Seismicity of the Sunda Strait: Evidence for crustal
extension and volcanological implications
Hery Harjono, Michel Diament, Jacques Dubois, Michel Larue, Mudaham
Taufick Zen
To cite this version:
Hery Harjono, Michel Diament, Jacques Dubois, Michel Larue, Mudaham Taufick Zen. Seismicity of the Sunda Strait: Evidence for crustal extension and volcanological implications. Tectonics, American Geophysical Union (AGU), 1991, 10 (1), pp.17-30. <10.1029/90TC00285>.
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TECTONICS,
SEISMICITY
FOR
CAL
VOL. 10, NO. 1, PAGES 17-30, FEBRUARY 1991
OF
CRUSTAL
THE
SUNDA
EXTENSION
STRAIT:
AND
EVIDENCE
VOLCANOLOGI-
IMPLICATIONS
HeryHarjono,
1MichelDiament,
2Jacques
Dubois,
2and
MichelLarue3
Laboratoirede G6ophysique,
Universit6de ParisSud,Orsay,
France
Mudaham Taufick Zen
LaboratoriumGeofisika,Bandung,Indonesia
Abstract. The Sunda Strait is located in the transitional
zone between two different modes of subduction: the Java
frontalsubduction
andthe Sumatraobliquesubduction.
This
settingimpliesthat the SundaStraitregionis a key to the
understandingof the geodynamicprocessesinvolved.In
order to studythe shallowseismicity,a microearthquake
surveywascarriedout in thatregion.Twelvestations,accuratelylocatedby the aim of satellitepositionning,
recorded
about300 local eventsin the summer1984.From this set, 174
shallowearthquakes
havebeenpreciselylocated.The results
of thisstudyrevealthat the crustalearthquakes
in the Sunda
Strait area occursin three main areas: (1) beneath the
Krakatau complex,where earthquakesare generatedby
double-couples
andare of tectonicorigin;(2) insidea graben
in the westernpart of the strait;and (3) in a more diffused
zoneto the southof Sumatra.The individualand composite
focal mechanisms from the events inside the strait show an
extensionalregime. A stresstensor,which have been deducedfrom the individualfocal mechanismsof earthquakes
of the Krakataugroupshowsthat the tensionalaxisis orient-
edN130øE.
Thisstudyconfirms
thattheSundaStraitis in an
extensional
tectonicregimeas a resultof the northwestward
movementof the Sumatrasliverplate alongthe Semangko
fault zone.
INTRODUCTION
The Cenozoic collision between the Indian continent and
the Eurasianplate producedthe displacementof several
blocksor plateseastwardor southeastward
[Molnar and
Tapponnier,1975;Tapponnieret al., 1982]whichmustbe
studiedin orderto resolvethe geodynamicsof the Indonesianregion. The displacementof sucha block along the
Semangkofault was proposedby several authors [e.g.,
Huchon and Le Pichon, 1984; Deplus, 1987] in order to
explainthe originof the trenchjunctionin front of the Sunda
Strait. Consequently,the SundaStrait, whichis alsohistoricallyfamousdue to the presenceof the Krakatauvolcano,is
a key area to understandingthe geodynamicevolutionof
western Indonesia.
The tectonicevolutionof the westernpart of Indonesiais
often related to a clockwiserotation of Sumatraby 20ørelative to Javawith an axisof rotation lyingcloseto the Sunda
Strait during Late Cenozoic time [Ninkovich, 1976]. The
openingof the Sundastrait would then be related to that
rotation[e.g.,Zen, 1983].Other authorsas HuchonandLe
Pichon[1984]proposed
that the SundaStraitis an extensional area whichresultsfrom the northwestwarddisplacement
of the southernblock of Sumatraalongthe Semangkofault
systemas a consequence
of oblique subductionin front of
Sumatra.
AlsoatPuslitbang
Geoteknologi
- LIPI,Bandung.
NowatInstitut
dePhysique
duGlobe
deParis.
AlsoatORSTOM,
Noum6a,
Nouvelle
Ca16donie.
Copyrightr901
by theAmericanGeophysical
Union.
Papernumber90TC00285.
0278-7407/90/90TC-00285510.00
Until recently,veryfew geologicalandgeophysical
data
were availablein thisarea.A joint French-Indonesian
study
of geologyandgeophysics
wascarriedout from June1983to
February 1985 in order to collect marine and field data in
andaroundthe SundaStrait. Marine geophysical
data were
intensivelyrecordedduring CORINDON IX and GEOINDON I cruisesof R/V Coriolis(in 1983and in 1984) and
duringKRAKATAU cruiseof R/V JeanCharcot(in 1985).
Furthermore,fieldworkincludingneotectonicstudiesand a
microearthquake
surveywasconductedall aroundthe Sunda
18
Harjonoet al.:Seismicity
andCrustalExtension,
SundaStrait
Strait. In this paper, we present the results of the microearthquakesurvey.
If we considerthe seismicityof the SundaStrait area in
the contextof the worldwidenetwork,the hypocenterlocations are poorly constrained,especiallyfor shallowearthquakes,becausethe neareststationwhichcouldcontrolthe
focal depthis locatedabout 125 km from the strait. More-
104e
108 ß
I0• ß
over, no focal mechanism was available in the Sunda Strait.
So,we carriedout the surveyin the summerof 1984in order
to constrainthe shallowseismicityin the SundaStrait area,
andto better definethe seismologically
activetectonicfeaturesand their possibleextensionin the SundaStrait. Furthermore,our goalwasto checkif the onlyactivevolcanoin
that area, Krakatau,presentedsomeseismicityof tectonic
originandin suchcaseto determinetherelationship
between
the possibletectonic seismicitybeneath or close to the
Krakatau complexand the tectonicfeaturesof the Sunda
Strait.
TECTONIC
SETTING
Figures1 and 2 showthe geodynamic
pattern and the
geologicalsettingof the area. The main structuralfeatures
are the JavaandSumatratrenchsystem,
the Semangko
fault
system,andthe volcanicline goingfrom Panaitanislandto
Sukadana
throughthe Krakataucomplex,
Sebesi,Sebukuand
Rajabasa.
Java-SumatraTrenchSystem
The Indian-Australian plate underthruststhe Eurasian
platenorthwardbeneaththe JavaandSumatraislandsalong
the Java-Sumatratrenches.The rate of convergenceof the
102'
106'
Fig. 2. Geologicalmap of the studyarea simplifiedfrom
Nishimura[1986].1, Alluvialdeposit,2, Quaternaryvolcanic rocks,3, Quaternarytuff, 4, Pliocenedeposit,5, Miocene
deposit,6, Basement,7, Locationsof oil explorationdrill
holes,8, Refractionprofries(A andB) of CORINDON IX.
P, Panaitanisland,K, Krakataucomplex,Si, Sebesivolcano, Su, Sebukuvolcanicisland,R, Rajabasavolcano,Sa,
Sukadana
basalt.Simplified
bathymetry
(every400m)after
M.Larue(unpublished
data,1983).
subducting
plateis 7 cm/yr [SclaterandFisher,1974]andthe
directions of relative convergence, deduced from the
India/Eurasia pole of the global model of Minster and
Jordan [1978] are N24øE off Java and N23øEoff Sumatra.
More recently,Jarrard[1986]proposedthat the directionof
I10'
Fig. 1. Geodynamicsettingof westernIndonesiaafter Hamilton [1979].BF is the Batee Fault. SFZ
correspondsto the SemangkoFault Zone. Solid and open trianglesrepresentactiveand inactive
volcanoes.
Hatchuredzoneis thevolcaniclinethroughtheKrakataucomplex(K). The arrowsare the
directions
of plateconvergence
[afterSclaterandFisher,1974].Bathymetry
in km.
Harjonoet al.:Seismicity
andCrustalExtension,
SundaStrait
plateconvergence
is nearlyN-S. So,according
to thesedirections of relative convergenceand sincethe azimuth of the
Javatrenchis roughlyN100øEand that of Sumatratrenchis
N140øE, the Sunda strait area has to be considered as a
transition between a frontal and an oblique subduction
[HuchonandLe Pichon,1984;Deplus,1987].
Note that, in additionto the variationin the trench direc-
tions,the maximumdepthof the Benioffzonechanges,
from
600 km beneathJavato 200 km beneathSumatra[Fitch and
Molnar, 1970; Hamilton, 1974; Newcomb and McCann,
1987].Moreover, southof Java the maximumdepth of the
trench is greater than 6000 m, and decreasestowardsthe
Sumatra trench.
SemangkoFault System
The Semangkofault zone (Figure 1) [Van Bemmelen,
1949] runsparallel to the long axis of Sumatraand offsets
right laterallythe Sumatraisland[Katili and Hehuwat,1967].
Accordingto Molnar and Tapponnier [1975], the origin of
the Semangkofault is a consequence
of the collisionbetween
the Indianand the Eurasianplates.The Semangkofault zone
is supposedto accomodatethe obliquity of the Sumatra
subductionzone [Fitch, 1972; Beck, 1983; Huchon and Le
Pichon,1984].
Hamilton [1979]assumedthat the fault continuessouthward and intersectsthe trench,while Huchon and Le Pichon
[1984]proposedthat the fault endsin the SundaStrait as a
graben. Indeed, there is no field indication that the fault
extendsto West Java[Nishimuraet al., 1986].However,the
interpretationof gravitydata from West Javaindicatessome
trendsnearly parallel to the trend of the Semangkofault
[Untungand Sato,1978].Evidenceof a NW-SE right lateral
faultwasrecentlyobservedduringgeological
fieldworkin the
westernpart of West Java(S. PramumijoyoandM. S6brier,
personalcommunication,
1988).So, the questionof the exact
locationof the activeSemangkofault systemis still openand
canbe addressed
with the useof seismological
data.
The Semangkofault systemis also consideredas the
limit betweenthe Eurasianplate and the so-calledSumatra
sliverplate or forearcplate to the south[Jarrard,1986].In
that casethe SundaStrait area wouldbe locatedjust to the
north of a triple junction between the Indian plate, the
Eurasianplate,and the Sumatrasliverplate.Furthermore,as
the Sumatra sliver plate movesnorthwestward,the Sunda
Strait area would act as a trailing edgeand shouldpresent
extensional
structuralpatterns[Huchonand Le Pichon,1984;
Jarrard,1986].
19
grabenclearlyrevealrapid subsidence
sincePliocene[Lassal
et al., 1989;V.Renard et al., unpublisheddata from Krakatau'85cruise,1985].Suchrapid subsidence
is alsoevidenced
by the borehole data from an oil exploration area to the
southeast
of Krakatau(Figure2) [Noujaim,1976].This drilling encountereda thick accumulationof Quaternary to
UpperPliocenesedimentseries(2450m).
Figure3 displaysthe epicentersfor the period1964-1981
as determined by the worldwide network. To obtain this
figurewe selectedshallowearthquakes(depthslessthan60
km) with magnitudesgreater than 4.5 and recordedby at
least 10 stations.Figure 3 revealsthat the seismicityis not
correlatedwith the bathymetry.For example,the graben is
not conspicuous
on Figure3. The mostpronounced
figureis
the N-S seismic belt which
104'
105'
i
i
5'
m (Figure2). In the easternpart of the straitthe seabottom
is shallowerthan 100 m and relativelyflat; exceptinsidethe
Krakataucomplex,wherethe depthsare about200 m but are
related to the calderas.The stratigraphicinterpretationsof
seismicreflection profiles acrossthe easternflank of the
with the Krakatau
6.
106'
i
ß SUMATRA
4
Sunda Strait
The main featureof the bathymetricpatternin the strait,
accordingto the detailedsurveyof CORINDON IX in 1983
[M.Larue, unpublisheddata from CORINDON IX cruise,
1983],is a N-S grabenwith a maximumdepthof about1800
coincides
volcanicline. This led Nishimuraet al. [1986] to interpret
that seismicbelt as a fracturezone.A well pronouncedduster of seismicityis alsopresentin the southof Sumatraand
seemsto showa N-S linearpattern.
The area of the SundaStrait is mostlycoveredby Quaternaryvolcanicproducts(Figure 2). The recentvolcanic
activityaroundthe SundaStraitoccurredalongthe Krakatau
volcanicline [Nishimura et al., 1986]. According to these
authorsit startedin the north by alkali basalticemplacement
at Sukadana, continued to the south through Rajabasa,
Sebesi,Sebuku, Panaitan, and finally ended at Krakatau.
Neverthelessit mustbe notedthat no datinghasbeen done
on Panaitan. The K-At age determinationof the Sukadana
basalt is between 0.8 and 1.2 Ma [Soeria-Atmadja et al.,
1986;Nishimuraet al., 1986].Comparedto the other Quarternaryvolcanicrocksin the region,the sourceof thisbasaltic plateauis more primitive.This mightbe connectedto the
ii
o
•D
JAVA
7'
20 km
"
I
..
II,
Fig. 3. Superficial(lessthan 60-km depth) seismicitybetween1964and 1981asgivenby NEIS.
20
Harjonoet al.:Seismicity
andCrustal
Extension,
Sunda
Strait
fact that the SukadanabasalthasbeenpreferentiallyemplacedalongNW-SEtrendingfractureswhichare subparallel
to the Semangko
fault systemandnot parallelto the Krakatau volcanicline. At present,the onlyactivevolcanoin the
areais theAnakKrakatau(childof Krakatau)volcano
which
wasformedin 1927,44 yearsafter the famousexplosionof
Krakatau.Accordingto variousauthors[e.g., Zen, 1983;
104'
105'
,
106'
,
KLI
A
ß
-
BTG
ß• ß TBG
Camus et al., 1987] Krakatau is a calc-alkaline volcano.
Neverthelesspetrologicalstudiesindicatesomedifferences
betweenthe chemicalcompositions
of the Krakatauproducts
andthoseof the JavaandSumatravolcanoes
nearby[Nishimura, 1986].This would suggestthat the Krakatauvolcano
differs from the other volcanoes in the area.
In summary,questionssuchas the existenceof seismologicallyactivefeaturesin the SundaStrait, their exactlocations,their relationship
to the openingof the straitand their
relationshipto the volcanicactivityandthe stressregimein
ß. •
"o '•.•% •
the area need to be studied in order to better constrain the
geodynamic
evolutionof thisregion.
DATA
AND
/ •.L
.
i
0
ANALYSIS
,
20 km • ,
•
a
.
i
i
Network
From July 7 to August 22, 1984, 10 portable seismographswereinstalledaroundthe SundaStrait area (Figure
4). Five of thesestations(TRM, BTG, TBG, CIL, CBL)
remainedin operationuntilthe endof September1984.Two
additionalstationsfrom the Volcanological
Surveyof Indonesiaand the Geophysical
and Meteorological
Agencywere
alsousedin order to completeour network.The first, KRK
station,is intalled on the Anak Krakatau with the signal
beingbroadcastedto PAS station,and the secondone is KLI
station.All stations,
exceptKLI, wereMEQ-800Sprengnether associated with Mark Product L4C vertical seismometer
with an eigen frequency of 1Hz. KLI is a short period
permanentstation(Kinemetrics).
The seismometer
therehas
a peak eigenfrequencyof 3 Hz. The mean distancebetween
two stationswasabout30 km exceptfor the gapbetween
TRM
and UJK in the west entrance
of the Sunda Strait
(Figure
4).Theearthquakes
wererecorded
either
onsmoked
i
paper or with ink at a drum speedof 60 mm/min or 120
mm/min dependingof the access
facilities.Drift of internal
clocksof all MEQ-800waschecked
usingWWV radiosignal
every2 or 4 days.It must be noted that the drift remained
linearandsmall,rangingfrom 8 ms/dayto 30 ms/day.The
accuracy
of the clockfor KLI is givenby a permanent
reception of WWV. The positionsof the stationswere located
usinga JMR-4 satellite receiver and, dependingon the
numberof satellitesavailable,were determinedto an accuracysuchthatthe resultingstandarddeviations
weresystematicallysmallerthan60 meters.
Hypocenter
Determination
The arrivaltimeswerereadusinga magnifying
lensand
the estimated errors are of 0.1 s for the P waves and 0.5 s for
the S waves. We also estimated the total amount of error on
timeslessthantwicethesevaluesespecially
dueto the small
drift of the clocks.Hypocenterswere located using the
Hypoinverse
routineof Klein [1978].
Fig.4. • eventsrecorded
duringthissu•ey. Full andopen
circlescorrespondto the depthof 0-20 km and 20-50 km
respectively.
andthe full andopensquarescorrespond
to
50-100 km and more than 100 kin. The Krakatau
area is
shown•th moredetailon Figure7. Sizeof a s•bol correspondsto the classof accuracy(A, larger symbols,B,
intermediatesizesymbols,
C, smallersymbols).Triangles
show the stations.
Vp/Vsratio.The depthdeterminationis sensitiveto the
choice of Vp/Vs ratio if S waves are introduced in the
hypocentercalculation[McCaffrey et al., 1985]. In the
present study we estimatedthe Vp/Vs ratio by plotting
variousP and S arrival timesfor severalpairs of stations.
Sixtyfiveeventswith at leastfour S readingsanddistributed
overtheentirestudiedareawereusedto estimatetheVp/Vs
ratio usinga least squaresmethod.The resultingvalue of
1.72 _+0.01wasadoptedfor all otherhypocenterdeterminations.
Crustalstructure.A flat layeredmodelwasadoptedfor
hypocenterdeterminations.
Two seismicrefractionlineswere
shotin the SundaStrait duringthe CORINDON IX cruise
[Larue,1983].Their locationsis shownon Figure2. Interpre-
tationof theseprofiles,according
to Fatwan[1983],is given
in Table 1. The maximumpenetrationwasnot largerthan7
km, thuswe do not haveanyinformationon the lower crust
velocitystructure.Therefore,the lowercrustandthe upper
mantlevelocitiesandthe depthof the deepinterfacesmust
be deducedfromthe resultsof tests.Thesetestswereperformedusinganinitialcrustalmodelfor whichthedepthsof
interfacesandthe velocities
weresubsequently
varied.We
locateda selectedset of events(35 eventsa priori located
insidethe network)with a large numberof crustalmodels.
We then selectedthe most appropriatemodelsusingthe
meanrms [Hatzfeld et al., 1986;Kiratzi et al., 1986].Of
coursethe preferredcrustalmodelstronglydependson the
Harjonoet al.: SeismicityandCrustalExtension,SundaStrait
21
A
TABLE la. VelocityModel:ProfileA
ß250
8.1\
\
Depth (km)
Vp(km/s)
0
2.6
2.2
3.1
4.9
4.0
6.8
5.7
ß240
ß250
\
,.
.220
\x\
Upper crustal model deduced from refraction data
[afterFatwan,1983].SeeFigure2 for locationof profileA.
.210
ß200
TABLE lb. VelocityModel: ProfileB
Depth(km)
ß19o
Vp(km/s)
Moho depth(km)
0
2.6
4.0
5.5
5.2
6.8
B
.250
Upper crustal model deduced from refraction data
[afterFatwan,1983].SeeFigure2 for locationof profileB.
o
TABLE
ß2•)
.220
lc. Crustal Model Used for
Determination
of Hypocenters
.2!0
Depth(km)
Vp(km/s)
0
3.1
4.0
5.5
9.0
6.8
22.0
7.8
.190
Lower
crustal
interface
(km)
Seetextfor explanation.
c
.$20
startingmodel.For the uppercrust,we adoptedan average
betweenthe two interpretations
of seismicrefraction.For the
upper mantle velocity we adopted a starting value of 8.1
km/s, a commonvalue for this parameter. The starting
Moho depthwastaken from a deeprefractionsurveyperformed in the southof Java (located at lat. 8øSand long.
108.5øE)[Raitt, 1967]whichgavea valueof 22 km. Note that
Kieckhefer[1981]useda Moho depthat 25 km andan upper
mantlevelocityof 8.1 km/s in order to modelthe resultsof a
microseismicity
networkinstalledin the north of Sumatrato
studytheSemangko
faultsystem.
As concernsthe upper part, resultsshowedthat the
o
.280
•
.26o
.220
variations of the rms remained small. This is due to the fact
that most of the rays are refracted wavestraveling in the
lowercrust.The bestuppercrustalmodelis givenin Table
lc.
We thenvariedthe Moho depthandthe uppermantle
velocities,the lower crustvelocityandthe deepercrustal
interface.Results (Figures 5a and 5b) showthat the best
uppermantle velocityis 7.8 km/s and the one of the lower
crustis 6.8 km/s. This lastvalueis coherentwith the refrac-
Lower crustal Vp(km/s)
Fig.5. (a) Variationof rmsversusMoho depthandupper
mantlevelocity.(b) Variation of rms versuslower crustal
interface.(e) Variationof rmsversuslowercrustalP velocity.
22
Harjono
etal.:Seismicity
andCrustal
Extension,
Sunda
Strait
tionresultsandappears
wellconstrained
(Figure5c).The
Moho depthappearsto be 22 km for the chosenupper
mantlevelocityand more than 24 km if a higherupper
A
104'
105'
106'
20-5*0
KM
O-2OKM
•.•
'
'
SUMATRA
ß
mantlevelodtyis taken(Figure5a). Figure5c showsthat the
lowercrustal
interface
liesbetween
8 and9 km.Theresulting
ß
:
crustalmodelchosento usefor the hypocenter
locationsis
givenin Table lc. Of course,accordingto theseresults,
ß
ß
ß
ee ß
ee
ee eeeß • ß
slightlydifferentcrustalmodel canbe used.Thereforein the
followingsectionwe will discuss
the displacements
of the
locationsaccordingto the crustalvelocitystructureand to
ß
•
the variousclasses
of accuracy.
Earthquakelocations.We classifiedour hypocentersas
A, B, and C dependingon the criteriaof statisticalerror and
stationdistribution(includingthe numberof phasesused
duringdeterminations).Severaltestsmadewith 10 selected
eventsand our averagecrustalmodel were performedin
D
IO0-:•00
KM
I
ß
order to define our classification. We chose 10 events located
insidethe Krakatau complex,in the middle of the network,
sincethe depthsof these eventsare well controlledby the
KRK station.These 10 eventswere first locatedusingdifferent setsof P arrivaltimes(associated
with S or not). In order
to get an idea of the accuracyof poorlyrecordedevents(for
example,outsidethe network), we also relocatedour test
eventsusingonly readingsfrom severallimited setsof stations,for examplethe Sumatrastationsand CIL but excluding KRK.
Results
of these various
tests showed that for events
occuringinsidethe networkandlocatedusingat leastseven
P arrival
times with a distribution
Fig.6. Recorded
seismicity
asa function
of depth.Sizeof a
symbol
corresponds
to theclassof accuracy;
(largersymbols,classA, intermediatesizesymbol,classB, smaller
symbols,
classC).
of stations in the four
quadrants,we still got a goodresult,that is, the hypocenters
were not shiftedmore than 2 kin. Using six P arrival times
only, the epicenterswere as closeas 1 km to those determinedusingsevenarrival timesbut the error on the depth
(ERZ) waslarger (about5 kin). For eventslocatedoutside
IO5.3'
,
IO5.5 ß
,
,
the network, we had to introduce at least one S arrival time
in order to obtainan acceptableaccuracy(about 5 kin) on
the depth.
Accordingto theseresults,we definedour classifications
o
of A, B, and C events as follows. A events are those deter-
mined usingat least eight phasesincludingat least one S
arrival time, with arms lessthan 0.3 s, recordedby stations
in at least three quadrantswith at least one station with an
epicentraldistance(D rain) lessthan2 timesthe focaldepth
(2Z), and horizontal(ERH) and vertical(ERZ) error less
than 5 km. B eventswere locatedusingat least six phases
includingat leastoneS arrivaltime if outsideof the network,
with rms lessthan 0.4 s and recordedin at least two quadrants with D min lessthan 3Z, and ERH and ERZ lessthan
6.2 ø
10 km. C classeventscorrespondto thoselocatedusinga
minimumof sixphasesnot necessarily
with S, arms lessthan
0.5 s, with ERH and ERZ lessthan 15 kin. With this classifi-
cationwe retainedonly 174 shallowearthquakesamongthe
300localonesrecorded(Figures4, 6 and7).
Finally, as mentionedabove,in order to quantify the
qualityof the locations,we investigated
the displacements
of
the hypocenters
whenchanging
the crustalstructure.For that
purposewe relocateda set of randomlychoseneventsfrom
classesA and B usingseveralcrustalmodels.These models
were obtainedby varyingthe depthsof interfacesand the
velocityof the lower crust with the limiting values deter-
,
I0 km
ß
,
i
i
ß
A
ß
C:
i
Fig. 7. Epicenterscloseto the Krakataucomplex.Also
shown
arethetwocalderas.
Thethicklinecorresponds
to
theoldercaldera
andthethinlineto theoneresulting
from
the 1883 explosion(redrawn from Camusand Vincent
[1983]).Sizeof a symbolcorresponds
to the classof accuracy.
Harjono et al.: SeismicityandCrustalExtension,SundaStrait
minedpreviously
in the crustalmodeldetermination.
Results
showthat for eventsA the horizontaldisplacementis about
1.5 km and the verticaldisplacement
about2.0 km, while for
eventsB the valuesare 3.0 km and5.0 km respectively.
RESULTS
Figures4, 6 and7 showthe spatialdistribution
of epicenters accordingto the three classes.If we compare these
figures with the one deducedfrom teleseismicnetwork
(Figure3), we find somesimilarities
andsomedifferences
in
the seismicity
patterns.We did not recordedthe earthquakes
forminga N20øEseismicbelt that coincides
with the Krakatau volcanicline as clearly shownby worldwidedata. Our
microearthquakedata showthat the eventsare clustered
mainlybeneaththe Krakataucomplex.Nevertheless
we note
that the microseismicity
shownhere wasrecordedduringa
verylimited time period as comparedto the one shownon
Figure 3. On the other hand, the N-S seismicitysouthof
Sumatrarevealedon Figure3 is clearlypresentin our results.
AlthoughFigure6 showsthat the shallowseismicity
southof
Sumatrais scattered,eventsseemto be roughlyalignedalong
23
of tectonicearthquakes[Minikami, 1974]. However, there is
commonlyevidencethat the volcanicearthquakes
(tremors)
are not generatedby a doublecouple[Shimizuet al., 1987]as
are the tectonicones.Concerningour data, we believethat
the eventsbeneath the Krakatau complex are of tectonic
origin sincethey clearly showedhigh frequenciesand all
stationsaround the strait recordedboth up and down first
motions.
These earthquakesshowa tight vertical distribution.
Vertical crosssectionsfrom severalazimuthsshowthat they
form a narrowcolumn(Figure8). If we considertheir distributionas a functionof depth,it appearsthat the earthquakes
are concentratedbetween2 and9 km with a higherconcentration in the 5-9 km range. The earthquakesare lessfrequentfor depthsgreaterthan 10 kin. The local magnitudes
(magnitudeduration,ML) were generallybetween2.0 and
3.0. Someearthquakeslocatedat a depthbetween3 km and
8 km showmagnitudes
largerthan3.0; one shockevenhad a
magnitudeof 4.4.
w
E
a N30øE-N40øE trend rather than the N-S one evidenced on
Figure3. Unfortunatelythat seismicity
is poorlyconstrained
since it lies outside our network.
Indeed our resultsrevealthree verypronouncedconcentrationsof shallowearthquakes:
a Krakataucluster,a graben
clusterand a clustersouthof Sumatra(Figure 6). The two
first clustersare alignedalonga N50øE-N60øEazimuththat
roughlycoincideswith the southeastern
flank of the graben,
which is underlined by a pronounced gravity anomaly
[Diament et al., 1987]. Therefore the Krakatau complex
appearsto lie just at the intersection
of the volcanicline with
a fault trendingN50øE-N60øE.This probablyexplainswhy
Anak Krakatau is presentlythe only activevolcanoof the
volcanic line.
The northwesternflank of the grabenis alsounderlined
by a shallowseismicitythat prolongatesthe trace of the
Semangkofault zone (Figure 6). This confirmsthat the
Semangkofault system does not cross the Sunda Strait
towardsJavabut seemsto end in the graben,as inferredby
HuchonandLe Pichon[1984].It is alsonoteworthythat we
did not recordcrustalearthquakeseither on Sumatra,except
on the Semangko
fault,or on Java.This confirmsthe seismicity patterndeducedfromtheworldwidedata(Figure3).
We discuss now the various clusters of shallow earth-
quakes,occuringat depthsbetween0 and 20 km, sincewe
are mainlyinterested
in the crustalseismicity.
Krakatau
Cluster
ß
ß
ß
2O
ß
10Km
Fig. 8. East-westcrosssectionacrossKrakatau.
Individual focal mechanismsof these eventsgenerally
showan extensional
pattern(Figures8 andA1). However,if
we considerthe focal mechanismsas a functionof depth,
thereis an apparentsystematic
change,from compression
to
extensional mechanism.
1. For depthsbetween0-4 km, there are only two solutionsof focal mechanisms,wichboth showa strike-slipwith
reversecomponents
(Figure9a).
2. Between4 and6 km depth,the fault planesolutionsof
threeearthquakes
(Figure9b) showthat the mechanisms
are
mostly controlled by extensionbut with some strike slip
components.
Most of the events which are located beneath the Kraka-
tau complexhavebeenclassified
asA events.Figure7 shows
that the seismicity
is mainlyconcentrated
insidethe Krakatau
complex.
A questionariseswhetherthe eventsbeneaththis active
volcanowere of volcanicor tectonic origin. Earthquakes
associated
with a volcanicoriginsuchas magmamovements
normalyhavelow frequenciesas comparedto tectonicones.
Note howeverthat volcaniceventsmighthavean appearance
3.For depthsgreaterthan6 km, the extensional
pattern
seemsto dominate(Figures9c and9d). Someearthquakes
shownearlypure dip-slip.It is noteworthythat theseevents
are locatedcloseto the flanksof the caldera(e.g.,events2, 6,
10 or 12 on Figure 9c). Thus we proposethat theseearthquakescorrespondto displacement
alongfaultscontrolledby
the deepgeometryof the calderas.Two events(4 and 9) in
this clusterare slightlydifferentand showsolutionsof lateral
slip movements.Earthquake 5 located at a depth of 20 km
24
Harjono
etal.:Seismicity
andCrustal
Extension,
Sunda
Strait
A
•0,5.4
B
( 4 Km
•,o?.4
4 - 6 Km
14
I0
km
•
IO km
,
10![4
105.4
i
6-10
Km
)20
Km
5
IO km
Fig.9. Focalmechanism
asa function
of depthfor events
of theKrakataucluster.
Notetheevolution
fromcompression
toextension
withdepth.
Lower
hemisphere
projection
andblack
zonecorrespond
to compression.
shows
a normalmechanism
witha direction
of faultplane
nearlyN-S (Figure9d).
We proposethat this clear variationof mechanismswith
depthis related to local structuraleffectssincethe crustal
structurebeneaththe Krakataucomplexis verydisturbed.
Apart from the probablemagmachamber,the existence
of
thevarious
calderas
andprobable
volcanic
intrusions
might
induceslocal variation in the stressfield.
To checkthisassumption
of localvariations
we showon
Figure10all P andT axesof eventsdeeperthan4 km.This
figureclearlyconfirms
anextension
roughly
east-west
for all
events.We then inverted all the focal mechanismsof this set
inordertoobtain
a stress
tensor
using
themethod
of CareyGailhardis
andMercier[1987].Thetwoveryshallow
events
withreverse
faulting
werenotusedsincetheysignificantly
differfromthe deeperonesandcannotbe includedin sucha
computation.
Results
aregivenin Table2 andonFigure11.
This methodallowsselectionof a set of preferredfault
planesfroma setof auxiliaryplanes.The preferredfault
planewaschosen
sothattheanglebetween
thepredicted
t
andobserved
s slipvectors
issmall(lessthanabout25ø)and
theR value(relative
ratioof principal
stresses
differences)
is
between
0 and! (seeCarey-Gailhardis
andMercier[1987]
for details).
In Table2, thepreferred
faultplanesareunderlined.Thethirteen
faultplanes
areshown
onFigure11b.
O2 ando3 are the compressional,
the intermediate,
andthe
tensional
principal
deviatoric
stress
values
respectively.
This
computation
shows
thatall theeventsare dominated
by a
stress
regimewitha N130øE
extension,
thatis parallelto the
Semangkofault. This is an indicationthat the stresstensor
computedusingonlyeventsbeneathKrakatauis controlled
by the regionalstressregime.Finally,it appearsthatlocal
variationsin the stressfield beneath Krakatau are suchthat
extension
takesplacealongmoreor lesshorizontalfaultsfor
the deepestevents,alongmorecloseto the verticalonesfor
the4-6kmdeepevents
(seeTable3) andthatcompression
dominates
in theuppermost
partof thecrust.
Harjonoet al.:Seismicity
andCrustalExtension,
SundaStrait
25
agreeswith bathymetricdata.Althoughverypoorlyconstrained(seeFigureA1), mechanism
18 yieldsP andT axis
closeto the previous
ones.The fourthsolution,19,givesrise
to twopossibleinterpretations
asshownon FigureA1. One
solutiongivesa T axisNW-SE and the other one is SW-NE
direction.Since,as shownon Figure 13, the T axisfor the
grabenclusterare compatiblewith the stresstensordeterminedfrom data of the Krakataucluster,we can assumethat
the eventsof the grabenare controlledby the samestress
pattern as the ones of the Krakatau cluster. In order to dis-
criminate
thetwofaultplanesolutions,
we testedbothusing
our tensor.One solution(shownon Figure12) appearedto
be compatiblewith thistensor,thereforewe keptit (see
Table3).
Fig. 10. Compression(solid diamonds)and tension(open
diamonds)axisfor theKrakataucluster.
ClusterSouthof Sumatra
During the survey, many earthquakeswere recorded
from the area southof Sumatra(Figures4 and 6) but their
magnitudeswere generallysmall.For that reasonmany of
the earthquakeswere recorded only by the three nearest
stations(BTG, SDM and TRM). Most of the eventsfrom
this clusterbelongto classC. As we alreadymentioned,this
cluster is aligned approximatelyperpendicular to the Semangkofault. Unfortunately,we were not able to calculate
focalmechanismssincethe locationof the earthquakesare
outsidethe network.However,shallowearthquakes
recorded
by the worldwidenetwork have been located closeto this
cluster.Their centroidmomenttensorsolutions(CMTS)
[e.g.,Dziewonskiet al., 1989]mostlyrepresentsstrike-slip
faultingwithan extensional
component
(Figure14).
The extensiondirectionis more or lessparallel to the
Semangkofault, that is, parallelto the directionof extension
The Graben Cluster
Most of the eventsoccuringin the grabenare classified
as B and C events. Their epicenters are clustered on the
easternflank of the graben(Figures4, 6 and 12). There is a
goodcorrelationwith the abrupt scarpsin the bathymetry,
eitherwith the alreadymentionnedsoutheastern
flank of the
grabenor with the east and southflanksof the triangulary
hill lyingin the middleof thegraben(Figures6 and12).
Figure 12 showsfour compositefocalmechanisms,
which
were computedusingonly B events.Three mechanismsshow
an extensionalpattern.Mechanism16 hasa nearlyvertical
plane strikingparallel to the Semangkofault. Solution17
showsa normalfaultwith a strikedirectionnearlyN-S, which
TABLE 2. ComputedDeviation (t,s) and R Values(seetext) on the Nodal Planesof the
Focal Solutions of the Krakatau Cluster
Nodal Planes
N1
Event
Strike
Computed(t,s) andR Values
N2
N1
Dip
Strike
Dip
(t,s) R
N2
(t,s)
R
1
N356øE
68ø
N241øE
50ø
4.8 0.46a
38.4 -1.25
2
N309øE
78ø
N091øE
17ø
.1...2 0.42
61.7 11,62
3
4
N007øE
N148øE
66ø
77ø
N147øE
N042øE
32ø
53ø
59.9 1.37
10.8, 0,50
!2.3 0,26
68.4 .47.82
5
6
N192øE
N345øE
58ø
82 ø
N349øE
N151øE
35ø
8ø
42.2
75.0
10.9 0.57
8.5 0.53
7
8
9
10
12
13
N033øE
N067øE
N!95øE
N241øE
N230øE
N298øE
63ø
73o
30ø
82ø
80ø
62ø
N256øE
N172øE
N066øE
N086øE
N050øE
N!86øE
36ø
57o
70ø
9ø
!0ø
58ø
!3,3 -0.50
ill
0.15
30,2-0.62
61,2 1.03
30,2 1.01
85.0 -0.66
15
N053øE
44ø
N212øE
48ø
0.6
a Underlining
represents
preferred
faultplane.
1.29
1.01
0.08
1,2.• 0.09
27.3 -0.06
4,6 0.65
0,3_.0,33
0.•..• 0,71
21.3.....
:0:66
6.9
-0.12
26
Harjonoet al.:Seismicity
andCrustalExtension,
SundaStrait
••Z•Z•ZZZZZ•ZZ•
•ZZZZZZZZZZZZZZZZZZ
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
ß--
•--
•--
•--
0
',--
•--
•--
',--
0
,,--
•--
0
0
•--
27
Harjono
etal.:Seismicity
andCrustal
Extension,
Sunda
Strait
104OE
106øE
_
- 5ø$
•'•8•
!
eFivge
•;s2•
•2•hl•t
pgO
rSai•
•nf
•l•lst
•r•Cchoann•t
fe•
r2eør7
•ose2;uCtt•
using
closely
located
earthquakes
withidentical
depth.
N
Fig.11.(a) Superimposition
of thetensional
rightdihedra
of the 13 focalmechanisms
of the Krakataudustershowing
a N130øEextension.The tensionalzone is definedby area
containing
thenumber13.The compression
is shown
by
thedottedarea,i.e. subvertical.
(b) Deducedseismic
fault
planesandprincipalaxisof the computed
stresstensor.
Arrowsdenotecomputed
slipon the faults.Seetextfor
furtherexplanation.
computed
fromtheKrakatau
cluster.
We propose
thatthe
mostprobable
direction
offaulting
istheonethatgives
rise
to a rightlateralfault.Indeedtheplanewitha direction
normalto theSemangko
Faultwouldbe a left lateralstrike
slipwhichseemsdifficultto reconcilewith the dextral
movement
of the Semangko
fault.So,we proposethatthis
dustercorresponds
to a NW-SEdextral
strikeslipfault.Such
a fault,theBateefault(Figure1) wasdescribed
byKariget
al. [1980]northof Sumatra.
Thesefaults,transverse
to the
fore-arc
basinprobably
delineate
blocks
thatrotatealongthe
Fig.13.P andT axisforallevents
insidetheSunda
Strait.
Full andopensymbols
correspond
to P andT axesrespec-
tively.Triangles
anddiamonds
correspond
to thegraben
andKrakatauclusters.
Squares
showthe P andT axisof
oneeventrecordedby the worldwidenetworkandwhich
occurredinsidethe strait.This figureshowsthat the stress
patternin thevicinity
ofKrakatau
isidentical
tothestress
patternin theSundaStrait.
Semangko
fault.
CONCLUSIONS
Theanalysis
of thelocalseismicity
recorded
bya microearthquake
survey
in theSundaStraitareacomplemented
bybathymetry
anddatafromtheworldwide
network
allowus
to drawthe followingconclusions:
1. Seismicactivityin the SundaStraitareais concentrat-
ed in three clusterscenteredon Krakatau, a grabenin the
western
partof thestraitandsouthofSumatra.
TheKrakatauandgrabendusters
arelinkedby a faultstriking
N50øE
N60øE.
2. Our resultsconfirmthat the SundaStrait area is under
an extensional
tectonicregime.The stresstensoranalysis
gives
a N130øE
direction
ofextension,
thatisparallel
to the
28
Harjonoet al,:Seismicity
andCrustalExtension,
SundaStrait
104•
105•
,
',
lOG•
,
,
1
2
3
4
9
10
11
12
13
14
16
17
18
19
,
zoSUMATRA
12127185
{
I 39
618184
1213185
ltat•/ JAVA
10122187
21 3125186
•//•.•.,,..•.._•,
12114185
20km
Fig. 14. Harvard centroid moment tensor solutions
(CMTS) for the dustersouthof Sumatra[e.g.Dziewonski
et al., 1989].Numbersgivethe depth(in km).
Semangkofault. This crustalstretchingshouldbe a resultof
the NW movement of the Sumatra fore-arc plate (sliver
plate) [Huchon and Le Pichon,1984;Jarrard, 1986]. This
result agreeswith other geophysicaldata suchas seismic
reflection[Lassalet al., 1989].
3. Seismicitysouthof Sumatrais interpretedas a fault
transverse
to the fore arc basin.But, obviously,
an additional
surveywhichwould alsoinvolveoceanbottomseismographs
is necessary
in orderto betterconstrainthat seismicity.
4. As a consequence
of the crustalextension,the Sunda
Straitis characterized
by importantvolcanicactivityas shown
by large depositsof Quaternarymaterial [Nishimuraet al.,
1986]. The fact that Krakatau differs from volcanoes
commonelsewherein Indonesiacanbe relatedto the specific, extensional,
tectonicsetting.The existence
of the presently
activeAnak Krakataualsoappearsas a consequence
of its
Fig.A1. Lowerhemisphere
equalareafaultplanesolution
asshownin Figures9 and 12. Compressional
and dilatational first motionsare shownas full and open circles
respectively.The squaresrepresentthe direct wavesand
dots the refracted waves.
Jusuf,EmanD. Ruswandi,A. TatangandSardiman)for
their help in operatingthe seismographs
and thank the
peopleof the area of the surveyandthe telecommunication
stationsof TanjungKarangandCilegon,KrakatauObserva-
location as the intersection of an active fault with the volcanic
toryin Pasauran
andPPAUjungKulon.We acknowledge
D.
line runningfrom Panaitanto Rajabasa.
5. Seismicity
beneaththe Krakataucomplexis controlled
by the regionalstressfieldbut we find a variationwithdepth
Hatzfeld, who suppliedus five MEQ instrumentsand the
Volcanological
Surveyof Indonesia
(VSI) andtheIndonesian
Meteorology
andGeophysical
Agency(BMG) for providing
dataof KRK and KLI. We thankE. Carey-Gailhardis
for
discussions
andfor providing
a programfortensoranalysis.
Themanuscript
benefitedof discussions
withC. Deplus,J.
that we relate to local effects.
Acknowledgments.
We dedicatethisworkto JeanBloyet,
who enthusiastically
participatedin the field work and the
preliminary interpretations. We thank M. R6gnier
(ORSTOM), C. Jouannic(ORSTOM), I. Suhardi(BPPT),
and S. Wirasantosaand M. E. Arsadi from Puslitbang,
Geoteknologi-LIPI
for their assistance
duringthe surveyand
in arrangingthe field work. We also thank the technician
group of PuslitbangGeoteknologi-LIPI (A. Sanyoto,D.
Rahayu, Suyatno,Y. Sudrajat, E, Junaidi, D, Rusmana,
Deverch•re, O. Lassal,P. Tucholka and from extensiveand
detailed
reviews
byH. Lyon-Caen
andD. Hatzfeld.Figure14
wasdrawnafter originalfiguresproducedat I.P.G. Paris
usinga software program of G. Ekstr6m. This work was
suported
byA.T.P. "G6ologie
et G6ophysique
desOc6ans"
of
C.N.R.S,IFREMER, ORSTOM, LIPI (Indonesian
Institute
of Sciences)
andBPP.Teknologi(Agencyfor theAssessment
andApplication
of Technology).
Harjonoet al.:Seismicity
andCrustalExtension,
SundaStrait
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J1.Ganesha
10,Bandung
40132,
Indonesia.
M. Diament, J. Dubois, H. Harjono and M. Larue,
Laboratoirede G6ophysique
(URA du CNRS 1369),B•t.
509,Universit6ParisSud,91405Orsay-Cedex,
France.
(Received
November
17, 1988;
revisedJanuary24, 1990;
acc•'pted
January25, 1990.)