
very similardirectionof platemotion.The RM2 modelof
MinsterandJordan(!978) predictsanazimuthof 53ø anda
rate of 2.7 cm/a, and the NUVEL-1 model(DeMetset al.,
anazimuthof 41ø andarateof 3.0cm/afor the
Fig,4. Comparison
of P andSH waveforms
for different 199.0)assigns
...models
listedin Table 1. Number next to the stationcodeis the
•ufft
fromthe source.
platemotionin theepicentral
region.
Our total seismic moment estimate is close to that of the
1020
Braunmiller
andN•ib•lek:The1989Macquarie
RidgeEarthquake
long-period
CMTsolution
(14x1020
Nm).Ourresults
show
DavidReinerr
drafted
thebaihymetric
map.Thisproject
was
that about 80% of this moment was releasedin the first 24 s
supported
by the NationalScienceFoundation•ant EAR-
of theruptureprocess.
Observable
directivitydueto rupture
8896187.
propagation
is surprisingly
small,partlydueto shortfault
lengthandpartlyto bilaterally
propagating
rupture.
A model
withrupturepropagating
unilaterally
towards
northeast
fits
betterthana modelwith a purelysouthwestward
rupture.A
bilaterally
propagating
rupturewitha smallvariation
in the
sourcemechanismsbetweenthe southernand northernseg-
ments
prov/des
thebestfit tothedata.Thelengthof thenortheastsegment
is about55 kmandtheseismic
moment
released
alongit is about5.5x1020
Nm. Thesouthwest
segment
is
shorter(35 km) and the seismicmoment slightly lower
(4.7x1020Nm).
Initially,onlya smallamount
of moment
wasreleased
in the
epicentral
region,butthereis a possibility
thattheregionreslippedduringlaterstages
of theruptureprocess.
Theinitial
lowmomentrelease
in theepicentral
regionandthesmalldifferencein dip of thetwo faultsegments
suggest
a possibility
thattheMacquarieRidgeearthquake
nucleated
neara geometric barrierformedby a smallchangein the dip of the fault.
King and N•b•lek (1985) and King (1986) showedthat
earthquakes
oftennucleate
nearfaultbendsor othergeometricalcomplexities
of thefaultzone.
References
Banghat,A. R., and L. R. Sykes,Focal mechanisms
of
earthquakes
in theIndianOceanandadjacent
regions,
J.
Geophys.Res., 74, 632-649, 1969.
Boyd, T. M., and J. L. Ntib•lek, Ruptureprocessof the
AndreanofIslandsEarthquake
of May 7, 1986,Bull. Seisra.
$oc. Am., 78, 1653-1673, 1988.
DeMets, C., R. G. Gordon, D. F. Argus, and S. Stein,
Currentplatemotions,Geophys.
J. Int., in press.
Duncan,R. A., andR. Vame, The age anddistributionof the
igneousrocks of Macquarie Island, Pap. Proc. R. $oc.
Tasm., I22(1), 45-50, 1988.
Kanamori, H., and D. L. Anderson,Theoretical basisof sou•
empiricalrelationsin seisinology,Bull. Seisin.Soc.Am.
65, 1073-1095, 1975.
King, G. C. P., Speculations
on the geometryof theinitiatim
andterminationprocesses
of earthquake
ruptureanditsrelation to morphologyandgeologicalstructure,Pageoph,124,
567-585, 1986.
The rupturelengthof 90 km associated
with the main
momentreleasebasedon the bilateral rupturemodel is
unusually
shortfor aneventof thismagnitude.
Thefightclus-
King, G. C. P., andJ. L. N/ib•lek,Theroleof bendsin faults
in the initiation and termination of earthquakerupture,
ter of the aftershocks
(Figure1) alsoindicatesthattherupture
Minster, J. B., and T. H. Jordan,Present-dayplate motions,
J. Geophys.Res., 83, 5331-5354, 1978.
Molnar, P., T. Atwater, J. Mamrnerickx, and S. M. Smith,
Magneticanomalies,bathymetry,andthe tectonicevolu•on
of the southPacific sincelate Cretaceous,Geophys.J. R.
probablydid notextendmorethan100 km. The stressdrop
associated
withthedominant90 km of therupture,takingthe
faultwidthof 20 km (twicethecentroidal
depth)anda seismic
momentof 10.2x1020Nm, is 180barandthe averagedisplacement
is 17 m. For a 100-kin-long
ruptureandthe total
seismicmomentof 13.4x1020Nm, thesevaluesare2!0 bar
and20 m. HencetheMacquarieRidgeearthquake
showsone
of thelargeststressdropsfor interplatestrike-slipeventsin
thismagnitude
range(seeKanamoriandAnderson,1975).
The relativelyshortfault lengthandlargestressdropof this
eventmay be relatedto the segmentation
of the Macquarie
Ridgein theepicentral
region.Bathymetric
chartsof theepicentralregion(Figure1) showthattheridgeis brokenup into
smallersegments,
on the orderof 50-100 km long. One of
thesetopographic
segments
coincideswith what we determinedto be theregionof theprimarymomentreleaseduring
the !989 event.Two thrustevents(BangharandSykes,1969;
HarvardCMT solution)thatoccurredin 1966 and 1984 seem
to brackettheregionthatbrokein thestrike-slip
fashionduring
the 1989 event,indicatingfault segmentationon the same
orderas seenin thetopography.
We submitthereforethatthe
observedridge segmentation
relatesto fault segmentation
whichconfinesrupturesto shortsegments.
Acknowledgements.
We thankDavidChavezfor providing
IDA data and Larry Ruff for providingGEOSCOPE data,
whichultimatelywerecontributed
by BarbaraRomanowicz.
Science, 228, 984-987, 1985.
Astr. $oc., 40, 383-420, 1975.
N/ib•lek, J. L., Determination
of earthquake
sourceparameters
frominversionof bodywaves,Ph.D. Dissertation,
360pp,
Massachusetts
Instituteof Technolog3r,
Cambridge,
1984.
Ruff, L. J., J. W. Given, C. O. Sanders,and C. M. Sperber,
Large earthquakesin the Macquarie Ridge Complex:
Transitional
tectonics
andsubducfion
initiation,Pageop,q,
129, 71-129, 1989.
Stock,J., andP. Molnar,Uncertainties
in the relativepositionsof the Australian,Antarctica,Lord Howe, andPac•.fic
platessincethe !ate Cretaceous,
J. Geophys.
Res.,87,
4697-4714, 1982.
Wiens,D.A., Bathymetric
effectson bodywaveforms
from
shallowsubduction
zoneearthquakes
andapplication
m
seismicprocesses
in theKurileTrench,J. Geophys.
Res.,
94, 2955-2972, 1989.
J. BraunmillerandJ. N•b•lek, Collegeof Oceanography,
OregonStateUniversity,Corvallis,OR 97331
(ReceivedMarch 19, 1990;
revisedMay 21, 1990;
accepted
May 21, 1990)