This document is downloaded from DR-NTU, Nanyang Technological University Library, Singapore. Title Neotectonics of the Sumatran fault, Indonesia. Author(s) Sieh, Kerry.; Natawidjaja, Danny. Sieh, K., & Natawidjaja, D. (2000). Neotectonics of the Citation Sumatran fault, Indonesia. Journal of Geophysical Research, 105, 28295–28326. Date 2000 URL http://hdl.handle.net/10220/8470 © 2000 American Geophysical Union. This paper was published in Journal of Geophysical Research and is made available as an electronic reprint (preprint) with permission of American Geophysical Union. The paper can be found at the following official URL: http://dx.doi.org/10.1029/2000JB900120. One print or Rights electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper is prohibited and is subject to penalties under law. JOURNALOF GEOPHYSICAL RESEARCH, VOL. 105, NO. B12, PAGES 28,295-28,326, DECEMBER 10, 2000 Neotectonicsof the Sumatran fault, Indonesia KerrySieh SeismologicalLaboratory, California Institute of Technology,Pasadena 1 DannyNatawidjaja Seoteknologi,Lembaga Ilmu PengetahuanIndonesia, Bandung, Indonesia Abstract.The 1900-km-long, trench-parallel Sumatran fault accommodates a significant amountof thefight-lateral component of obliqueconvergence between the Eurasian and Indian/Australianplates from 10øN to 7øS.Our detailed map of thefault, compiled from topographicmaps and stereographic aerial photographs, shows that unlike many other great strike-slipfaults, the Sumatran fault is highly segmented. Cross-strike width of stepovers betweenthe 19 major subaerial segments is commonlymany kilometers. The influence of thesestep overs on historical seismic source dimensions suggests that the dimensions of futureevents will alsobe influenced by faultgeometry. Geomorphic offsets along the fault rangeas high as -20 krn andmay represent the total offset across the fault. If thisis so,other structuresmust have accommodated much of thedextral component of oblique convergence duringthe past few millionyears. Our analysisof stretchingof theforearc region, near the southerntip of Sumatra,constrains the combined dextral slip on the Sumarran and Mentawai faultsto be no more than 100 km in thepast few millionyears. The shape and location of the Sumatranfault and the active volcanic arc are highly correlated with the shape and character ofthe underlying subducting oceanic lithosphere. Nonetheless, active volcanic centers of the Sumatranvolcanic arc have not influenced noticeably the geometry of theactive Sumatran fault.On the basis of its geologichistory and pattern of deformation,we dividethe Sumatran platemargin into northern, central and southern domains. We supportprevious proposals thatthe geometry and character of the subductingInvestigator fracture zone are affecting the shapeand evolution of the Sumatranfault systemwithin the central domain. The southern domainis themost regular. The Sumatran fault there comprises six fight-stepping segments. Thispattern indicates that the overall trend of thefault deviates 4 ø clockwise from the slip vectorbetween the two blocksit separates.The regularity of thissection and its association withthe portion of thesubduction zone that generated the giant (Mw 9) earthquakeof 1833 suggestthat a geometricallysimple subducting slab results in bothsimple strike-slip faulting andunusually large subduction earthquakes. 1. Introduction roughly coincidentwith the active Sumarranvolcanic arc !.1. Plate Tectonic Environment (Figure 1). On its northeasternside is the southeastAsian plate, separatedfrom the Eurasianplate only by the slow TheSumatran fault belongsto a classof trench-parallelslipping Red River fault of Vietnam and southernChina strike-slipfault systems that work in concertwith subduction [Allen et al., 1984]. On its southwesternside is the Sumatran zonesto accommodateobliquely convergent plate motion "forearcsliver plate" [Jarrard, 1986], a 300-km-widestrip of [Yeatsetal., 1997,Chapter 8]. Otherstrike-slip faults that lithospherebetween the Sumarranfault and the Sumatran occurin similarsettings include the left-lateralPhilippine deformation front. At its northwestern terminus the Sumarran fault(parallel tothe Luzon and Philippine trenches), Japan's fault transformsinto the spreadingcenters of the Andaman right-lateralMedian Tectonic Line (parallel to theNankai Sea [Curray et al., 1979]. At its southeasternend, in the trough),and Chile's Atacama fault (parallel to the South Sunda Strait, the fault curves southward toward the Americantrench). deformation front [Diament et al., 1992]. Forits entire 1900-km length the Sumarran fault traverses The basic kinematic role of the Sumatran fault is rather thehanging wall blockof the Sumatransubduction zone, simple:It accommodatesa significantamount of the strike- slip componentof the oblique convergencebetween the Australian/Indianand Eurasianplates. The pole of rotation 'NowatDivision ofGeological andPlanetary Sciences, California for the relative motion between the Australian/Indian and InstituteofTechnology, Pasadena. Eurasian plates is in east Africa, ~50ø west of Sumatra Copyright2000by the American Geophysical Union. [Prawirodirdjoet al., this issue,?rawirodirdjo, 2000; Larson et al., 1997]. NorthernSumatra is closerto thispole than is Papernumber 2000JB900120. southernSumatra. Thus the orientationand magnitude of the 0148-0227/00/2000JB900120509.00 relative-motionvector vary significantlyalong the Sumatran 28,295 28,296 SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS 95OE 100øE 105øE 110ø.E 10ON Andaman Sea South Ch/na Sea 5ON 0 o 5os 10os Figure 1. Regionaltectonic setting of the Sumatranfault. The Sumatranfault (SF) is a trench-parallel,right-lateral strike-slipfault that traversesthe hangingwall blockof the Sumarransubduction zone from the SundaStrait to the spreadingcenters of the AndamanSea. It separatesa forearcsliver plate from the southeastAsian plate. Triangles are active volcanoesof the Sundaarc. Arrow is relativeplate motion vectors determined from GPS. Topography andbathymetry are from Smithand Sandwell[1997]. WAF is the West Andamanfault. MF is the Mentawai fault. portionof the plate boundary(Figure 1). At 6øS, 102øEit is Fitch[1972] suggested that the right-lateral component of 60 mm/yr, N17øE [Prawirodirdjo et al., this issue]. At 2øN, this obliqueconvergence is the causefor the right-lateral 95øE, it is 52 mm/yr, N10øE. Furthermore,because the shape Sumarran fault. McCaffrey [1991, 1992] added more of the plate boundaryis arcuate,the natureof relativeplate substanceto this hypothesiswith his discoverythat slip motionchanges markedly along its strike. At the longitudeof vectorsof moderateearthquakes along the Sumatran portion central Java the strike of the subductionzone is nearly of thesubduction zone are nearly perpendicular to the strike orthogonalto the direction of relative plate motion, so any of theplate boundary. He notedthat if thesevector directions component of strike-slip motion need not be large are representativeof long-term slip trajectoriesalong the [McCaffrey,1991]. At the latitudesof Sumatra,however, the subductioninterface, then subduction itself is only slightly strike-slip component of relative plate motion must be obliqueand most of thedextral component of plate motion significantbecause the direction of relative plate motion is must be accommodated elsewhere. substantiallyoblique to the strikeof the subductionzone. The Sumatranfault is the most obviouscandidate for SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS 28,297 accommodationof theremaining component of dextral slip. localities to determinewhether or not the actual slip rates TheMentawai fault, discovered offshore by Diamentet al. conform to current kinematic models. Such rates would also [1992],complicates thisslightly. This major, submarine, serve as a long-termaverage for the interpretationof geodetic trench-parallelfaultlies between the Sumatran fault and the data from Global PositioningSystem (GPS) networksthat trenchand may also have accommodated a significant amount now spanthe fault [Genrichet al., this issue]and historical ofthe dextral component of plate motion. triangulationdata [?rawirodirdjo et al., this issue]. Thecombination of an arcuateplate boundary and a distant poleofrotation suggests thatthe rate of dextral slip along the 2. A Modern Map of the Fault Sumatranfault increasesnorthwestward [Huchon and Le Pichon,1984; McCaffrey, 1991]. Observationsnear the To mapthe Sumatranfault efficiently and reliably, we have northwesternandsoutheastern termini of theSumatran fault reliedprimarily upon its geomorphicexpression. Geomorphic supportthiscontention. Curray et al. [1979]suggested that expressionis especiallyreliable for mappinghigh slip rate therate of openingacross the spreadingcenters of the faults, where tectoniclandforms commonly develop and are AndamanSea (Figure 1) hasaveraged about 37 mrn/yrfor the maintained at rates that exceed local rates of erosion or burial past11 Myr. They proposed thatmost of this motion has been [Yeats et al., 1997, Chapter 8]. Examples of carriedto thesoutheast by the Sumatranfault. Reanalysisof geomorphologicallybased regional maps of active faults thesedata yields the same rate; total opening in thepast 3.2 includeactive fault mapsof Japan,Turkey, China, Tibet, and Myris ~1I8 km (J.Curray, written communication,