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Title Neotectonics of the Sumatran , .

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 ,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 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 arcuate plate 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, 1999). Mongolia[Research Group for ActiveFaults, 1980; Saroglu Theslip rate inferred for theSumatran fault near its southern et al., 1992; Tapponnierand Molnar, 1977] as well as most terminus,however, appears to be far lower than 37 mrn/yr. mapsof submarineactive faults. Bellieret aI. [1999] calculatea rate of ~6 mm/yr near the Admittedly,the geomorphicexpression of activefaults southernend of the fault from an offsetchannel incised into a withslip rates that are lower than or nearlyequal to localrates datedPleistocene tuff. of erosionor burialis likely to be obscure.This is especially likelyif thefaults are short, have small cumulative offset, or 1.2. Motivation of This Work have no componentof vertical motion. Becauseof our Despiteits ranking as one of Earth'sgreat strike-slip faults, relianceon geomorphicexpression, our map of the Sumarran itshigh level of historicalseismic activity and its major role in fault undoubtedlyexcludes many short,low-rate active fault strands. theactive and seismic hazard of SoutheastAsia, the Sumatranfault has not been well characterized. What 2.1. Resources and Methods attentionthe fault hasreceived has been predominantly from a greatdistance, mostly at platetectonic scales. Until recently, The grossestfeatures of theSumatran fault have long been thegeometry of thefault was known only to first-order(see, knownfrom analysisof small-scaletopographic and geologic forexample, the small-scale maps of Fitch [ 1972],BelIier et maps. More detailedsmall-scale maps of the fault, based al. [1997]or McCaffrey[ 1991]. More detailedstudies have uponanalysis of satelliteimagery, have been produced more beenlimited to local studies,such as Tija's [1977] and Katili recently[Bellier et al., 1997; BeIlier and Sebrier, !994; andHehuwat's [1967] work on exemplaryoffset drainages. Detourbetet al., 1993]. The unavailabilityof stereographic The Sumatranfault has generated many historical imagery,however, limited the resolution and the reliability of earthquakeswith magnitudes M> 7, butbecause most of these thesesmall-scale maps. Specifically,the lack of stereoscopic happenedmore than a half a centuryago, they have not been coverageprecluded the recognitionof importantsmall well documented.Reid [1913] used geodeticmeasurements tectoniclandforms, unless they were favorablyilluminated. from before and after the 1892 Sumatran earthquakeas Conversely,inactive faults lacking small, late Pleistocene and supportfor his conceptof elasticrebound. Berlage [1934] Holocenetectonic landformsmay have been mapped as describedthe effects of the 1933earthquake in southSumatra. active,based upon the presenceof olderand larger tectonic Visser [1927] described the effects of the 1926 landforms. Padangpanjangearthquake in westSumatra, and Untung et al. Ourmapping of theSumatran fault is basedprimarily upon [1985]and Natawidjaja et al. [1995]recently reported dextral inspectionof l:50,000-scaletopographic maps and l:100,000- offsetsformed during the nearby 1943 Alahanpanjangscale aerial photographs. Where these were not available or earthquake. were of unsuitablequality, we utilized l:250,000-scale Thepaucity of detailedmaps of thefault, the scarcity of geologicmaps and radar imagery. Figure2 displaysthe dataon historical large earthquakes, and the lackof reliable coverageof materialsthat we used. estimatesof slip rates are unfortunate. They seriouslyhamper Figure3 displaysrepresentative stereographic pairs of the attemptsto forecast the seismicproductivity of thefault and 1:100,000-scaleaerial photographs. These photos display the effortsto understandquantitatively its role in the oblique fault at about0.3øS, where it offsetsstream channels that are convergenceof the Sumatranplate boundary. deeplyincised into a thickpyroclastic flow deposit.After Ourfirst task in this study,then, has been to constructa interpretingthese and otherstereopairs, we compiledour modemmap of theactive components of the Sumatran fault. interpretationsonto l:50,000-scaletopographic maps (or To be of use in seismic hazard assessmentand in l:250,000-scaletopographic maps, where the larger-scale understandingtheneotectonic role of thefault, the scale of the maps were unavailable).Where stereographicaerial mapneeded to be largeenough to clearlydiscriminate major photographswere unavailable,we interpretedactive fault faultstrands and the discontinuitiesand changes in strike geometryand sense of slipdirectly from the l:50,000-scale betweenstrands. topographicmaps. Oursecond task, which will be describedin a futurepaper, Thesedata were then digitizedand attributed,using the willbe to determinethe sliprate of the faultat several GeographicInformation System (GIS) software,Arc/Info. 28,298 SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS

100 0 I00 200 300 400 Kilometers

The Sumatran Fault Lake Coveragesof 1:250,000topographic maps Centersof aerial photographs Coveragesof 1:50,000 topographic maps Figure2. Dataupon which our map compilation isbased. Most of ourmapping is basedon inspection of 1:50,000- scaletopographic maps produced by BAKOSURTANAL& JANTOP,the national mapping agencies for Indonesia, andl:100,000-scale aerial photographs. Other data sources include smaller-scale geologic and topographic maps.

The resultingG1S databaseincludes fault geometry,sense of 2.3. Major Segmentsof the Sumatran Fault fault slip, and photo centers. Plate 1, constructedfrom the Superimposedupon the broad sinusoidalgeometry of the database,depicts all of the salient featuresof the Sumarran Sumarranfault are more than a dozen discontinuities,ranging plate boundarythat we mappedand compiled. in width from -5 to 12 km (Plate 1). Major local changesin strikealso occur. Most of the discontinuitiesare right stepsin 2.2. Geometry of the Fault the fault trace and tht•s representdilatational step overs. The overall shape of the Sumarranfault acrossSumatra is However, a few contractional bends also occur. sinusoidal(Figure 1). The northernhalf of the fault is gently Theoretically,these discontinuities and bendsin the faultare concave to the southwest, whereas the southern half of the large enoughto influencethe seismic behaviorof the fault fault is concave to the northeast. Over the 1650-kin [Harris et al., 1991; Harris and Day, 1993]. The relationship subaeriallyexposed length of the fault, the "amplitude"of the of historicalruptures to thesegeometrical segment boundaries sinusoidal trace is -55 km. will be the subjectof a futurepaper (D. Natawidjajaand K. Ornamentingthe broad, sinusoidalshape of the Sumatran Sieh, manuscriptin preparation,2000). fault are numeroussmaller irregularities. Though smaller, We haveused these second-order geometric irregularities these have dimensions of the order of tens of kilometers and to dividethe Sumarranfault into 19 segments(Figure 4 and are thereforetectonically and seismologicallysignificant. Table 1). Each segmentbears the name of a majorriver or The greatestof theseis a featurethat we call the Equatorial bay alongthe segment. In so namingthe varioussegments, B ifurcation (Figure 1 and Plate 1). This forceps-shaped we haveabandoned the usual practice of retainingnames that feature is present between the equator and about 1.8øN have precedence in the scientific literature. The latitude. It is characterizedby the bifurcationof the Sumatran nomenclatural morass inherited I¾om numerous earlier studies fault toward the southeastinto two principal active strands. includesmany fault names derived from nearbycities, The two strands are distinct from each other even at their districts, basins, and rivers. These include Banda Anu, point of bifurcation(about 1.8øN).The greatestseparation of Lam TeubaBaro, Reuengeuet Blangkejeren, Kla-Alas, Ulu- these two branches is -35 km, near 0.7øN. The western Aer, Batang-Gadis,Kepahiang-Makakau, Ketahun, Muara branch of the bifurcation does not rejoin the easternbranch Labuh,and Semangko [e.g., see Katili andHehuwat, 1967; farther south; instead, it dies out geomorphicallyat about Cameronet al., 1983; Durham, 1940]. Sincemany of these 0.35øN. overlapour geometricsegment boundaries or includeonly Other large irregularities include subparallel partsof oursegments, we haveabandoned them in favorof a geomorphicallyexpressed fault traces at about 5.5øN, 4øN, moresystematic and precisenomenclature. and 3.5øS. The Batee fault, a right-lateralfault that may have Forthe entire group of activefault segments, from Aceh in displacedthe island's western shelf-150 km since the thenorth to theSunda Strait in thesouth, we havechosen the Oligocene[Karig et al., 1980], divergessouthward from the name"Sumarran fault," first usedby Katili and Hehun'at Sumatranfault at about 4.6øN. A 75-km-long fold-and-thrust [1967]. This namerepresents best the dimensionof the belt, exhibiting clear geomorphicevidence of youthfulness structure.Earlier names for the faultare "Semangko"and lies about 40 km west of the Sumarran fault at about 1.3øN. "Ulu-Aer,"suggested by VanBemmelen [1949] and Durham All of these features are described in section 2.3. [1940]; but these refer to local features. "Great Sumatran

SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS 28,301

segmentsalong the Sumatranfault, we suggestthe particular namesin Figure4. In sections 2.3.1-2.3.19, we describe each segment, beginningin the south. Each descriptionfocuses on the geomorphicexpression of the segmentand its terminations. Discussionof importanthistorical earthquakes is minimal becausethe associationof earthquakeswith segmentswill be the focusof a futurepaper. Likewise, we do not focuson the slip ratesof the varioussegments because this alsowill be the principaltopic of a futurepaper. Plate 1 displaysthe fault at a scale that is appropriatefor the detailed discussionthat follows. (This plate and it's databaseare alsoavailable as postscriptand GIS (ArcView) files at www.scecdc.scec.org/geologic/sumatra). 2.3.1. Sunda segment (6.75øS to 5.9øS). Bathymetric maps of the SundaStrait, betweenJava and Sumatra,reveal that the southernmostportion of the Sumarran fault is associatedwith two prominentsouth striking fault scarpson the seafloor [Nishimura et al., 1986; Zen et al., 1991; Pramumijoyo and Sebrier, 1991]. These scarps form a submarinegraben, ranging in depth to 1800 below sea level 0 2000 m (Figure 5). The large vertical displacementsof the seafloor and the orientationand locationof the faults suggestthat their senseof slip is normaland dextral. Focal mechanismsfrom a local seismicnetwork [Harjono et al., 1991] and from the Harvard Centroid Moment Tensor (CMT) cataloguesupport this interpretation. They show normal-fault mechanismson the westernside of the graben. Furthermore,faults appear on both sides of the graben in three seismic reflection profiles [Lassal et al., 1989]. The grabenwidens southward, toward the subductionzone, but losesbathymetric expression ---130 km from the trench, near where one would expect it to intersectthe floor of the Sumatranand Javanforearc basins (Figure 5). A belt of fault scarpsand foldsof the innertrench slope continues across the southwardprojection of the graben, but the outer-arcridge and forearc basin that are prominent in the offshore of Sumatraand Java are absentin this region. Instead, these features appear to converge upon each other and to be replaced by a narrow, 150-km-long plateau across the projectionof the graben. The lesseningof sliver-platewidth occasionedby the absenceof the forearc basin and outer-arc ridge appearsto be accommodatedby a landwarddeflection Figure3. An exampleof the approximatelyl:100,000-scale of the trenchaxis (Figures 1 and 5). aerialphotographs we usedto compilemost of our mapof the Huchon and Le Pichon [1984] were the first to propose Sumatranfault. These two setsof stereopairsshow channel that the disappearanceof the outer-arcridge and the forearc offsets of *-720 m. The channels cut a late Pleistocene basin acrossthe southernprojection of the Sumarranfault pyroclasticflow depositat about 0.3øS. The flat upland indicatesstretching parallel to the Sumatranfault. They also surfacesare the unincisedtop of theflow. Theseoffsets yield speculatedthat the subtlebending of the trenchtoward the aaverage slip rateof- 11 mm/yr. Sunda Strait indicates arc-normal thinning of the region betweenthe trailingedge of the Sumarranforearc sliver plate and the crust offshore from Java. This would be consistent with the northwestwardtranslation of the forearc sliver plate troughsystem" was first usedby Westerveld[1953]. Since along the Sumarran fault. We attempt to quantify this "gmat"is not used for other faults of similar dimension, we stretchingin section3. suggestthat it not be used for the Sumarranfault. In keeping 2.3.2. Semangko segment (5.9øS to 5.25øS). From withconvention generally accepted in California,where "San beneaththe watersof SemangkoBay at about5.9øS to a 6- Andreasfault system" refers to theSan Andreas and its many km-wide dilatationalstep over that has producedthe Suoh auxiliaryfaults, we use"Sumarran fault system" (SFS) for the Valley at about5.25øS, the principaltrace of the Sumarran Sumarranfault and other structuresthat are related to the fault runs almost linearly along the southwesternside of accommodationof strike slip along the Sumatranplate SemangkoBay and the SemangkoValley (Plate 1 and Figure margin.These would include the Batee fault, the Toru fold- 4). The prominentnortheast facing escarpment along the 65- and-thrustbelt, and the Mentawai and the West Andaman km lengthof this segmentattests to a significantcomponent faultsinthe forearc region (Figure 1). Fordiscrete, individual of dip slip, southwestside up. An earthquakeon July 26, 28 302 SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS

NORTHWEST

,..,, . C' ß

% % SOUTHEAST

KUMERING SEMANGK0

INDIAN OCEAN

t• Lake ßVolcanic crater Volcanicedifice majorvalleys Figure4. Map of 20 geometricallydefined segments o1: the Sumatran fault system and their spatial relationships to active volcanoes,major graben,and lakes.

1908, may have involved rupture of all or most of this The northwesternmost15 km of the Kumering segment segment[Berlage, 1934]. deviateswestward from the trend of the rest of the segment 2.3.3. Kumering segment(5.3øS to 4.35øS). This 150- and is part of a 10-km-widecontractional jog. This portion km-long segmentruns between the dilatationalstep over at of the segmentdisplays a significantcomponent of reverse Suoh Valley to a contractionaljog at 4.35øS. Near the center slip, as evidencedby a high escarpmentand a mountainous of this segment, the waters of Lake Ranau occupy a late anticline north of the fault trace. Aerial photography Pleistocenecaldera and conceal about 9 km of the trace (Plate available to us did not reveal the continuation of the fault i and Figure 4). The southernpart of the Kumeringsegment tracenorthwest of 4.35øS,through the restof thecontractional traversesthe drainagesof the Werkuk and upper Semangko bend. rivers. A less active southeastward continuation of this High intensitiesindicate rupture of many tensof kilometers segmentmay form the northeasternflank of the Semangko of the Kumeringsegment during the Ms 7.5 Liwa earthquake Valley [Pramumijoyoand Sebrier, 1991],but we did not have of June24, 1933[Berlage, 1934]. Deadlyphreatic explosions adequatematerials to determineits activitythere. occurred2 weeksafter the earthquake within the Suoh Valley North of Lake Ranau, a 40-km-long reach of the fault [Stehn, 1934]. traversesthe headwatersof the Kumering River. The trunk A geomorphicallyless prominent subparallel strand of the streamof this large river doesnot crossthe fault; instead,its fault exists2.5 km to the southwestof the principalactive two major tributariesflow toward one anotheracross the trace tracesouth of LakeRanau [Natawidjaja, 1994; Widiwijayanti of the fault and flow northeastwardaway from the fault from et al., 1996].The devastatingMw 6.8 Liwa earthquakeof their confluence.This relationshipof large streamchannels to 1994 wasgenerated by this lessprominent trace. The most the fault is common along much of the Sumarranfault; not sev6redamage and the aftershock region coincided with a 25- uncommonly,the headwatersof a principal stream are near km reachof this secondarytrace. the fault, and none of the larger channelsof the drainage 2.3.4. Manna segment(4.35øS to 3.8øS). This85-km network cross the fault trace. In these cases, dextral offsets of segmentdeviates only a kilometeror two from being the streamchannels are either ambiguousor small. recfilinearbut has rather obscureterminations on bothends SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS 28,303

o o

o u,'% o o o CD 0 u"% o u% o 0 u"', o o • o,1 ',,ID u"', 0,1 00 0 28,304 SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS

SUMA.'RA deformationfront x, JavaSea outer-arcridge axis

---T--forearc basinaxisnormal thrust'• fault inferredanticline %$ JAVA bathymetrycontour (in meters) ' depreasion/basin a, active volt. dnO

%0 INDIAN OCEAN

Figure 5. Sumatranfault and relatedstructures near the SundaStrait and bathymetricmap of the portionof the SundaStrait and surroundingseafloor. The Sundasegment of the Sumarranfault formsan 1800-m-deepgraben that widenssouthward, toward the deformationfront. Northwestwardmovement of the forearcsilver plate alongthe Sumatranfault appearsto have causedthinning of the regionbetween the trenchand the strait. Bathymetryis Digital ElevationModel ETOPO02 and bathymetricsurveys [Smith and Sandwell, 1997].

(Figure 4). The Manna segmentappears discontinuous on stratovolcano, Bukit Kaba. Stream channels cut into the Plate I becausethe trace is obscurelocally on the aerial youngestflows there are offset -700 m. We have usedthese photographsand topographicmaps. The southernend of the channelsto determinethe slip rate of 11 mm/yr for theMusi segmentabuts the contractionalbend mentionedabove. The segment (D. Natawidjaja and K. Sieh, manuscriptin northernend of the segmentis obscurebeyond about 3.8øS preparation,2000). but appearsto be within a geometricallycomplex right The destructive,Ms 6.6 Kepahiangearthquake occurred (dilatational)step in the fault. alongthis segmentat about3.6øS on December 15, 1979. We Exceptionally clear 2.4 + 0.2 km dextral offsets of two heardeyewitness accounts of minor crackingalong the fault large streams(Air Kanan and Air Kiri) exist on the dissected whenwe visitedin 1993, but we saw no convincingevidence westernflank of an extinct volcanos,.utheast of Pajarbulan of tectonicsurficial ruptures from 1979. (Plate 2). We encounteredsurprisingly we!l-preserved small 2.3.6. Ketaun segment (3.35øS to 2.75øS). This 85-km- tectonic landforms beneath the jungle canopy during an long segment consists of a linear trace with several excursionin 'thedrainages of thesetwo streams. discontinuitiesand stepovers of about a kilometer in A destructiveearthquake occurred in the vicinity of this dimension(Plate 1 and Figure 4). The segment'ssouthern segmenton June 12, 1893. The area of greatestdamage end is at a 6- to 8-km-widedilatational step over ontothe coincidedwith the centralpart of the Manna segment[Visser, Musi segment.An inactiveor lessactive continuation of the 1922]. Ketaunsegment naay extend beneath the stratovolcanicedifice 2.3.5. Musi segment (3.65øS to 3.25øS). This 70-kin of Bukit Kaba. This possibilityis suggestedby thepresence segmentof 'the Sumatranfault comprisesseveral highly of a geomorphicallysubdued fault, southeastof the volcano discontinuousfault segments(Figure 4 and Plate 1). Despite and ---25km eastof the centralMusi segment.The northern good coveragewith l:100,000-scaleaerial photography,we end of the Ketaun segment is within a 6-km-wide couldnot identify clear geomorphic traces along much of this contractionalstep over. Within this contractionalstep oxer segment. the topographyrises several hundredmeters above the The longestcontinuous trace that we were able to map surroundinglandscape. traverses the southwesternflank of the large, active Twomajor rivers cross the Ketaun segment, the Ketaun in SIEHAND NATAWIDJAJA:SUMATRAN FAULT NEOTECTONICS 28,305

thesouth and the Seblat in thenorth. The Seblat river valley southeasternterminus of the segment[Natawidjaja et aI., appearstobe offset dextrally -17 km, and the Ketaun river 1995]. valleymay be offset -23 km. A moderateearthquake on 2.3.10. Sumani segment(1.0øS to 0.5øS). This 60-kin- March15, 1952 (M 6.2,U.S. GeologicalSurvey (USGS)), longsegment runs northwestward from the volcanicterrane of producedhighintensities along the Ketaun segment [Kraeff, Lake Diatasto the southwesternflank of Lake Singkarak, 1952]. which occupiesa structuralgraben, rather than a volcanic 2.3.7. Dikit segment(2.75øS to 2.3øS). This is a caldera(Figure 4 andPlate 1). Two opposingarcuate normal predominantlylinear,60-km-long segment with several short, oblique faults form topographic scarps that rise400 m above obscuresections along its northernfew kilometers(Plate 1 the surfaceof the lake (Plate 3). Ancient uplandsurfaces, andFigure 4). It sharesa contractionalstep over with the with drainagesflowing away from the lake, are clearly Ketaunsegment on its southeasternend. Its northwesterntruncated by the steepscarps bounding the lake basinand thus terminationis at oneof thelarger dilatational step overs along appearto have been faulted down below the waters of the theSumarran fault. On the southwesternflank of this11-km- lake. widestep over, the Dikit segmentdisappears into the edifice Failureof the Sumanisegment produced the secondof two ofthe small stratovolcano Kunyit. This is oneof thefew clear large earthquakes(Ms 7.4 [Pachecoand Sykes, 1992]) on associationsof a di!atationalstep over and a volcanoalong the June9, 1943 [Natawidjajaet al., 1995]. Shakingintensities Sumatranfault. indicatethat the northwesternend of the fault rupturewas Thesmall diamond-•haped caldera of Dipatiampatisoffset beneaththe lake. Eyewitnessaccounts led Untung et al. ~500m by thefault. Justnorth of the smallcaldera lake, at [1985] to concludethat right-lateraloffsets of up to 2 m about2.65øS, the main trace appearsto form an enigmatic occurrednear the town of Solok,but Natawidjaja et aI. [1995] d0gleg.The Dikit River Valley follows the fault for -20 km. couldonly verify offsetsof ~1 m. Analysisof geodeticdata Weare not convincedthat this representsa dextral offset of supportsa meter or so of dextralslip [Prawirodirdjoet al., 20kin, because the constructionof two largevolcanic edifices this issue]. hasundoubtedly obscured older drainageson the block High intensitiesin the vicinity of Lakes Dibawah and northeastof the fault. Diatassuggest that the entiresoutheastern part of the segment 23.8. Siulak segment (2.25øS to 1.7øS). Clear alsoruptured, and perhaps even the northwesternpart of the dilatationalstep overs demarcate the terminationsof this 70- Suliti segment. kin-longsegment (Figure 4 and Plate 1). The 11-kmwide The first of two largeearthquakes on August4, 1926 was stepoverat the southeastern end is thewidest dilatational step most severein the narrow zone along the Sumani segment. overalong the Sumatranfault, but our aerialphotography did Anotherearthquake, on October1, 1822, was most severe notreveal its structural details. The northern terminus of the betweenthe Marapi andTalang volcanoes (Wichman, as cited Siulaksegment is a 4-km-widestep over on the westernflank by Visser [1927]). Thus this earthquakemay well have of the great active stratovolcanoKerinci. West dipping involvedrupture of the Sumanisegment. Genrich et al. [this normalfaults cut lavas of Melenggok volcano there, and issue]show that strainaccumulation during the early to mid- appearto transfer slip from the Siulak segmentto its 1990sis consistentwith 23 + 5 mngyr of dextralslip on this northwesternneighbor. segment. Along the Siulak segment's southeasternreach, Lake 2.3.11. Sianok segment (0.7øS to about 0.1øN). This Kerinciand the alluvium of a broad valley obscurethe fault predominantlystraight and continuoussegment runs -90 km tracefor-30 km. Two largeearthquakes have caused severe from the northeastshore of Lake Singkarak,along the damagealong the Siulak segmentof the Sumatranfault. On southwestflank of the greatstratovolcano Marapi to a 10-km- June3, 1909,most of the regiontraversed by this segment wide right stepover at the equator(Plate 1 and Figure4). Its wasdevastated by an earthquakejudged to havea magnitude southern18 km, on the flank of Lake Singkarak,is arcuate ofabout Ms 7.7 [Abe, 1981]. The zone of greatestdamage and must have a significantcomponent of normalfaulting duringthe M 7.0 earthquakeof October6, 1995,was within downtoward the lake. Geomorphicexpression of the fault is thebroad valley northwestof Lake Kerinci (Indonesian particularlyinteresting along the Sianok segmentbecause it newspaperKompas, October 7, 1995). traversesthe flank of Marapi volcanoand the young,200-m- 2.3.9. Suliti segment(1.75øS to 1.0øS).This 95-km-long thick pyroclasticflow depositof Maninjou volcano. Stream segmenthas a comparativelystraight fault trace,which channelsflowing off Marapi displayclear dextra!offsets that terminateson both the northwest and southeast at di!atational rangefrom -120 to 600 m. The trunk channelof the Sianok stepovers within volcanicedifices (Figure 4 and Plate 1). River is incisedinto the ManinjouTuff and displayoffsets of Thenorthwestern step over, at Lake Diatasand Talang ~700 m (Figure 3). We havebeen able to usethese offsets to volcano,is 4 km wide. The details'Of'the central reachesof determinea dextralrate of slip of- 11 mm/yr (D. Natawidjaja thesegment are obscure because the fault traverses the narrow andK. Sieh,manuscript in preparation,2000). valleyof the Suliti River headwatersfor more than 50 km. The secondof two large earthquakeson August4, 1926, Howmuch of thiscourse of thefault along the Suliti River wasmost severe along the southeasternportion of the Sianok valleyrepresents a dex•raloffset is unknownbecause the segment.This is consistentwith Visser's[1927] observation trunkstream does not cross the fault. Alongthe southernmost of fault rupturebetween Bukittinggi and Singkarak. Genrich partof thissegment, tributaries of the Liki River are offset et aI. [this issue] show that strain accumulation acrossthis severalhundred meters. segmentin the early to mid-1990sis consistentwith dextral Thefirst of twolarge earthquakes of June 9, !943 (Ms7.1 slip of 23 + 3 mngyr. [PachecoandSykes, 1992]), may have involved rupture of the 2.3.12. Sumpur segment(equator to 0.3øN). Dataalong northernpart of the Su!iti segment,judging from serious this 30-km-long segmentand its northwesternneighbor are damageto Muaralabuhvillage, 25 km northwestof scant.Our mapis basedpredominantly upon 1:250,000-scale 28,306 SIEH AND NATAWIDJAJA' SUMATRAN FAULT NEOTECTONICS SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS 28,307 geologicmaps[Rock etal., 1983, Aspden etal., !982] and the valleys of the Gadis and Angkola Rivers, between poorlyreproduced 1:50,000-scale topographic maps. Malintangand Lubuk Raya volcanoes[Visser, 1922]. Bothtermini of the Sumpursegment are at large 2.3.15. Toru segment(1.2øN to 2.0øN). Major bendsin dilatationalsteps. Thus, between the Sianok and Barumun the fault trace delimit this segmentof the Sumatranfault segments,the fault experiences a 35-km-wide, double- (Figure 4 and Plate !). We definethe southernterminus to be dilatationalstep over. The northwesternstep is associatedat a regionalbend of 15ø at !.2øN. The topographichigh east witha highwest facing escarpment and the adjoiningwide of the bendsuggests that this is a contractionalbend. valleyofthe Sumpur-. The northwesterntermination of the Tom segmentoccurs at a 2.3.13.Barumun segment (0.3øN to 1.2øN). This ! 15- 15ø regionalbend in the fault, which is coincidentwith a 2.5- Ian-longsegment is broadlyconcave toward the southwestkm dilatationalstep over. We canbe confidentthat this bend andforms most of the northeasternleg of the Equatorial is dilatationalbecause the segmentto the northwestdoes not Bifurcation(Figure 4 andPlate 1). Thesoutheastern 40 km of displaynet verticaldeformation across the fault andthe bend theBarumun segment forms the boundarybetween a high coincideswith theTamtung depression. westfacing escarpment and the broaddepression of the Northwest of Sibual-buali volcano, a 30-kin-wide caldera SumpurRiver. We interpretthis escarpment and adjacent northeastof the fault is truncatedby the fault. The otherhalf depressionto be evidenceof a significantcomponent of of the caldera, southwest of the fault, must be concealed extensionaldip slip on thisportion of the Barumunsegment. beneathyoung volcanic deposits. The geomorphicexpression Weplace the northwesternend of the Barumunsegment of the fault in the vicinity of thetruncated caldera is unusually somewhatarbitrarily at an abrupt15 ø bendin the traceof the complex. Significantcomponents of dip slip occuron faults fault,near the headwaters of theBarumun River. that splay northwardfrom the main trace into the caldera. 0nlyalong its northernmost35 km havewe beenable to The Tom segment has not produced a major historical inspectl:100,000-scale aerial photography. There the fault earthquake,but right-lateralslip near the northernend of this tracesdisplay clear geomorphic evidence of strikeslip. The segmentdid generatethe Ms 6.4 PahaeJahe earthquakeof channelof the BarumunRiver may be offsetabout 20 km, but 1984. thisoffset is not compellingbecause the trunk streamdoes not 2.3.16. Renun segment(2.0øN to 3.55øN). This longest crossthe fault. segmentof the Sumarranfault traversesthe westernflank of 2.3.14. Angkola segment (0.3øN to 1.8øN). The the 80-km-long Toba caldera, alleged to be the largest southwesternbranch of the EquatorialBifurcation consists of Quaternarycaldera on Earth [Chesneret al., 1991]. Much of a continuousfault with an abrupt30 ø bend at about0.65øN theRenun segment traverses the thick pyroclastic flow deposit (Figure4 and Plate 1). Geomorphicexpression is particularly of that 73,000-year-olderuption. The regionalexpression of clearbetween about 0.8øN and 0.5øN. Katili and Hehuwat this 225-km-longsegment is linear,except for a doglegalong [1967]used offsets of tributariesto the Angkola River at its northwesternmost30 km, where the segmentforms the about0.55øN to demonstrateright-lateral offsets ranging from southwesternflank of the Alas Valley graben. This graben, 200to 1200m alongthis segment.The northern30 km of the 45 km long and 9 km wide, is oneof the largestgraben along Angkolasegment consists of a set of discontinuousfaults on the Sumarran fault. West of , the fault consistsof thesouthwestern flank of the Sarullagraben. Althoughlarge- several30- to 40-km-longstrands, arranged en echelon,with scaleaerial photographsdo show minor, discontinuous across-strikeseparations of only a kilometeror so. Although faultingat about 0.35øN, the lack of through going the right-steppingnature of the en echelonpattern suggests geomorphicexpression of the westernbranch south of 0.5øN that the fault is experiencinga minor componentof showsthat the fault is significantlyless active there. The transtensionin the uppercrust, the stepovers are associated westernsegment does not rejoin the northeasternstrand just with horsts,not graben. northof the Equator. Geologic mapping supportsthis The southeasternmostpart of the Renunsegment exhibits a interpretation,and suggeststhat total slip on the western well-defined2-km offsetof the 73,000-year-oldToba Tuff, branchcannot be large [Rock et al., 1983]. Geodetic whichwe haveused to determinea 27 mm/yr sliprate for the measurementsspanning the early to mid-1990ssuggest that fault [Sieh et al., 1991; D. Natawidjaja and K. Sieh, modemstrain rates are higherin the vicinityof theAngkola manuscriptin preparation,2000]. GPS measurementsacross segmentthan on the mainsegment farther east [Genrich et al., thesouthern portion of thissegment suggest slip rates of 24 +_ thisissue]. Combined slip at depthat a rateof 23 _+4 mm/yr I mm/yrbelow --9 km. Acrossthe northernportion of the satisfiesthegeodetic measurements. Renunsegment, geodetic rates appear to be 26 +_2 mm/yr TheAngkola segment of the Sumarranfault produced the [Genrich et al., this issue]. famousearthquake of 1892,during the establishment of the The Renun segmentwas the sourceof three major firstprimary triangulation network in theregion. Differences earthquakes early in thetwentieth century. Accounts of these inangles measured just beforeand after the earthquakeevents are very sparse, however, and the limitsof therupture enabledM•iIler [ 1895]to calculatethat coseismic fight-lateral can only be guessedfrom poorlyconstrained isoseismal dislocationsof at least2 m hadoccurred along a northwest contours. Visser [1922] reportsthat shakingduring the trendingline coincident with that portion of thefault trace February22, 19!6, earthquakewas very strongin the between0.45ON and 1.2øN. These geodetic data, along with Tamtungvalley and that the radius of strongshaking was thosefrom the 1906San Francisco earthquake and 1891 -200 km. TheJanuary 24, 1921,earthquake had a regionof Mino-Owariearthquake inspired, Reid [1913] to formulate the severeshaking similar to that of theearthquake of 1916. The theoryof elasticrebound [Yeats et al., 1997, Chapter8]. radiusof shakingfor theearthquake of April !, 1921,was Prawirodirdjoet al. [thisissue] have reanalyzed the Dutch twiceas large [Visser, 1922]. dataand conclude that the dextral slip was 4.5 + 0.6m. The 2.3.17.Tripa segment(3.2øN to 4.4øN). Marked mostserious damage reported in 1892was along the fault in irregularityand curvature,mountainous terrain, and 28,308 SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS spectaculardextral offsets of major rivers characterizethis and Plate 1). The southeasterntwo thirdstraverse 180-kin-longsegment (Figure 4 and Plate 1). The locationof mountainousterrain and are well expressed byaligned major the main traceof the fault is well constrainedby spectacular rivercanyons and stream offsets. Dextral separations of--25 offsetsof the Kuala Tripa and MeureuboRivers. Each of and20 km on theGeumpang and channels are. thesedeeply entrenched rivers displaysa clear offsetof-21 not compellingevidence for offset,but theyare similar in km (Figures6 and 7 and Tables 2 and 3). magnitudeto the size of clearoffsets of the Tripaand The segment's southeasternterminus is the northeastern MeureuboRivers farther southeast(Figure 7). The flank of the extensionalAlas Valley graben. Its northwestern northwesternportion of theAceh segment traverses a region limit is a 9-km-wide restrainingbend, which displayssouth- of low relief and is obscureon l:100,000-scaleaerial side-upfaults with a significantcomponent of reverseslip. photographs.Geomorphic expression of the lhult is subtle Onecould argue that an appreciablecontractional jog at 4.0øN andstream offsets appear to be absentthere. Although some and a major changein strike at 3.85øNjustify dividingthis publishedmaps show the Sumarranfault running along the segmentfurther. southwesternflank of theAceh Valley and continuing into the Parallelto and ---15km northeastof the centralportion of seaacross the northwestern coast [Curray et al., 1979;Page this segment (between 4.0 ø and 4.25øN) is another active et al., 1979], we see no geomorphicevidence of active strike-slip fault. This 55-kin-long fault trace is also well faultingwithin 25 km of the coastline.Therefore, we, arenot definedby alignedriver valleys and streamoffsets. Stream convincedthat the fault is active northwestof about5.4øN. patternssuggest that this fault may convergewith the main Geomorphicevidence for inactivity is compatiblewith activetrace at the northwesternterminus of theTripa segment. geodeticobservations that strain is accumulatingat no more However, we could find no clear large-scalegeomorphic than a few millimeters per year acrossthe fault [Genrichet evidence of this, nor does the l:250,000-scalegeologic al., this issue]. mappingsuggest it [Cameron et al., 1983]. 2.3.19. Seulimeum segment (5.0øN to 5.9øN). This An earthquakeon September 19, 1936, occurredalong the segmentrepresents the principal active trace of the Sumarran southeasternmostpart of the Tripa segment(M., 7.2 [Newcomb fault throughnorthern Aceh province(Figure 4 andPlate 1). and McCann, 1987]). A smaller, more recent shock (rob6.0, The active trace is markedby sharpescarpments and dissected November 15, 1990) occurred near the middle of this young volcanic deposits on the southwesternflank of segment. SeulawahAgam volcano. Small tributariesof the Seulimeum 2.3.18. Aceh segment (4.4øN to 5.4øN). This 200-kin- River are clearly offset a few hundredmeters. Alongthe long segmentof the Sumarranfault has a smooth sinusoidal central part of this segment,young folds appearto be offset shapeand lacks major discontinuitiesor sharpbends (Figure 4 •-20 km (Figures6 and7 andTables 2 and 3).

'6 ½" 7 'k s . I 10

Northern part of Sumatra..:...... d. kJ.

GSF - Volcano Lakes ./•" River • • Select•Large Off•ts Select•Small Off•L• 17Ob•med channel off2u (•eTable 3) t...... b ] (SeeTable2) i Linespointing,o the exit

- 32

19• 2021 22 23' 24 25 26 283031

I..,•' E• flF• •_•. I '.•;,:•.'•.'

Southern part of Sumatra

Figure 6. Map of small and largegeomorphic offsets along the Sumarranfault. See Tables2 and 3 for more information.The largestoffsets indicate that total slip acrossthe fault is at least20 km. SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS 28,339

NORTH SUMATRA , • • • 2!------'•, /

• activehult zone •21 • offset(•) • Anticline• SynclineV•leys 0 20 100 km smalllargeriverriver • .,.,.-,Drainage Divide centers and cones of active volcanoes

Figure7. Two of themost compelling large geomorphic offsets along the Sumatran fault, the 21-km dextral offsets of the Tripa and MeureuboRivers in northSumatra. The headwatersof the nearbyWoyla River and folded Quaternarysediments near 6øN also appear to be offset by this amount. These offsets appear to representthe total dextraloffset since initial uplift of thispart of theBarisan Mountains several million years ago.

Clearevidence of recentactivity along the southeastern22 the geomorphicevidence for recentdextral slip along the kmof this segmentis absentfrom our aerial photos,but we Seulimeumsegment, Gertrich et al. [this issue]show that inferthat the fault continuesthrough the long, narrowvalley strain accumulationacross this segmentin the early 1990s ofthe Baro River alongthis reachto an intersectionwith the could be nil. Acehsegment at about 5øN. Northwest from the coastline, bathymetry[Curray et al., 1979; J. Curray, written 2.4. Other Related Structures communication,1999], focal mechanisms (Harvard CMT catalogue),geomorphic expression of faultingon Weh Island, 2.4.1. Batee fault. The Bateefault is a major right-lateral andevidence on a seismicreflection profile [Peter et al., strike-slipfault that diverges from the Sumarran fault at about 1966;Weeks et al., 1967] suggestthat the fault continues 4.65øN. Betweenits intersectionwith the Sumarranfault and underwater. the coastline, the fault traverses the 1000-m-high It is interestingthat dextral movementalong the southwesternescarpment of the Barisanrange. Kariget al. Seulimeumsegment has produced no deflectionof theAceh [1980] have shownthat this structurecontinues onto the segmentat their intersection. It is difficult to imagine how continentalshelf and offsets the edge of the continentalshelf manykilometers of dextralslip on the Seulimeumsegment -150 km andthe eastern edge of theouter-arc ridge -100 km. couldhave occurred without at least a broad deflection in the One strandof the Batee fault terminatesbefore reachingthe Acehsegment. northernpart of NiasIsland (Plate 1). Anotherstrand runs A largeearthquake in 1936 (M 7.1-7.3 [Newcomband alongthe northern coast of Niasand appears to offsetthe McCann,1987; Soetardjo et al., 1985])severely damaged the innertrench slope and outer-arc ridge (Plate 1). Exceptvery cityof BandaAceh, but the sourceof theevent is unknown. locally,the Batee fault does not appear to be activeon the An earthquakein 1964 (Ms 6.5, NationalEarthquake mainlandof Sumatra.Although several large river channels InformationCenter (NEIC)) damagedKmeng Raya more displaydextral deflections of up to 10km, smaller ridge lines severelythan . SinceKrueng Raya is closerto and channelsexhibit no offset. We suspectthat tl,½selarge theSeulimeum segment, the Seulimeumsegment of the deflectionsare, indeed,dextral offsets, but the lackof clear Sumarranfault may have generated this event. In contrastto smalloffsets suggests either no activity in thepast few tens of 28,310 SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS

Table 2. SelectedSmall OffsetsAlong the SumatranFault (From North to South)

River/Lake Name Offset, m Comments

a 750-1000 offsetof severalstreams that incised young volcanic depositson thesouthwest flank of SeulawahAgam volcano b 1700-2100 excellentoffset of severalstreams deeply cut into the 73,000 year old Toba Tuff c Angkola River 1200-1400 offsets of a few streamson the northeastflank of the Sorik Merapi volcano d AngkolaRiver 1000-1300 offsetsof severaltributaries of the AngkolaRiver (Ringkitbranch) e Sianok River 700 excellentoffsets of severalcrossings of the Sianok River, deeply incisedinto the 60,000 yearold ManinjauTuff f Anai River 600 offsetsof severalchannels on the southwest flank of Merapi volcano g Lake Dipatiampat 500 offset of north sidewall of the caldera lake

h 700 excellentoffsets of tributariesto the Musi River, on the southwest flank of Kaba volcano i Manna River 2400 offsetof Air Kiri andAir Kanan(Plate 2) whichdrains an eroded volcanic edifice Werkuk River (Menjadi,Pisai, 300 offsetsof threechannels that are deeply incised into the thick, Rebu branches) QuaternaryRanau Tuff

thousandsof yearsor activityat a ratemuch lower than along 3. Discussion,Interpretations, and Speculations the Sumatranfault. This interpretationconflicts with the 12 +_ 5 mm/yr estimateof dextralslip rate of BeIlier and Sebrier In this paper, we have defined the geometryand [1995]. We questionthe validity of their approach,which geomorphologyof the Sumatranfault. Thereare now several usesan empiricalrelationship of channellength and age to questionsthat theserefinements allow us to address.These derivean agefor a channel.This ageis thendivided into the includethe implicationsof the fault's historicbehavior and measured offset to determine a rate. geometryfor the evaluationof future seismichazard and 2.4.2. Tom fold and thrust belt. Between about 1.0ø and questionsabout the total' offset across the Sumatranfault and 1.5øN lies a geomorphologicallyremarkable set of active its role in obliqueconvergence during the pastmany millions foldsand faultsthat strikeroughly parallel to the Sumatran of years. Other questionsconcern the geometricand fault but lie 15 to 40 km farther southwest(Plate !). The kinematic relationshipof the Sumatran fault to the principal manifestationsof this fold-and-thrustbelt are a neighboringsubduction zone and the relationshipof arc northweststriking anticline and sync!inc. The syncline volcanismto strike-slipfaulting. We addresseach of these underliesa 25-by-10-kinswamp, and the anticline appears as four questionsin turn,below. a 30-by-15 km fold in Mio-Pliocene sediment. The Gadis 3.1. Historical and Future Seismicity Riverand its tributariesmeander across the syncline and then traverse the anticline as an antecedent stream. In thepreceding discussion, we havedescribed very briefly In addition, several smaller northweststriking reverse what is knownabout large earthquakesalong the Sumamn faults appearto break the anticline (Plate 1). The anticline fault. Even thesehighly abbreviated accounts suggest that alsois cutby smallnorth striking strike-slip faults. However, geometricsegmentation influences seismic rupture of the thesefaults are so smalland closelyspaced that theydo not Sumatran fault. In contrast to the San Andreas fault in appearon Plate 1. California [Lawsonet al., 1908; Sieh, 1978], the Sumam

Table 3. ProposedLarge Offsets Across the SumarranFault

Features Offset,km Quality Description

Quaternaryfolds 20 fairlygood offsetof a few fold axeswhich deformedPliocene, Miocene,and Oligocene strata Meureubo River 21 excellent dextraloffset is clearlyindicated by thedeflection of the trunk channel 21 excellent dextraloffset is clearlyindicated by thedeflection of the trunk channel Singkarakgraben 20-22 N/A basedon an interpretation of the graben opening (Figure7) SeblatRiver 17 good clearlyshown by a sharpdeflection of themain channel

KetahunRiver 23 good clearlyshown by a sharpdeflection of themain channel 28,311

E

1'

..i

o o

E•o o

, ,...•

.,..• 28,312 SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS fault appearsto produceearthquakes with rupturelengths no Thetwo major offsets between 5ø and5.5øN provide the greaterthan a hundredkilometers or so. We speculatethat mostcompelling evidence from stream channels forlarge this contrastin behaviorresults from the contrastin continuity offsetalong the Sumatran fault (Figure 7). Thedeeply incised of the two fault systems:The SanAndreas fault hasonly one trunkchannels ofboth streams cross the fault at a high angle stepover discontinuitywith a cross-strikewidth greaterthan a andhave long, straight courses along the faulttrace. The kilometer(near San GorgonioPass [Allen, !957]), whereas neighboringWoyla River drainage also appears to beoffset the Sumarranfault has at least 12. The SanAndreas has only -21 km,but this offset is lesscertain because the match across two large bends (near Monterey Bay and at Tejon Pass) the fault is of trunk channelto tributarychannels. The [Jenningsand Saucedo, 1994], whereas the Sumatranfault drainagedivide between the Woyla andGeumpang Rivers hasabout eight. alsoappears to be offsetby ~20 km. A more preciseand detailedevaluation of the relationship Onecould propose 40- to 50-kinoffsets for the deeply of these irregularitiesand their relationshipto historical entrenchedchannels of the Tripaand Meureubo Rivers, but rupturesis warrantedbut is beyondthe scopeof this paper. thiswould leave implausible mismatches in the surrounding We havebegun a thoroughanalysis of the historicalaccounts topography. Our proposed20- to 30-kin offset of an and hopeto interesta seismologistin studyinginstrumental anticline/synclinepairat about6.4øN, which is basedupon a recordsin order to assessmore fully the role of geometric plausibleoffset of foldedPliocene, Miocene, and Oligocene segmentationin controllingrupture parameters. Until this rocks [Bennettet al., 1981] (Figure 7), supportsthe futurework is completed,one can obtaina crudesense of the interpreted20-21 km offset of the Tripa and Meureub0 influence of fault segments on historical ruptures by Rivers. comparingKati!i and Hehuwat's[1967] compilationof the Anotherlarge offsetthat we will considerin moredetail is felt regionsof historicalearthquakes with our map of the onewe can infer from the geometry of thenormal faults along fault. Bellier et al. [1997] have redrawnKati!i andHehuwat's theSingkarak graben at about1.4øS. This is morespeculative map and include a few more recentearthquakes in their thanthe geomorphicoffsets described above. In mostcases, compilationof historicalfelt areas. the lengthof a pull-apartgraben along a strike-slipfault probablydoes not representthe total slip acrossthe faultzone

3.2. Offsets Across the Sumatran Fault and the Evolution (for example, the 7-km-long step over mappedby Zachariasen and Sieh [1995] between two faults in California of Dextral Slip Along the Sumatran Margin hasonly 300 m of totaloffset across it). The particularnature 3.2.1. Exemplary small to moderate offsets. Noneof the of the faultsbounding the Singkarak graben suggests that it geomorphicoffsets across the Sumatranfault are greaterthan may be an exception. ~20 km, and most are far smaller (Figure 6 and Tables2 and Althoughthe dextralfault segmentscoming into the step 3). The smallestknown offsetsalong the Sumarranfault are over from the northwestand southeastare misalignedby only thoseassociated with particularhistoric fault ruptures.These ~3.5 km, the normalfaults bounding the lake are separatedby include offsets of a meter or two on the Sumani segment as much as 7.5 km (Plates 1 and 3). Becauseof their salad- (0.75øS), during the 1943 earthquake[Untung eta!., 1985] tong geometry,we surmisethat the normal faults represent and up to about 4.5 m during the 1892 earthquake,on the collapse of shallow crust into the expandingrectangular Angkola segment(about 0.6øN) [Miiller, 1895; Reid, 1913; regionthat is beingproduced by dextralslip on themisaligned Prawirodirdjoet al., this issue]. Our bestexamples of dextra! lateral faults. offsets in the range of hundredsof meters to a couple The predominanceof volcanic rocks of Plio-Pleistocene kilometersare on or near the flanks of young volcanoes: age on the flanksof the grabenindicate that the grabenis no Channelson the southwestflank of Kaba volcano (3.6øS) are morethan a few millionyears old. Bellierand Sebrier[1994] offset~700 m. The walls of Dipatiampatcaldera (2.65øS) are proposedthat the Singkarakbasin is an extinct pull-apart offset •-500 m. Stream channels incised into the southwest graben,inactivated when the trace of the Sumatranfault cut flank of Marapi volcanodisplay offsetsranging from 120 to acrossthe lake. The very steep scarpsand youthful 600 m. The Maninjou Tuff (0.4øS) has been offset 700 m topographyassociated with the graben-boundingnormal faults (Figure 3), and channelscut into the Toba Tuff (2.2øN) are strongly suggest, however, that accommodationspace offset about 2 km (Table 2). We have usedthree of theseto continuesto be createdby dextral slip on the en echelon determinethe modem slip rate of the Sumatranfault, but full Sumaniand Sianok segments. Furthermore, the locationof documentationof theserates is the subjectof a manuscriptin the 1943rupture is inconsistentwith a competingmodel for preparation. the evolutionof the faultby Bellier and Sebrier[1994]. As one would expect,highly dissectedvolcanic landforms We hypothesizethat the normalfaults shouldonly be are offset more than their younger neighborsare. The two activeadjacent to founderingcrust within the accommodation offsetstreams cutting a dissectedvolcanic edifice at 4.2øSare spacegenerated by dextralslip along the en echelon faults. A goodexamples of this. They are offsetabout 2.5 km (Plate2). hypotheticalevolution of thesenormal faults as the strike-slip 3.2.2. Largest geomorphicoffsets. The largestplausible displacementgrew is depictedin Figure8. Therefore,we geomorphicoffsets along the Sumatranfault are ~20 km propose,that the totaloffset on thesetwo misalignedstrike- (Table 3, Figures6 and 7, and Plate 1). Theseinclude right- slipsegments is -23 km,the length of thearcuate normal fault lateral deflection of the channels of the Ketaun River channel zones on either side of the lake. at 3.2øS,the SeblatRiver channelat 2.9øS,and the Tripa and Thisis, of course,not the only plausible evolution for the Meureubu River coursesat 4.1 ø and 4.4øN (Plate 1). Late Singkarakpull-apart graben, but it is one that is consistent Cenozoic folds at 5.25øN may also be offset-20 km. with -20 km of totaloffset along the Sumatranfault. One Furthermore,we speculatebelow that the Singkarakgraben could,for example,accept our inference that the lengths of (at 0.6øS)has developed in responseto 23 km of offset. thenormal faults reflect the fault-parallel length of actively SIEH AND NATAWIDJAJA:SUMATRAN FAULT NEOTECTONICS 28,313

^ B

Lake

o

o ß ',. Smgkarak ,' X

13 km 18 km 23 km offset offset offset 0 et

Figure8. Hypotheticalevolution of theSingkarak graben and bounding normal faults showing how the length of the normal-obliquefaults might represent the totaloffset on theSianok and Sumani segments. Profile E showsthe currentgeometry of the graben.

founderingcrust but hypothesizethat the length of the accruedonly a few tensof kilometersof offset. An example foundedngregion has remainedunchanged at --23 km since is Turkey's 1500-km-longNorth Anatolianfault, whichhas a thefaults initiated. This would imply that the lengthof the total offsetof only 85 km [Annijo et al., 1999]. Second,in a founderingregion has no bearingon the amountof total strict sense, the Sumatra fault is not one fault; rather, it is a offset.We favorour hypothesisbecause it is consistentwith fault zone that consistsof many segments,which range in otherevidence for---20 km of total offset. lengthfrom 60 to 220 km. Many strike-slipfaults with 3.2.3. Total offset. Why are the largestgeomorphic lengthsas shortas thesehave accruedonly a few kilometers offsetsno greaterthan ---20km? Is it possiblethat these to a few tens of kilometers of offset (for example, the San representtotal strike-slip offset along the Sumatranfault? Or Jacinto fault in California is a zone with 24 km of dextral isthere a limitto thesize of geomorphicoffsets related to the offset that consistsof many disjunct segments,tens of susceptibilityof landformsto erosionand burial? We will kilometerslong). givereasons below why 20 km mightwell be thetotal offset Anotherreason to suspectthat total slip would be >20 km acrossthe fault, but we will also show that a total offset as is the transformationof the Sumarranfault into the spreading greatas -,-100 km can not be ruled out at this time. centers of the Andaman Sea [Curray et al., 1979]. This Indirectarguments for offsetmuch greater than 20 km are suggeststhat offsetcould equal the 460 km of spreadingthat asfollows: One mightexpect that the greatlength of the hasoccurred there in the past 10 Myr. But we will seebelow Sumarranfault requires substantially greater total offsets than that much of this offset has been carried by faults that splay acouple tens of kilometers.It is certainlytrue that many very into the forearc, west of the Sumarranfault zone. longstrike-slip faults, such as the Alpine (New Zealand) and Regardless of plausible analogues and the fault's SanAndreas (California) and manyoceanic ridge-ridge connection to the spreadingcenters of the Andaman Sea, transformfaults displaygeologic offsets of hundredsof direct geologicevidence for total oftket acrossthe Sumatran kilometers[Yeats et al., 1997,Chapter 8]. fault is sparseand equivocal. McCarthy and Elders [1997] Butthis is nota strongargument for largeoffset, for two suggest150 km of dextralslip, on the basisof similaritiesin reasons.First, many other very long strike-slipfaults have isolatedoutcrops of crystallinebasement on both sidesof the 28,314 SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS

Ocean. The flow of the SumpurRiver, between about 0.10 and0.75øN, has also been strongly influenced by subsidence alongthe fault; majortributaries flow into andacross the N 3S SumputValley before flowing eastward toward the Java Sea from their confluence at the fault. 0.SN '• A thirdlimit to thesize of geomorphicoffsets is imposed by thespacing of majordrainage channels that cross the fault. Cumulativeoffsets are unlikely to be greater than the spacing betweenmajor river channelsbecause piracy occurs as trunk 80 km channelsof one drainagesystem are offset to positions upstreamfrom neighboring trunk channels [see, e.g., Prentice, YoungVolcanic Cover 1988;Allen et al., 1984;Yeats et al., 1997,Chapter 8]. Along TertiaryAndesites onlya smallpercentage of the Sumarranfault are major I 4s streamchannels spaced more than a coupletens of kilometers Paleogene apart(Plate 1). Piracyof the headwatersof theAlasijani Triassic River by the Manna River, for example,may haveoccurred at about 4.1øS (Plate 1). Furthermore, where the Sumarran drainage divide is within just a couple kilometersof the •00•0,0• • Pertoo-CarboniferousSumarranfault, large trunk stream channelsdo not crossthe fault trace. In these places,the Sumarranfault traversesonly '•4"'•'C9/._tt' •" "-,...... GeologicalGranitesoffsets ., ...... about 20 to 25 km smaller tributary drainages. Because tributaries are more closely spaced, geomorphic interference will result where Plate 4. Geologicmaps of offset bedrockunits alongthree offsetsexceed a few kilometers. Thus only abouthalf of the sectionsof the Sumarranfault suggestingthat the total offset acrossthe fault is only -20 km. Reproducedfrom Katili and Sumarranfault mightbe expectedto expressoffsets greater than a few kilometers. Hehuwat [ 1967]. Nonetheless,there is reasonto favor the hypothesisthat the largestgeomorphic offsets are, in fact, the total offsetacross the fault. The 20- to 21-km offsetsof deeply incisedchannels fault in central Sumatra. Katili and Hehuwat [1967], in northern Sumatra probably record total offset sincethe however, infer that total dextral offset at three localities (near initiation of uplift of the Barisan mountain range in this the equator,3øS, and 4øS) is only 20 to 25 km on the basisof region, and that uplift is quite old. The age of initiationof regional-scalemaps of late Paleozoicto early Cenozoicrocks uplift is poorly constrained,but sedimentationhistory of the (Plate 4). Cameron et al. [1983] suggesta 20-kin dextral forearc basin suggeststhat Sumarransediment sources began offset of Oligocenebeds at about 4.1øN. Neither the larger erodingin late mid-Miocene time (about 10 Ma) [Karig et al., nor the smalleroffsets are adequatelydefended by sufficiently 1979; Harbury and Kalagher, 1991], and Cameronet al. detailedmapping. [1980] document major activity of a range-boundingfault The geologic setting of the Sumarranfault supportsthe about 10 Ma. If this is true, then incision of the Tripa and notion that geomorphicoffsets might be limited to lessthan a MeureuboRivers would also have begun about 10 Ma, and few tens of kilometers and that these values could be the 21-km offsets would necessarilyreflect total offsetsince significantly less than the total offset. The abundanceof that time. The nearby 20-km offset of an Oligocene young volcanic cover, the spacing of major river channels, sedimentaryunit proposedby Cameronet al. [1983] suggests and the length of individual fault segments all limit the thatthis may be the totaloffset since the Oligoceneas well. accumulation of geomorphically evident offset. Let us Our analysisof the Singkarakgraben also suggests that 23 km considervolcanic cover first. More thana quarter(-450 km) is the total offset across the Sumarran fault since formation of of the 1650-km-longSumatran fault traversesyoung volcanic the two boundingfaults, the Sumaniand Sianoksegments. If edifices and their thick pyroclasticdeposits. Most or all of the total offset acrossthe fault were greater,proof would thesevolcanic constructions are probablyfar less than a half require discovery of an older fault, hidden beneaththe million years old, given their generally undissectednature. youngersediments of the region. Even if the Sumatranfault carriedall the dextralcomponent 3.2.4. Evidenceof stretchingnear the Sunda Strait. A of the relativeplate motionvector (-30 mm/yr), no morethan simple structuralanalysis of the forearc region near the -15 km of offset could have accumulated since their southernterminus of the Sumarranfault providessupport for deposition. Burial of older offsets would have obscuredor ---100 km of total offset across the Sumarran fault system. eliminatedtheir clear geomorphicexpression. However,as we will show,not all of this,nor evena majority Clear geomorphicoffsets are also limited by the lengthof of it, need be associated with the Sumarran fault. individualfault segments,which range in length from-35 to Two earlier papersdiscuss stretching of the forearcnear 220 km (Table 1). Since the majority of the fault segments the southern terminus of the fault. Huchon and Le Pichon are right-stepping,graben are commonalong the fault. These [1984]were the first to suggestthat arc-parallel stretching of grabenform intramontanevalleys that occupyabout-•350 km theforearc region near the Sunda Strait is relatedto strikeslip of the fault. As these basins form, streams divert into them. along the Sumatranfault. They hypothesizedthat the The Alas graben, between 3.1ø and 3.9øN, has probably landward bend in the subduction deformation front and the enabled such a diversion. The drains a 130-km absenceof an outer-arcridge and forearc basin south of the reachof thefault into the 50-km-longgraben before breaching SundaStrait (Figure 5) indicatearc-parallel stretching of the the graben wall and flowing southwestwardto the Indian forearcregion. However,they did not usethis to calculate SIEHAND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS 28,315

plausibleamounts ofoffset along the Sumatran fault. Instead, Sunda Strait. Thesetwo featuresdisappear near the strait, and theyaccepted sparse and equivocal evidence for-100 km of thedeformation front bows landward. Following Huchon and totaloffset along the fault and attempted to demonstrate that Le Pichon[1984], we interpretthis as an indicationof fault- thisoffset is consistentwith reasonableestimates of arc- parallelstretching and fault-normalnecking of the forearc parallelstretching. They did not attempta rigorousregion. Extensiveseismic reflection studies and structuraland assessmentof the implicationsof the forearcgeometry on stratigraphicinformation from the forearc and outer-arc totaloffset along the Sumatran fault. regionsnorth of theequator show that the paired forearc basin LassaIet al. [1989] also attemptedto quantify the andouter-arc ridge developed throughout the Mioceneepoch stretchingof the forearcregion south of the SundaStrait. but grew particularlyrapidly during the Plioceneepoch Theyshow three seismic reflection lines from a 80 x 50 km [Samuelet aI., 1997;Samuel and Harbury, 1996]. Thuswe areain andon the flanks of thegraben at the westernentrance infer that the deformation of these features has occurred tothe strait. They annotatethese with five stratigraphic withinjust the pastfew millionyears. boundaries,whose geometry and ages they defend by We beginwith an estimateof the boundariesof the volume referenceto unpublishedwork. They claim (without that has been stretched.The concavityof the deformation discussionor argument)that an allegedly upper Miocene front and mergingof the outer-arcridge and forearcbasin stratalpackage contains reef deposits (an indicator of shallow suggestthat the currentlength of the deformedregion, L, is water).They assume an uppermostMiocene (5 Ma) agefor ~356 km (Figure 9). Hypocentraldepths on or near the thereefs and then usethe depthof this packetof sedimentto subductioninterface constrain the northeastdipping base of calculatethe "stretchingfactor" since 5 Ma. This factor is the deformedforearc wedge. The deformationfront and the describedby Le Pichon and Sibuet [1981], who apply a baseof the continentalslope define the seawardand landward stretchingmodel of McKenzie[1978] to passivecontinental boundariesof the deformedregion. margins.The use of thismodel seems wholly inappropriate to Using these boundaries,we calculatethat the deformed ussince the parametersneeded to calculatestretching are crustalwedge has a volumeV, of about1.01 x 106km 3. We mostlyunknown for the SundaStrait. LassaIet al [1989], assumethat this volumeis equalto the original,unreformed concludeby asserting,without any discussionor calculation, volumeVo. By furtherassuming that the cross-sectionalareas thatthis stretching factor "probablyexplains the openingof of the current southeasternand northwesternedges of the thestrait since 5 Ma ago,with a maximumdisplacement of 50 deformed region, A and B, have not changed since to70 km alongthe centralSumatra fault." Their paperis, in deformationbegan, we can calculatethe originalarc-parallel fact,so sparse on dataand documentationthat its conclusions lengthof the deformedregion. A and B are currently2870 areleft undefended. and4970 km2: We proposea simple measureof extensionacross the Lo = 2*V / (A+B) =258km. (1) grabenof the Sundasegment, which establishes a minimum mountof dextralslip on the Sumatranfault. If we assume The total amountof northwest-southeaststretching is: thatthe faults bounding the grabendip 60ø, we cancalculate the horizontal extension across the faults in the direction of A L = L - Lo= 356 km - 258 km = 98 km. (2) the Sumatran fault. We calculate a 6.5-km lower bound on extensionof the grabenparallel to the Sumatranfault if we Sincethe Sumatranfault forms the northeasternboundary of assumethat the 2-kin heightof the scarprepresents vertical the forearcsliver block, we are temptedto concludethat this throwacross the faults. This assumptionis manifestlyan estimateof stretchingof the forearc equalsthe amountof underestimateof total vertical throw, sincehundreds of meters right-lateralslip along the Sumatranfault. However,in fact, of depositswithin the graben are clear on the seismic this 100 km is only an upperbound on offsetof the pastfew reflectioncross sections. Thus 6.5 km is probablyseveral million yearssince there is anotherstructure in the forearc kilometersless than the actual amount of extension acrossthe regionthat could also have accommodatedsome of this Sundagraben. Several more kilometers of dextralslip could stretching. The Mentawai fault [Diament et al., 1992], probablyalso be addedto totalslip along the Sumarran fault if located between the forearc basin and the outer-arc ridge thegeometry and timingof faultingfarther east within the (Figures1 and8 andPlate 1), couldalso have accommodated strait and buffed beneath >2000 m of volcanic debris some of this motion. The linearity of this large structure (summarizedby Huchon and Le Pichon[1984]) were known suggestsa significant component of strike-slipmotion, but the better.In summary,extension of theSunda graben and filled magnitudeof strike-slipmotion, if any, has not been grabenfarther east is consistentwith dextralslip of the order documented. of10 km alongthe Sumatranfault. However,more detailed 3.2.5. Plausible evolution of dextral slip along the stratigraphicand structural data will be necessaryto calculate Sumatranmargin. Althoughknowledge of the geologyof extensionacross the graben more precisely. the Sumatran fault and other faults of the Sumatran fault Letus now attempt a quantitativeanalysis of stretchingof systemis incomplete,enough information exists to attempta theforearc region, to providea maximumlimit to dextral slip reconstructionof the system'sdeformational history over the onthe Sumatran fault during the past few million years. This pastfew million years (Figure 10). Theprincipal constraints analysissimply carries the geometricalobservations of on this historyare: (1) the magnitudeand timingof the Huchonand Le Pichon[1984] to theirlogical conclusion. discrepancy between spreading in the AndamanSea and Fromsimple volumetric balancing of theforearc wedge, we stretchingnear the Sunda Strait; (2) a rangeof plausibletotal calculate~!00 km of stretchingof theforearc parallel to the offsets for the Sumatranfault; (3) the timing, style, and Sumatranfault. magnitudeof deformation in the Sumatr•.• forearc region; and Aswe discussedin section2.3.1. (Figure5), the forearc (4) a southeastwarddecrease in thecurrent rates of slipalong basinand outer-arc ridge are attenuated in the region of the the Sumatranfault. These constraintssuggest that the 28,316 SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS

SUMA'I-., •,RA X'X • deformationfront . \ JavaSea • outer-arcridgeaxis ,,--;..-• ,.._,• • '• !• •. • ___.i.__forearcbasinaxis • • •' •""••a?,,.,,,y>•--.-- • • •. -.---,,-, • Jakaru % bathymetry(in meters)contour ,,r -.:• g • • " depression/basin ' * VA '----...... 5O-kin ••.... isoba/h KRX'•Xa;X,, ; • J• ' activevolcano

• • [ .....

• - •56 km - - -

INDIAN OCEAN

Figure 9. Stretchingof the forearc sliver plate near the Sunda Strait, which appearsto have thinned the forearc wedgeperpendicular to the deformationfront. By volumetricbalancing, we calculatethat ---I00 km of stretchingof the forearc silver hasoccurred parallel to the Sumatranfault sinceformation of the outer-arcridge and forearcbasin. This would be a maximumvalue for northwestwardtranslation of the part of the torearcsilver plate that is southof the equator.

Sumarranfault systemhas evolved significantlyin the past discrepancybetween deformation in the Andaman sea and several million years and that the current configurationof SundaStrait during the past 3 Myr may be very smallor deformation is not representative of pre-Quaternary nonexistent. deformation. Nonetheless,the currentrate of slip on the Sumatranfault One hundred kilometers of motion near the Sunda Strait appearsto diminish significantlyfrom northwestto southeast. contrastsmarkedly with the 460 km of openingsuggested by Although new geodetic evidence suggeststhat there is no Curray et al. [1979] for the Andaman spreadingcenters significantdecrease between about løS and 2øN [Gertrichet (Figure 1). The contrast disappears,if one compares al., thisissue], geologic slip ratesacross this sectionsuggest a Andamanextension and Sundan offset for similar periodsof muchlarger decrease in rate, from 27 mm/yr (near2.2øN)to time. Only about 118 km of Andamanextension may have 11 mm/yr (near 0.4øS) [Sieh et al., 1991, 1994; D. accumulated in the past 3 Myr (J. Curray, written Natawidjajaand K. Sieh, manuscriptin preparation,2000]. communication,1999). This does not differ greatly fi'omthe Bellier and Sebrier's[ 1995] estimationsof slip ratealong the 100 km of stretchingof the forearc near the SundaStrait for fault, basedupon correlations of streamlength with age,also aboutthe sameperiod of time (i.e. sincethe rapid rise of the decrease from northwest to southeast. Sumatranouter-arc ridge in the early Pliocene). Hence the If thetotal offset along the Sumatran fault is only-20 km

Figure10. (opposite)A plausible(but nonunique) history of deformationalong the obliquely convergent Sumatran platemargin, based upon our work and consistentwith GPS resultsand the timingof deformationin the forearc region. (a) By about 4 Ma, the outer-arc ridge has formed. The former deformation front and the Mentawai homoclineprovide a set of referencefeatures for assessinglater deformations. From 4 to 2 Ma, partitioningof obliqueplate convergenceoccurs only north of the equator. Dextral-slipfaults on the northeastflank of the forearc sliverplate parallel the trenchin northernSumatra but swingsouth and disarticulatethe forearcbasin and outer-arc ridge north of the equator. (b) Slip partitioningbegins south of the equatorabout 2 Ma, with the creationof the Mentawaiand Sumarran faults. Transtension continues in theforearc north of theequator. (c) In perhapsjust the past 100 yr, the Mentawaifault hasbecome inactive, and the rate of slip on the Sumatranfault northof 2øN has morethan doubled. This differencein slip rate may be accommodatedby a new zoneof transtensionbetween the Sumarranfault andthe deformationfront in theforearc and outer-arc regions. SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS 28,317

A

•r m• m•r

Z' Ma %,..•..•.•.•.._... -• •

• (76km) • •• • ...... 40mnVyrJ

• (57kin) •' 42m•yr •'47m•v• 7 m y •48'•mmvr

/ ..... • _l . / % / z• , I %

m•r m•r m•r

ß •, ßß -,.._•• N.. Zoneof transtension •,• 37mm/yr '• •.•"•'"'•"%•'-'"-'---• -,•

• o 7Ma

present

• • (25k)

25 10 10

m•r m•r m•r 200 0 200 400 600 800 1000 •!ometers 28,318 SIEH AND NATAW!DJAJA: SUMATRAN FAULT NEOTECTONICS andslip rates have been constant, then the northern part of the thatthe strand of the Batee fault northwest ofNias offsets the fault zonewould be lessthan a millionyears old. Southof the ancientdeformation front ~50 km, from northwest ofNias to Equatorial Bifurcation, where late Quaternary slip rates a positionwest of Nias(Plate 1). Farthersouth on the inner appearto be --10 mrn/yr,20 km of slipmight have accrued in trenchslope, between Nias and SiberutIslands, the --2 Myr. deformationfront may be offset by about an additional 50km Our calculationof-100 km of fault-parallelstretching of alonganother north striking fault. the forearcnear the SundaStrait suggests that eitherthe total Dextraloffset of the eastern edge of the forearc basin by offsetalong the Sumatranfault is muchlarger than 20 km or the Bateefault is -150 km [Kariget al., 1980]. From that another structure in the Sumatran fault system has paleonto!ogicallyconstrained seismic stratigraphy, Matson accommodated-80 km of the stretching.The only plausible andMoore [1992] show that the Batee fault was active from othercandidate for dextralslip wouldbe the Mentawaifault, thelate Miocene through the Pleistocene epochs. Twenty to well constrained from seismic reflection data to run between thirtykilometers of dextralslip appear to haveoccurred onthe the outer-arcridge and the forearc basin [Diament et al., nearbySingkel fault in the late Mioceneepoch. Thusit is 1992]. The linearity of the feature suggeststhat it is reasonableto suggestthat the first few tensof kilometersof principallya strike-slipfeature. Diamentet al. [1992] also the 150-kmdextral offset on the northernportions of the arguethat the structureof the fault zone indicatesthat its Bateefault accrued in thelate Miocene. However, the bulk of senseis primarily strike-slip. In our opinion,the structural the slip must be late Plioceneand youngerbecause the argumentis a less compelling one becausewe are not Pliocenehomocline of Nias Island is offset -100 kin. This convinced that the Mentawai fault zone exhibits the "flower" offsetmust have accrued over at least1.5 Myr, sincea shorter structurecharacteristic of strike-slipfaulting. In fact, the durationwould requirerates of dextralslip in excessof the positionof the fault, on the northeasternflank of theouter-arc rateof relativeplate motion. ridge, is consistentwith the fault beinga backthrust,along Plate1 alsoshows a disruptionof the outer-arcridge and whichthe outer-arcridge has risen. The existenceof a large inner trenchslope south of Nias Island, at the Pini basinand homoclinein the sameposition relative to the forearcbasin between Tanabala and Siberut Islands. The Pini basin andouter-arc ridge northof the equator[Karig et al., 1980] experiencedrapid subsidencebeginning about 4 Ma. This (Plate 1) supportsthis interpretation.So it is with some subsidenceis probablycontemporaneous with activityof reluctancethat, in the evolutionarymodel below, we use the northstriking faults that bound the basin [Matson and Moore, Mentawaifault as a strike-slipelement of the Sumarranfault 1992]and with minor north striking dextral-slip faults on Nias system. [Samueland Harbury, 1996]. A disruptionin theinner trench A final constraint on the evolution of the Sumatran fault slopefarther south, along strike of the Pini basin,may systemis the Mio-Pliocene history of the forearc and outer- representa 40- to 50-km dextral offset of the sameancient arc regions. The Andamanspreading centers were actively deformation front mentioned above. spreadingat ~40 mm/yr during this period,yet we have no Figures 10a-10c depict a plausible evolutionof the evidence of contemporaneousdextral deformationof the Sumatranfault and other structures of theplate boundary that forearcsliver plate southof the equator. How and where,in is consistentwith available geologic, geodetic,and Pliocene and late Miocene time (about 2 to 10 Ma), was the seismographicdata. Variationsof this historyare als0 dextral componentof oblique convergenceaccommodated? possible;our principal intention is to show that the fault Matson and Moore [1992] suggest that some of this systemevolved significantly in the past few millionyears. discrepancycan be accommodatedby the dextral-normal The main characteristicsof this speculativehistory are as faults of the forearcregion near Nias Island (Figure2 and follows:(1) thecurrent 15 mm/yrdifference in Sumarranfault Plate 1). We considerthis possibilitybelow. sliprate north and south of theequator is veryyoung (perhaps Stratigraphicand structuralstudies by Samuelet al. [1997] onlyl00,000 years old), and (2) active normal- and dextral- and Samueland Harbury [1996] show that broadeningand slip (transtensional)faulting in the forearc and outerarc uplift of the outer-arcridge occurredearly in the Pliocene betweenløS and 2øN is an ancient(and perhapscurrent) epochthroughout the Sumatranforearc region. This is critical analogueto the stretchingat the southernend of theSumatran to reconstructingdeformation of the forearc sliver plate fault. becausethe early Pliocene growth of the outer-arcridge Figure10a showsthe geometryof the regionat about4 producedan elongatefeature that has beendeformed in the Ma. Justprior to this time, relief betweenthe forearcbasin subsequentseveral million years. The ridge is clear in the and the outer-arc ridge increasedgreatly acrossthe bathymetryof Plate 1. Southof about1 øS, it is regularand 60 homoclinalfold betweenthe forearc basin and outer-arcridge to 80 km wide. Its northeasternboundary is the Pliocene [Karig et al., 1980; Samuelet al., 1997;Samuel and Hatbury, Mentawaihomoclinal flexure. On the southwestthe ridgeis 1996]. We speculatethat as the outer-arcridge grew,the boundedby a plateauthat sits at a depthof-2400 m. We subductiondeformation front jumped southwestward to its speculatethat this plateau was formerly a part of the presentlocation, from a deformationfront still visiblein the Australianplate and that its northeasternedge is the former bathymetry,closer to theouter-arc ridge (Plate 1). From4 to deformation front of the subduction zone. Similar features are 2 Ma, dextralslip on theAceh segment and the Batee fault also presentbetween about 1.5øN and 3øN, near Simeulue occurredat 37 mrn/yr,and the homocline, outer-arc ridge, and Island. innertrench slope were offset 37 km by a curvedsouthern Between1.5øN and 2øS,the outer-arcridge, the homocline, extensionof theBatee fault, off thenorth coast of NiasIsland, and the ancientdeformation front and plateauare markedly and 37 km moreacross the Pini basin. This is consistentwith disarticulated.Karig et al. [!980] observedthat the Pliocene the stratigraphyof Matsonand Moore[1992]. Several homoclineon the eastside of Nias is dextral!yoffset- 100 km kilometersof arc-parallelelongation of Nias Islandalong by two strandsof the Bateefault. We infer from bathymetry northstriking dextral-slip faults and conjugate sinistra!-slip 28,3

6N

'9

4N N

2N

Equator

2S

6S

Plate$. Geometricand structural details of the Sumatran fault, the forearc basin, outer-arc ridge, and volcanic arc, suggestingthedivision ofthe Sumatran platemargin into northern, central, andsouthern domains. The simplest outerarc, forearc, and Sumatran fault geometries arein the southern domain. The coincidence ofthis structural domainwith the source region of thegiant (Mw 9) subduct•onearthquake of 1833 suggests thatgeometrical simplicityencourages largeruptures. Thecentral domain appears tohave been the source region ofthe great (Mw 8.4)subduction earthquake of 1861. Fragmentation ofthe central domain appears tohave been caused by subductionofthe Investigator fracture zone during thepast 5Myr. The locus ofimpingement ofthe fracture zone onthe deformation front was calculated byassuming thecurrent relative plate motion vector and the forearc deformationhistoryof Figure 10. Contours inred are the top of the Benioff-Wadati zone.Bathymetric contour interval is 200 m. 28,320 SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS lhultsalso occurred during this period[Samuel and Harbury, 3.3. TectonicModel of theSumatran Plate Margin 1996]. Subructionsouth of the equatorwas parallelto the Transtensionalnecking of theforearc region between lOS relative plate motion vector and highly oblique to the and2øN during the past 4 Myr hashad a profoundeffect on deformationfront. Subructionnorth of the equatorwas all of themajor elements of theplate margin there. The inner mostlyor wholly dip slip becausemost or all of the dextral trenchslope, outer-arc ridge, and forearcbasin have been componentof plate motion was occurringalong the Batee- Aceh fault. fragmentedby this process. Even the shapesof the subductioninterface, the active volcanicarc, and the About2 Ma (Figure 10b), both the Mentawaifault andthe Sumatranfault appear to havebeen affected. In fact,we can Sumatranfault formed. From 2 Ma to 100 ka, they carried divide the Sumatranplate boundaryinto three tectonic -40 mm/yrof the dextralcomponent of obliqueconvergence domains,based upon their relationship to thisPlio-Pleistocene south of the equator, and the subruction interface transtension(Plate 5). The southerndomain, which we accommodatedonly the dip-slip component. North of the suggesthas been part of the forearcsliver plate only for the equator,40 mm/yr of dextralslip was accommodatedby the Sumatranfault (10 mm/yr)and Aceh-Batee fault (30 mm/yr). past2 Myr,is themost simple geometrically and stmcturally. Figure 10c depicts our suggestionfor the current Thecentral domain, which comprises all the transtensionally fragmentedpieces, is the mostcomplex. neotectonicpartitioning of deformation. The Aceh-Batee fault is no longer active or is only minimally so. The Thesouthern domain has the following characteristics: (1) the Sumatranfault displays a right-steppingen echelon Sumatranfault is slipping~15 mm/yr fasternorth of about patternand courses above the 100- to 135-kmisobaths of the 2øN than south. The massbalance problem caused by this subduction interface, (2) the locus of volcanism is discrepancyis beingtaken up by a nascentbelt of deformation predominantlynortheast of or near the fault, (3) the forearc that crosses the outer-arc ridge at the equator. This deformationbelt is superjacentto Fauzi et al. 's [1996] swath basinis remarkablysimple, ~2 km deepand unbrokenby majorfaults, (4) the outer-arcridge is relativelynarrow, forms of exceptionallyhigh seismic activity in the down going a singleantiformal high, and is geometricallysimple, (5) the oceanicslab. It alsoencompasses the activeTom foldsof the mainlandcoast, two young faults on and southof Nias and Mentawaifault and homocline,which separatethe basinand north-southgraben that bathymetrysuggest may existon the ridge, are unbrokenand relativelystraight, and (6) theinner inner trenchslope (Plate 1). Figure 10c is consistentwith trenchslope is relativelyuniform and possessesa prominent recentmeasurements of geologicallymeasured Sumatran fault plateauabout half way betweenthe active deformationfront slip ratesbut is inconsistentwith the ratesof geodeticstrain and the outer-arcridge. The sourceof the giant (Mw9) measuredby GPS southof the equator. subductionearthquake of 1833 was the subductioninterface If the Sumatranfault is carryingonly ~ 10 mm/yrof dextral beneathmuch of this domain [Newcomband McCann, 1987; slip southof the equator[Sieh eta!., 1994; Bellier et al., Zachariasenet al., 1999]. Strains measuredby GPS in the 1999], the remainderof the dextralcomponent of slip mustbe earlyto mid-1990sshow that the outer-arcislands are moving taken up along either the subductioninterface or by a fault parallel to the relative plate motion vector and that the within the forearc sliver. The GPS data show no sharp subductioninterface beneath the southerndomain is currently gradientsin shear in the forearc region, so the remaining fully locked[Prawirodirdjo et al., 1997;McCaffrey et aI., this dextralcomponent is probablyaccommodated by slip on the issue]. The Sumatranfault appearsto be slippingat a rateof subductioninterface [McCaffrey et al., this issue]. This about 10 mm/yr in the Southerndomain [Sieh et al., 1991, portionof the dextralcomponent, x, wouldbe -27 mm/yr(x = 1994; Belllet et al., 1999]. 58 mm/yr*sin 41 ø - 10 mm/yr, where 58 mm/yr is the The northerndomain is characterizedby thesefeatures: (1) magnitudeof relative plate motion and 41ø is the angle a geometricallyirregular Sumatran fault, with bothreleasing betweenthe plate motionvector and the trenchnormal and 10 and restrainingbends, which residesabove the 125- to 140- mm/yr is the slip rate on the Sumatranfault). Slip vectorsfor km subductionisobaths, (2) a volcanicarc on and northof the earthquakeson the subductioninterface deviate from the Sumatranfault, (3) a 1- to 2-km-deepforearc basin, (4) a very trench normal by -20 ø, on average. These suggestthat the broad, structurallyand bathymetricallycomplex outer-are dextralcomponent on the interfacewould be a bit lessthan ridge, (5) a homoclinealong its southernmostfew hundred our modelpredicts, only •-16 mrn/yr. kilometers that is similar to the Mentawai structure of the The historydepicted in Figure 10 is consistentwith the southerndomain, and (6) a very narrowinner trench slope. timingof activityon faultsboth offshore and onshore Nias The centraldomain is distinguishedby thesefeatures: (1)a [Kariget al., 1980;Matson and Moore, 1992; Samuel and 350-km-longsection of the Sumatranfault that is markedly Harbury,1996]. It alsoincorporates our observation that the discordantwith the subductionisobaths, (2) a volcanicarc Bateefault is not currentlyactive alongmost of its exposed that cuts dramaticallyacross the Sumatranfault, (3)a trace but retains clear evidence of 5-km dextral offsetsof a topographicallyshallow (0.2-0.6 km deep)forearc basin, few of thelargest channels that cross it (Plate1). Restoration which has been fragmentedinto severalblocks during of ~80 km of slip on thefaults between løS and2øN in the oblique-normalfaulting, (4) a fragmentedouter arc, (5)a offshoreregion eliminatesthe dimplein the subductionfragmented homocline between the outer-arc ridge and forearc deformation front west of Nias and Simeulue, just as basin,and (6) a fragmentedinner trench slope. The giant (Mw restorationof-80 km of slip on the combinedSumatran and 8.5) subductionearthquake of 1861and numerous other large Mentawaifaults nearlyeliminates the dimplewest of the historicsubduction earthquakes originated within this domain SundaStrait. Thus we suggestthat the concavitiesof the [Newcomband McCann, 1987]. Strainsmeasured by GPS in deformationfront west of Nias and west of the SundaStrait theearly to mid-1990sindicate that the hanging wall block are featuresinherited from Plio-Pleistocenedextra! strike-slip acrossthe central domain is currentlymoving parallel to the motionin the forearcregion. subductiondeformation front [Prawirodirdjo et al., 1997; SIEHAND NATAWIDJAJA:SUMATRAN FAULT NEOTECTONICS 28,321

McCaffreyet al., thisissue]. The geologic rate of slipof the A more logical propositionmay be that transtensional Sumatranfault increasesmarkedly from southeastto neckingof the central domain has led to bendingof the northwestacross the central domain, from -11 mm/yrto -27 subductingslab Trench-orthogonalthinning of the forearc rnm/yr[Sieh et al., 1991]. appears to have drawn the deformation front and trench We suspect that transtensionalfragmentation has northeastward,tens of kilometerscloser to the mainlandcoast. dominatedthe central domain becausethe Investigator If this processhad not also drawn the deeperparts of the fracturezone has been subducting beneath the central domain subductingslab northeastward, the dip of the interfacein the forthe past several million years. Its locusof impingementon forearcand outerarc wouldbe steeperthan in the southern thedeformation front has migratedfrom the northernto the domain. The isobathsshow the contrary,that the subduction southernmargin of the centraldomain during the past 5 Myr zone beneaththe centraldomain has a very similar cross- (Plate5). Thismay be significant because fault activity in the sectionalprofile to that beneaththe southerndomain. One hangingwall block of theforearc region appears to havebeen test of this hypothesiswould be to determineif the active restrictedduring this period to thecentral domain (Figure 10). volcanicarc in thecentral domain is substantiallynortheast of Furthermore,the orientationsof faults in the centraldomain thelate Miocene and Pliocene arc. If so,it wouldsuggest that arepredominantly north-south, parallel to thetopographic and thesubduction isobaths have moved northeastward in the past structuralgrain of the underlyingInvestigator fracture zone. few million years. Wehypothesize therefore that the topographicheterogeneity of theInvestigator fracture zone beneaththe centraldomain hasled to disruptionof the forearcand outer-arcregions. 3.4. Relationshipof the SumatranFault Currently,the Investigatorfracture zone is also associated to the Modern Volcanic Arc witha bandof intenseseismicity within the downgoing slab Many havenoted the proximity of the Sumatranfault to the in themiddle of the central domain (Plate 5) [Fauzi et al., volcanicarc andhave suggested that it formedthere because 1996]and an abruptchange in the azimuthof GPS vectorson of the effectof magmatismon the lithosphere[e.g., Fauzi et theouter-arc ridge [Prawirodirdjo et al., 1997,McCaffrey et al., 1996; Tikoff, 1998]. Sumatra aside for the moment, al.,this issue]. however, most trench-parallelstrike-slip faults are not Thesubduction interface curves broadly across the Central coincident with their volcanic arcs. The Median Tectonic domain(Plate 5) [Fauzi et al., 1996]. The closeassociation of Line (Japan)does not have an associatedarc; the Denali fault this curve with the other elements of the central domain (Alaska) lies much farther from the trench than the Alaskan suggestscause and effect or at leasta sharedcause. Could arc volcanoes; the Atacama fault (Chile) lies between the flexureof thedowngoing slab have been produced by necking trenchand volcanicarc; and the Philippinefault is tens of of the hangingwall block? Or did deformationwithin the kilometersfrom the major Philippinearc volcanoes[Yeats et downgoingslab lead to transtensionin the forearc sliver al., 1997]. Furthermore,most volcanic arcs along obliquely plate?We suggestthe former. convergentmargins do not sportlarge strike-slipfaults. This The existenceof the 1500-km-wide boundarybetween generallack of associationsuggests that the alignmentof the Indianand Australian plates offshore western Sumatra and the Sumatranvolcanic arc and the Sumatranfault is purelya AndamanIslands gives reason to suspectthat the downgoing coincidence.In fact, McCaffreyet al. [this issue]have used slabwest of theInvestigator fracture zone is deforming.This finite elementmodeling of stressesacross the obliquely broadregion of deformationabuts all of the central and convergentSumatran plate boundaryto showthat formation northerndomains. Gordon et al. [1990] calculatethat the two of the trench-parallelSumatran fault did not requirethe oceanicplates are converging north-south at an angularrate of presenceof themagmatic arc. Nonetheless,Tikoff[1998] has 0.3ø/Myrabout a poleof rotationin the centralIndian Ocean. suggestedthat faults suchas the Sumatranfault form above At the Sumatran deformation front this translates into a the locus of greateststrain gradient in the lower crust or nominal13-km north-south shortening of'the downgoing slab mantle, occasionedby the magmatismof the volcanicarc. in the past 3 Myr. The actual nature of lithospheric BetIier and $ebrier [1994] have claimed that numeroussmall deformationwest of the deformationfront is quiteuncertain, and large volcanic cones and calderasoccur at both current however.Simple north-south buckling is unlikely. Focal andancient releasing step overs along the Sumarranfault. mechanismsand structure indicate a predominanceof north- We can test directly whether or not magmatismhas southleft-lateral slip on north-southfaults [Depluset al., influencedthe locationof the fault or, conversely,whether or 1998]. To accommodatenorth-south contraction, these not faulting has influenced the location of volcanismand structureswould need to be rotatingclockwise, domino-like, magmatism. Plate 1 allows us to searchfor a relationship to enableeastward extrusion of lithosphere[Gordon et aI., between the volcanic arc and the Sumatran fault, since it 1990].The precise loci of suchdeformation is unknown, and displaysnot only the mostprominent traces of the Sumatran soits impact on the overriding central and northern domains is fault but also the youngestvolcanoes. We mappedthese hardto assess. Nonetheless, it is plausible that the contrast in volcanicfeatures using the samesources we usedto map the natureof the southernand northernhanging wall domains fault (Figure 2). We limited our mappingto thosefeatures couldhave arisen,at least in part, from subductionof thathave suffered minimal erosion, since highly eroded,older deformingoceanic lithosphere beneath the northern domain. volcanic constructs are harder to recognize It ishard to imagine,however, how dextrat transtension on geomorphologicallyand mapping would have required a more northstriking faults within the central domain could be related substantialeffort. The featureswe mappedexhibit very little tosinistral slip and clockwise rotation on north striking faults erosional modification of their constructional landforms. in thesubjacent subducting lithosphere, unless eastward Many have been active historically. Those that have been extrusionof the oceanic lithosphere has led to northwestwarddated radiometrically are typically<100,000 yearsold (e.g., extrusionof the forearc sliver plate, as plate collision has done Toba caldera,73 ka [Chesneret al., !991], and Maninjou inTurkey and Tibet. caldera,60-90 ka [Nishimura,1980]). In additionto mapping 28,322 $IEH AND NATAWIDJAJA: $UMATRAN FAULT NEOTECTONIC$

lOO NE

J' 80

60 jV.olcanodiameter large(> 15km• j 40 I I

20

20 ...... , ...... i -' ' 5 ' j ' SW 4.0 ''

NW

Figure 11. Plot of the distanceof volcaniccenters from the Sumatranfault showingthat the volcanic arc hasnot influenced the location of the fault. However, 9 of the 50 volcanic centers are within 2 km of the fault. Most of theseare associatedwith extensional(right) stepovers in the fault. Large (15-km diameter) volcanicedifices are listedalong the horizontalaxis. Smallervolcanoes mentioned in the text are named.

craters and calderas, which are indicators of volcanic source The local centerline of the volcanicarc variesalong the vents, we also mapped the edges of the volcanic cones in strike of the Sumatran fault. It is a few kilometers northeast order to displaya crudemeasure of the outputof individual of thefault between5.5øS and 0.4øS, swings southwest of the sources. Sumatranfault between0.4øS and about 2øN, andthen swings At first glance,the moststriking relationships between the to a position-25 km northeastof the Sumarranfault between Sumatranfault and the youngarc volcanoesare that: (1) the 2øN and5.5øN. This broaddisparity between the localcenter averagecenter line of the active arc is decidedlylandward line of the volcanic arc and the Sumatran fault is another (northeast)of the Sumatranfault and (2) the local centerline indicationthat modernarc magmatismhas not guidedthe of the young volcanic arc switchesback and forth acrossthe formation of the fault. traceof the Sumatranfault as it traversesthe 1650-kmlength It also does not appearthat individualvolcanic conduits of Sumatra. Figure 11 showsthese relationships. The 10-km have influencedthe locationof particularfault segments. separationsnortheast from the $umatran fault are common, Onlyrarely do individualsegments of thefault bisect volcanic 25-km distances are not rare, and a few volcanoes are even centersor bendin their vicinity (counterexamplesare Kaba farther northeast. Only two volcanoesare more than 10 km and Dipatiampat). However,we would not expectsuch an southwestof the Sumatranfault. From Figure 11 one can association,since the volcanoesthat we havemapped are far estimatethat the averagedcenter line of the largestvolcanic youngerthan the age of initiation of the mappedfault edifices is --10 km northeast of the Sumarran fault. This segments.We suspectthat mostof the unerodededifices are skewed distribution of volcanoes relative to the Sumatran lessthan 100,000 years old, whereas we havemade a casethat fault suggeststhat the modernmagmatic arc has not createda thefault planes we have mapped are probably -2 Myrold. If weak crustal zone that has favored the concentration of shear. the locusof faultingwere influenced by magmaticsoftening Perhapsthe active volcanic arc has failed to influence the of the crust, the magmaticplumbing that led to the locusof faultingbecause the volcanicconduits "soften" only a concentrationof strains beneath the Sumatran fault would small percentageof the length of the arc. Alternatively, haveformed long before the young volcanoes on Plate 1. To perhaps magmatic plumbing beneath the Sumatran fault, test the hypothesisthat magmaticconcentration of shear associatedwith an unmapped, extinct volcanic arc, did stresses led to the formation of the fault within the arc, one influence the location of the fault. would need to map the Pliocene and early Pleistocene SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS 28,323 volcaniccenters. We may attempt this at a futuredate, but it by Engdahlet al. [1998]) andas determinedby Fauzi et al. isbeyond thescope of ourcurrent efforts. [1996] in their local seismicsurvey in the regionof Lake Despitethe lackof influenceof activemagmatism on Toba. tectonism,tectonism is influencing magmatism, but only to a From about 6øS to the equator,the relationshipis minorextent. This conclusion contrasts with that of Bellier particularlyregular; the subductioninterface lies 100 to 135 andSebrier [1994], who proposed that extensional pull aparts km belowthe Sumarranfault, except along the southemmost alongthe Sumatran fault have affected the location of the (Sunda)segment (Plate 1 andTable 1). Betweenabout 3.5øN volcanoes.In fact, our map shows that only 9 of the50 young and 6.0øN the subduction interface is 125 to 140 km below volcanicvents shown on Plate1 arelocated within 2 km of a the Sumatranfault, exceptbeneath the northern(possibly mappedtrace of theSumatran fault (Figure 11). Theseare, inactive)part of theAceh segment. These depths in thenorth from southeastto northwest,Suoh, Seminung,Kaba, are, on average,~20 km greaterthan depthssouth of the Dipatiampat,Kunyit, Melenggok, Talang, Sibual-buali, equator. The relationshipof subductionisobaths to the SeulawahAgam, and Pulau Web. Kaba,Kunvit, Meleggok, Sumatranfault is markedlyaberrant between the equatorand Talang,Sibual-buali, Seulawah Agam, and Pulau Web are about 3.5øN. There the traces of the Sumarran fault and the stratovolcanoesgreater than about 10 km in diameterand, subductionisobaths are markedly discordant; the depthof the thus,embody the most substantialvolumes. Suoh,Kaba, interfacebeneath the Sumatran fault ranges from-100 to 175 Kunyit,Melenggok, Talang, and Sibual-bualiare located km. withindilatational stepovers or on one of the boundingfaults Becauseof thewell-behaved relationship of Sumatranfault of a dilatationa!step over. One of these(Suoh) is a large to isobathsin thenorthern and southern domains, we propose phreaticexplosion crater that formed 15 daysafter the large that the Sumatranfault formed first in those two domains,as Semangkosegment rupture of 1933 [Stehn,1934], most two separatestructures. As displacementon the faults has convincinglyin association with tectonic activity. Bellier and grown,they haveformed a linkageacross the centraldomain Sebrier[1994] proposed that Toba and Ranau calderas also andwill oneday become a singlestructure. formedat extinct extensionalstep overs along the Sumatran faultzone, but thesehypotheses are not well founded. They arebased solely on the use of SPOT imageryto map more 4. Summary, Conclusions,and ancientfault strandsin the vicinity of these two calderas. Remaining Questions Althoughlinearions may exist along thesealleged ancient We have used stereographicaerial photographyand faults,their documentation of the lineationsis scant,and they topographyto map 1650 km of the Sumatranfault (Figures2 presentno geologicmapping to confirmtheir existence or to and 3). The resultingmap showsthat the fault comprises quantifythe style, age, or amountof shearalong them. numerous segments separated by dilatational and We suspectthat the associationof just 9 of the 50 young contractionalstep overs and abruptchanges in trend (Plate 1 volcanoeswith the Sumatran fault is a random occurrence. If andFigure 4). This segmentationappears to haveinfluenced onepeppered an elongaterectangle (with the 1700-by-50km the rupturedimensions of historicallarge earthquakesand dimensionsof the volcanicarc) with a randomdistribution of limitedtheir magnitudes to ~7.5. 50points and then ran straightlines randomly through its long The largest geomorphicallyevident offsets along the dimension,several points would typically be within2 km of Sumatranfault are between17 and 23 km (Plate 3, Figures7, eachline. Thus the close association of several volcanoes and 9 and Table 3). Theseare predominantlydeeply incised withthe Sumatran fault zone doesnot, by itself, demonstratea fiver channels,but one apparentoffset of a fold pair and the geneticrelationship. The closeassociation of six of the eight accumulatedoffset acrossa major step over also fall within closeencounters with dilatationalstep overs does,however, this range. A lack of detailedand completemapping along suggestthat tectonic step overs are influencingthe locations the fault precludesconfident matching of geologicunits of a few of the arc's volcanic centers. acrossthe fault, but rock offsetssuggested by Katili and Hehuwat [1967] and Cameron et al. [1983] supportthe 3.5.Relationship of the SumatranFault contentionthat the 20-km geomorphicoffsets represent the to the Subduction Zone total offset across the fault. Thegeneral shape of the Sumatranfault mimics that of the The distention of forearc structures and the trench near the deformationfront offshoreso faithfully that one wonders SundaStrait suggests ~ 100 km of arc-parallelstretching of the abouta genetic relationship between the subductioninterface forearcsliver plate since the early Pliocene (Figures 5 and8). andthe strike-slip fault (Plate5). Northof theequator, both We proposethat 20 km of thiswas accommodated by dextral structuresare concave toward the southwest. South of the slipon theSumatran fault and that the Mentawai fault, a long, equator,both are broadly concavetoward the northeast. linear structurewithin the forearcregion, accommodatedthe Alongthe entirelength of the Sumatranfault on land,its remainingdextral slip. horizontaldistance from the deformationfront variesno more Our synthesisof datafrom the Sumatran fault, the volcanic than~10% from 290 km (Table1 andPlate 5). arc, and the forearcregion shows that the Sumarranforearc A similarcoincidence exists between the shapeof the sliver plate consistsof three tectonicdomains with very Sumatranfault andthat of the subductioninterface downdip distinct tectonichistories (Plate 5). The southerndomain fromits trace. This is clearfrom Plates1 and5, whichshow (from7øS to 1øS) is the simplestand may have been accreted the 50-, 100-, and 200-km isobathsof the subduction to the forearcsliver plate only about2 Myr ago by the interface.The contours are drawn on the top of theWadaft- creation of the Sumatran and Mentawai faults. The northern Benioffzone, as definedby hypocentrallocations in the domain(north of 2øN) is morecomplex, and its northernpart InternationalSeismological Center (ISC) catalog (as relocated has been experiencing arc-parallel translation for at leastthe 28,324 SIEH AND NATAWIDJAJA:SUMATRAN FAULT NEOTECTONICS past10 Myr. The centraldomain is the mostcomplex of the Ourmap of theSumatran fault can serve as a jumping-off three and has been a region of transtensionbetween the pointfor careful analysis of theseismic hazard posed by this northernand southerndomains since at least4 Myr ago. majorstructure. To whatdegree does the historical record of Geodetic measurementssuggest that slip across the largeearthquakes along the Sumarranfault demonstratethat Sumatran fault between about 0.8 ø S and 2.7øN is nearly large structuralirregularities constrain rupture lengths? uniformat about25 mm/yr[Genrich et aI., thisissue]. These Would primitive instrumentalrecords help constrainthe ratesare incompatible with the 27 andl1 mm/yrgeologic slip sourceparameters of theselarge events of thefirst half of the rates that we have determinedat 2.2øN and 0.3øS [Sieh et al., twentiethcentury? Whetheror not segmentationof the 1991, 1994; D. Natawidjajaand K. Sieh, manuscriptin Sumatranfault has markedly influenced ruptures, answering preparation,2000). We proposethat the geologic difference these questionscould profoundlyaffect our general in rateshas arisenin just the past 100 ka or so, because understandingof the importanceof structuralgeometry on structural evidence for accommodationof the 15 mrn/yr seismicrupture processes. differenceis obscure. We suggestthat a belt of auxiliary, transtensional deformation between the Sumatran fault and Acknowledgments.This work began with an invitationby the trench is the nascentmanifestation of this rate change Yehuda Bock and his colleaguesat the IndonesianNational CoordinationAgency for Surveying and Mapping (Figure10). This beltincludes the western(Angkola) branch (BAKOSURTANAL) to visit Sumatra. They were interestedin of the EquatorialBifurcation, the Tom fold-and-thrustbelt establishinga better geologic context for theirGPS geodetic studies, alongthe mainlandcoast, and submarine faults in theforearc supportedunder NSF grantsEAR-8817067 and EAR-9004376.Our basin,outer-arc ridge, andinner trenchslope. initial reconnaissance,in 1991, supportedby donationsfrom the Caltech Associates,convinced us that geologicalwork aimedat Althoughthe Sumatranfault andthe Sumarranvolcanic arc understandingthe activetectonics of the faultcould be veryfruitful. sharethe samejungle, neither appears to havefundamentally In 1992, we initiated our own NSF sponsoredproject (EAR- affected the location of the other. Rather than being 9205591) to map and characterize the Sumatran fault. coincident,the fault and the arc intertwine(Figure 11). The BAKOSURTANAL, through Bock, supplied most of our topographiccoverage of the fault. We are gratefulto Suparkaand averagedcenter line of thevolcanic arc is distinctlynortheast Hery Harjono at the IndonesianInstitute of Sciences(LIPI)for of, not at, the Sumatranfault. Nevertheless,the few volcanic helpingus acquiremany important aerial photographs and arranging centersthat are on or very near the Sumatranfault are fieldwork. Rudy Bachruddinat the VolcanologicalSurvey of predominantlyat major extensionalstep overs, which may Indonesia(VSI) and GunawanBurhan at the GeologicalResearch well haveattracted a smallpercent of the arcvolcanism. The and DevelopmentCenters (GRDC) also helpedus by providing dramatic bend in the modem volcanic arc between 0.7øN and essentialaerial photographsalong portions of the Sumarranfault. Various private companiesalso allowed us accessto topographic 2.5øNis mostprobably the result of transtensionalthinning of mapsand aerial photographs. Paul Tapponnier (IPG Paris)shared the forearcsliver plate in the past4 Myr. We can not rule out topographicmaps from the Singkarakregion. We alsobenefited the possiblilitythat the Pliocene and Miocene volcanicarc from conversationswith Rob McCaffrey and Bob Kieckheferand were lesssinuous and closer to the locusof later strike-slip from an initial GIS compilationof our data by CarolynWhite. Reviewsof anearly manuscript by YehudaBock, Joseph Curray, and faulting. RobMcCaffrey were very helpful. Finally, we greatlyappreciate the The broad similarityin shapeof the Sumatranfault and thorough,thoughtful, and carefulreviews of Jeff Freymuller,Eli subductioninterface suggests a geneticrelationship. The Silver, and Paul Tapponnier. This is CaltechSeismological broad, low-amplitudesinusoidal shape of the subduction Laboratorycontribution 8625. interfaceis mimickedby the Sumatranfault, and along most of its trace the Sumatran fault lies above the 110- to 140-km References isobathsof the subductioninterface. These relationships are Abe, K., Magnitudesof large shallowearthquakes from 1904to particularlyregular north of 3.5øNand south of the equator,in !980: Phys.Earth Planet.Inter. 27, 72-92, !981. the northernand southerndomains. We suggestthat the Allen, C., A. Gillespie,H. Yuan, K. Sieh,Z. Buchun,and Z. Sumatranfault first formedas two separatefaults in thesetwo Chengnan,Red River andassociated faults, Yunnan Province, China: Quaternarygeology, slip rate and seismic hazard, Geol. domains,and are in the processof linkingtogether through Soc.Am. Bull., 95, 686-700, 1984. the central domain and across the volcanic arc. We ascribe Allen,C.R., SanAndreas fault zone in SanGorgonio Pass, southern the disruptednature of the centraldomain's outer-arc ridge California, Geol. Soc.Am. Bull., 68, 315-350, 1957. Armijo,R., B. Meyer,and A. Hubert,Westward propagation ofthe and forearcbasins to its locationabove the Investigator North Anatolianfault into the northernAegean: Timing and fracturezone throughout the past 5 Myr. kinematics,Geology, 27, 267-270, 1999. Not unexpectedly,this work has generatedas many Aspden,J.A., W. Kartawa,D.T. Aldiss,A. Djunuddin,R. questionsas answers:What are the detailsof the creationand Whandoyo,D. Diatma,M.C.G. Clarke,and H. Harahap,The evolution of the three tectonic domains of the forearc sliver geologyof the Padangsidempuanand Sibolga Quadrangle, Sumatra,report Geol. Res. and Dev. Centr., Bandung, Indonesia, plate? How, for example, did deformation in the 1982. transtensionalcentral domain evolve through the pastseveral Barka,A., TheNorth Anatolian fault zone, Ann. Tectonicae, 6, 164- millionyears? Why did the Sumarranfault form whereit did, 195, 1992. 290 km from the subduction deformation front and 100 to 150 Bellier,O., andM. Sebrier,Relationship between tectonism and km above the subductioninterface? Would careful, detailed volcanismalong the Great Sumatran fault zone deduced bySPOT imageanalyses, Tectonophysics, 233, 215-231, 1994. mappingconfirm total Sumatran fault offsets of only-20 km? Bellier,O., andM. Sebrier,Is theslip rate variation on the Great When did the contrastin slip ratesalong the Sumatranfault Sumatranfault accommodated by fore-arc stretching?: Geophys. begin? Why is this gradientin rates not apparentin the Res.Lett., 22 1969-1972, 1995. Bellier,O., M. Sebrier,S. Pramumidjojo, T. Beaudouin, H. Harjono, geodeticdata? Is it plausiblethat the Mentawaifault hasa I. Bahar,and O. Fomi,Palcoseismicity andseismic hazard along strike-slipcomponent as large as 80 km? Did the two faults theGreat Sumatran fault (Indonesia), J. Geodyn., 24, 169-183, originatea mere2 Myr ago? 1997. SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS 28,325

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