Seismicity of the Sunda Strait Evidence for Crustal Extension And

TECTONICS, VOL. 10, NO. 1, PAGES 17-30, FEBRUARY 1991 SEISMICITY OF THE SUNDA STRAIT: EVIDENCE FOR CRUSTAL EXTENSION AND VOLCANOLOGI- CAL IMPLICATIONS HeryHarjono, 1Michel Diament, 2Jacques Dubois, 2 and MichelLarue 3 Laboratoirede G6ophysique,Universit6 de ParisSud, Orsay, France Mudaham Taufick Zen LaboratoriumGeofisika, Bandung, Indonesia Abstract. The Sunda Strait is located in the transitional INTRODUCTION zone between two different modes of subduction: the Java frontalsubduction and the Sumatraoblique subduction. This The Cenozoic collision between the Indian continent and settingimplies that the SundaStrait regionis a key to the the Eurasian plate producedthe displacementof several understandingof the geodynamicprocesses involved. In blocksor plateseastward or southeastward[Molnar and order to study the shallow seismicity,a microearthquake Tapponnier,1975; Tapponnier et al., 1982]which must be surveywas carried out in that region.Twelve stations, accu- studiedin order to resolvethe geodynamicsof the Indone- ratelylocated by the aim of satellitepositionning, recorded sianregion. The displacementof sucha block along the about300 local eventsin the summer1984. From this set, 174 Semangkofault was proposedby several authors [e.g., shallowearthquakes have been precisely located. The results Huchon and Le Pichon, 1984; Deplus, 1987] in order to of this studyreveal that the crustalearthquakes in the Sunda explainthe originof the trenchjunction in front of the Sunda Strait area occurs in three main areas: (1) beneath the Strait. Consequently,the SundaStrait, whichis alsohistori- Krakatau complex,where earthquakesare generated by callyfamous due to the presenceof the Krakatauvolcano, is double-couplesand are of tectonicorigin; (2) insidea graben a key area to understandingthe geodynamicevolution of in the westernpart of the strait;and (3) in a more diffused western Indonesia. zone to the southof Sumatra.The individualand composite The tectonicevolution of the westernpart of Indonesiais focal mechanisms from the events inside the strait show an often related to a clockwiserotation of Sumatraby 20ø rela- extensionalregime. A stresstensor, which have been de- tive to Javawith an axisof rotation lying closeto the Sunda ducedfrom the individualfocal mechanismsof earthquakes Strait during Late Cenozoic time [Ninkovich, 1976]. The of the Krakataugroup shows that the tensionalaxis is orient- openingof the Sundastrait would then be related to that ed N130øE.This study confirms that the SundaStrait is in an rotation[e.g., Zen, 1983].Other authorsas Huchonand Le extensionaltectonic regime as a resultof the northwestward Pichon[1984] proposed that the SundaStrait is an extension- movementof the Sumatrasliver plate along the Semangko al area whichresults from the northwestwarddisplacement fault zone. of the southernblock of Sumatraalong the Semangkofault systemas a consequenceof oblique subductionin front of Sumatra. Alsoat Puslitbang Geoteknologi - LIPI, Bandung. Until recently,very few geologicaland geophysical data Nowat Institut de Physique duGlobe de Paris. were availablein this area.A joint French-Indonesianstudy Alsoat ORSTOM, Noum6a, Nouvelle Ca16donie. of geologyand geophysicswas carried out from June1983 to February 1985 in order to collect marine and field data in and aroundthe SundaStrait. Marine geophysicaldata were Copyrightr901 intensivelyrecorded during CORINDON IX and GEOIN- by theAmerican Geophysical Union. DON I cruisesof R/V Coriolis(in 1983 and in 1984) and duringKRAKATAU cruiseof R/V JeanCharcot (in 1985). Papernumber 90TC00285. Furthermore,fieldwork including neotectonic studies and a 0278-7407/90/90TC-00285510.00 microearthquakesurvey was conducted all aroundthe Sunda 18 Harjonoet al.:Seismicity and Crustal Extension, Sunda Strait Strait. In this paper, we present the results of the micr- 104 e 108 ß I0• ß oearthquakesurvey. If we considerthe seismicityof the SundaStrait area in the contextof the worldwidenetwork, the hypocenterloca- tions are poorly constrained,especially for shallowearthquakes,because the neareststation which couldcontrol the focal depth is locatedabout 125 km from the strait. More- over, no focal mechanism was available in the Sunda Strait. So, we carriedout the surveyin the summerof 1984in order to constrainthe shallowseismicity in the SundaStrait area, and to better define the seismologicallyactive tectonic fea- tures and their possibleextension in the SundaStrait. Fur- thermore,our goal was to checkif the only activevolcano in that area, Krakatau,presented some seismicity of tectonic originand in suchcase to determinethe relationshipbetween the possibletectonic seismicitybeneath or close to the Krakatau complexand the tectonicfeatures of the Sunda Fig. 2. Geologicalmap of the studyarea simplifiedfrom Strait. Nishimura[1986]. 1, Alluvial deposit,2, Quaternaryvolcan- ic rocks,3, Quaternarytuff, 4, Pliocenedeposit, 5, Miocene TECTONIC SETTING deposit,6, Basement,7, Locationsof oil explorationdrill holes,8, Refractionprofries (A andB) of CORINDON IX. Figures1 and 2 showthe geodynamicpattern and the P, Panaitanisland, K, Krakataucomplex, Si, Sebesivolca- geologicalsetting of the area. The main structuralfeatures no, Su, Sebukuvolcanic island, R, Rajabasavolcano, Sa, are the Javaand Sumatra trench system, the Semangkofault Sukadanabasalt. Simplified bathymetry (every 400m) after system,and the volcanicline goingfrom Panaitanisland to M.Larue(unpublished data, 1983). Sukadanathrough the Krakataucomplex, Sebesi, Sebuku and Rajabasa. subductingplate is 7 cm/yr [Sclaterand Fisher, 1974] and the Java-SumatraTrench System directions of relative convergence, deduced from the The Indian-Australian plate underthruststhe Eurasian India/Eurasia pole of the global model of Minster and platenorthward beneath the Javaand Sumatraislands along Jordan [1978] are N24øE off Java and N23øE off Sumatra. the Java-Sumatratrenches. The rate of convergenceof the More recently,Jarrard [1986]proposed that the directionof 102' 106' I10' Fig. 1. Geodynamicsetting of westernIndonesia after Hamilton [1979].BF is the Batee Fault. SFZ correspondsto the SemangkoFault Zone. Solid and open trianglesrepresent active and inactive volcanoes.Hatchured zone is the volcanicline throughthe Krakataucomplex (K). The arrowsare the directionsof plateconvergence [after Sclater and Fisher, 1974]. Bathymetry in km. Harjonoet al.:Seismicity and Crustal Extension, Sunda Strait 19 plate convergenceis nearlyN-S. So, accordingto thesedirec- grabenclearly reveal rapid subsidencesince Pliocene [Lassal tions of relative convergenceand since the azimuth of the et al., 1989; V.Renard et al., unpublisheddata from Kraka- Javatrench is roughlyN100øE and that of Sumatratrench is tau'85 cruise,1985]. Such rapid subsidenceis also evidenced N140øE, the Sunda strait area has to be considered as a by the borehole data from an oil exploration area to the transition between a frontal and an oblique subduction southeastof Krakatau(Figure 2) [Noujaim,1976]. This drill- [Huchonand Le Pichon,1984; Deplus, 1987]. ing encountereda thick accumulationof Quaternary to Note that, in additionto the variationin the trench direc- Upper Pliocenesediment series (2450 m). tions,the maximumdepth of the Benioffzone changes,from Figure3 displaysthe epicentersfor the period1964-1981 600 km beneathJava to 200 km beneathSumatra [Fitch and as determined by the worldwide network. To obtain this Molnar, 1970; Hamilton, 1974; Newcomb and McCann, figurewe selectedshallow earthquakes (depths less than 60 1987]. Moreover, southof Java the maximumdepth of the km) with magnitudesgreater than 4.5 and recordedby at trench is greater than 6000 m, and decreasestowards the least 10 stations.Figure 3 revealsthat the seismicityis not Sumatra trench. correlatedwith the bathymetry.For example,the graben is not conspicuouson Figure 3. The mostpronounced figure is SemangkoFault System the N-S seismic belt which coincides with the Krakatau volcanicline. This led Nishimura et al. [1986] to interpret The Semangkofault zone (Figure 1) [Van Bemmelen, that seismicbelt as a fracturezone. A well pronounceddus- 1949] runs parallel to the long axis of Sumatra and offsets ter of seismicityis alsopresent in the southof Sumatra and right laterallythe Sumatraisland [Katili and Hehuwat,1967]. seemsto showa N-S linearpattern. Accordingto Molnar and Tapponnier [1975], the origin of The area of the SundaStrait is mostlycovered by Qua- the Semangkofault is a consequenceof the collisionbetween ternary volcanicproducts (Figure 2). The recent volcanic the Indianand the Eurasianplates. The Semangkofault zone activityaround the SundaStrait occurredalong the Krakatau is supposedto accomodatethe obliquity of the Sumatra volcanicline [Nishimura et al., 1986]. According to these subductionzone [Fitch, 1972; Beck, 1983; Huchon and Le authorsit startedin the north by alkali basalticemplacement Pichon,1984]. at Sukadana, continued to the south through Rajabasa, Hamilton [1979]assumed that the fault continuessouth- Sebesi,Sebuku, Panaitan, and finally ended at Krakatau. ward and intersectsthe trench, while Huchon and Le Pichon Neverthelessit mustbe noted that no datinghas been done [1984]proposed that the fault endsin the SundaStrait as a on Panaitan. The K-At age determination of the Sukadana graben. Indeed, there is no field indication that the fault basalt is between 0.8 and 1.2 Ma [Soeria-Atmadja et al., extendsto West Java[Nishimura et al., 1986].However, the 1986;Nishimura et al., 1986]. Comparedto the other Quar- interpretationof

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