Neotectonics of the Sumatran Fault_ Indonesia

8/13/2019 Neotectonics of the Sumatran Fault_ Indonesia

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28,296 SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS

95OE 100E 105E 110.E

10ON

5ON

0 o

AndamanSeaSouth Ch/na Sea

5os

10os

Figure 1. Regional ectonic etting f the Sumatranault. The Sumatranault (SF) is a trench-parallel,ight-lateralstrike-slip ault that traverses he hangingwall blockof the Sumarran ubduction one rom the SundaStrait o thespreading enters f the AndamanSea. It separates forearcsliverplate rom the southeast sianplate. Trianglesare active volcanoes f the Sundaarc. Arrow is relativeplatemotionvectors eterminedrom GPS. Topographyandbathymetryare from Smithand Sandwell 1997]. WAF is the West Andaman ault. MF is the Mentawai fault.

portionof the plate boundary Figure 1). At 6S, 102E t is60 mm/yr, N17E [Prawirodirdjo et al., this issue]. At 2N,95E, t is 52 mm/yr, N10E. Furthermore, ecausehe shapeof the plate boundary s arcuate, he natureof relativeplatemotionchangesmarkedlyalong its strike. At the longitude fcentral Java the strike of the subductionzone is nearlyorthogonal o the direction of relative plate motion, so anycomponent of strike-slip motion need not be large[McCaffrey,1991]. At the latitudesof Sumatra, owever, hestrike-slip component of relative plate motion must besignificantbecause he direction of relative plate motion ssubstantiallyblique o thestrikeof the subductionone.

Fitch 1972]suggestedhat he ight-lateralomponentfthis oblique onvergences the cause or the right-lateralSumarran fault. McCaffrey [1991, 1992] added moresubstanceo this hypothesisith his discoveryhatslipvectors f moderatearthquakeslong heSumatranortionof thesubductionone renearly erpendicularo the trikeof theplateboundary. e noted hat f these ector irectionsare representativef long-termlip trajectorieslonghesubductionnterface,hensubductiontself s onlyslightlyobliquendmost f thedextral omponentf platemotionmust be accommodated elsewhere.The Sumatran ault is the most obviouscandidateor


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SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS 28,297accommodationf the emainingomponentf dextrallip.TheMentawaiault, iscoveredffshorey Diamentt al.[1992],omplicateshis lightly. hismajor,ubmarine,trench-parallelaultiesbetweenheSumatranault nd hetrenchndmay lso ave ccommodatedsignificantmountof he extralomponentfplatemotion.The ombinationf anarcuate lateboundarynda distantpolef otationuggestshathe ate fdextrallip longheSumatranault ncreasesorthwestwardHuchon nd LePichon,984;McCaffrey, 991]. Observationsear henorthwesternndsoutheasternermini f theSumatranaultsupporthis ontention.urraytal. [1979] uggestedhattherate of opening cross he spreading enters f theAndamanea Figure ) hasaveragedbout 7 mrn/yror thepast1Myr. hey roposedhatmostf hismotionas eencarriedo thesoutheasty the Sumatranault. Reanalysisftheseata ields hesame ate; otalopeningn thepast3.2Myr s~1 8 km J.Curray, ritten ommunication,999).The lip ate nferredor theSumatranaultneartssouthernterminus,owever, ppearso be far lower than37 mrn/yr.Bellier t aI. [1999] calculate rate of ~6 mm/yr near thesouthernndof the ault from an offsetchannelncisednto adated leistoceneuff.1.2. Motivation of This Work

Despitets ankingsoneof Earth's reat trike-slipaults,its ighevel f historicaleismicctivity nd tsmajor ole ntheactive ectonicsand seismic hazard of SoutheastAsia, theSumatran ault has not been well characterized. Whatattentionhe aulthas eceived asbeenpredominantlyrom agreatistance,ostly t plate ectoniccales.Untilrecently,the eometryf the aultwasknown nly o first-ordersee,forexample,hesmall-scale aps f Fitch [ 1972],BelIieretal. [1997]or McCaffrey 1991 . More detailedstudies avebeenimited o local studies, uchas Tija's [1977] and KatiliandHehuwat's1967] work on exemplary ffsetdrainages.The Sumatran ault has generated many historicalearthquakesithmagnitudes> 7, butbecause ostof thesehappenedore hana half a century go, heyhavenot beenwelldocumented.Reid [1913] used geodeticmeasurementsfrom before and after the 1892 Sumatran earthquakeassupportor his concept f elastic ebound.Berlage 1934]describedheeffects f the 1933earthquaken south umatra.Visser [1927] described the effects of the 1926Padangpanjangarthquaken westSumatra,ndUntung t al.[1985] ndNatawidjaja t al. [1995] ecentlyeported extraloffsetsormed during the nearby 1943 Alahanpanjangearthquake.Thepaucity f detailedmapsof the ault, hescarcity fdata nhistoricalargeearthquakes,nd he ackof reliableestimatesf slip ates reunfortunate. hey seriously amperattemptso forecasthe seismic roductivityf the ault andeffortso understanduantitativelyts role in the obliqueconvergencef theSumatran lateboundary.Our irst ask n this study, hen,hasbeen o constructmodem apof theactive omponentsf theSumatranault.To be of use in seismic hazard assessmentand inunderstandingheneotectonicoleof the ault, hescale f themap eededo be arge nougho clearly iscriminateajorfault trandsnd he discontinuitiesndchangesn strikebetweentrands.Our econdask,whichwill be describedn a future aper,willbe to determinehe sliprateof the faultat several

localities to determinewhether or not the actual slip ratesconform to current kinematic models. Such rates would alsoserve s a long-term verageor the nterpretationf geodeticdata from Global PositioningSystem GPS) networks hatnow span he fault [Genrichet al., this issue]and historicaltriangulation ata ?rawirodirdjoet al., this ssue].

2. A Modern Map of the FaultTo map heSumatranaultefficiently nd eliably,we havereliedprimarily pon ts geomorphicxpression. eomorphicexpressions especiallyeliable or mappinghigh slip ratefaults, where tectonic andforms ommonlydevelopand aremaintained at rates that exceed local rates of erosion or burial[Yeats et al., 1997, Chapter 8]. Examples ofgeomorphologicallyasedregionalmaps of active faultsinclude ctive ault mapsof Japan, urkey,China,Tibet, andMongolia Research roup or ActiveFaults,1980;Sarogluet al., 1992; Tapponnier nd Molnar, 1977] as well as mostmapsof submarinective aults.Admittedly, he geomorphicxpression f active aultswithslip ateshatare ower hanor nearly qual o ocal atesof erosion r burial s likely to be obscure.This s especiallylikely f the aults reshort, avesmallcumulativeffset,orhave no component f vertical motion. Becauseof ourrelianceon geomorphicxpression,ur map of the Sumarranfault undoubtedlyxcludesmany short, ow-rateactive aultstrands.2.1. Resources and Methods

The grossesteatures f theSumatranaulthave ongbeenknown rom analysis f small-scaleopographicndgeologicmaps. More detailed mall-scale apsof the fault, baseduponanalysis f satellitemagery, avebeenproduced orerecently Bellier et al., 1997; BeIlier and Sebrier, 994;Detourbetet al., 1993]. The unavailabilityof stereographicimagery, owever,imited he esolutionnd he eliability fthese mall-scale aps. Specifically,he ackof stereoscopiccoverageprecluded he recognition f importantsmalltectonic andforms, nless hey were favorably lluminated.Conversely,nactiveaults acking mall, atePleistocenendHolocene ectonic andformsmay have been mapped asactive, ased pon he presencef olderand arger ectoniclandforms.Ourmapping f theSumatranault s based rimarily poninspectionf l:50,000-scaleopographicaps nd :100,000-scale erialphotographs. here hesewerenotavailable rwere of unsuitable uality, we utilized l:250,000-scalegeologicmapsandradar magery. Figure2 displayshecoveragef materialshatwe used.Figure displaysepresentativetereographicairsof the1:100,000-scaleerialphotographs.hesephotos isplay hefault at about0.3S,where t offsetsstreamchannels hat aredeeplyncisedntoa thickpyroclasticlow deposit.Afterinterpretinghese nd otherstereopairs,e compiled urinterpretationsnto l:50,000-scaleopographic aps (orl:250,000-scaleopographic aps,where the larger-scalemaps were unavailable).Where stereographicerialphotographsere unavailable, e interpretedctive aultgeometryndsense f slipdirectlyrom he :50,000-scaletopographic aps.Thesedata were then digitizedand attributed, sing heGeographicnformation ystem GIS) software, rc/Info.


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100 0 I00 200 300 400 Kilometers

The Sumatran FaultCentersof aerial photographs

Lake Coveragesf 1:250,000 opographic apsCoveragesf 1:50,000opographicaps

Figure . Datauponwhich urmap ompilationsbased.Mostof ourmappings based n nspectionf 1:50,000-scaleopographicaps roducedy BAKOSURTANAL JANTOP,henational appinggenciesor ndonesia,and :100,000-scaleerialphotographs.therdata ourcesnclude maller-scaleeologicnd opographicaps.

The resultingG1S databasencludes ault geometry,sense ffault slip, and photo centers. Plate 1, constructedrom thedatabase,depicts all of the salient featuresof the Sumarranplate boundary hat we mappedand compiled.2.2. Geometry of the Fault

The overall shape of the Sumarran ault acrossSumatra ssinusoidalFigure 1). The northernhalf of the fault is gentlyconcave to the southwest, whereas the southern half of thefault is concave to the northeast. Over the 1650-kinsubaerially xposed engthof the fault, the "amplitude" f thesinusoidal trace is -55 km.

Ornamenting he broad, sinusoidalshapeof the Sumatranfault are numeroussmaller irregularities. Though smaller,these have dimensions of the order of tens of kilometers andare therefore ectonicallyand seismologically ignificant.The greatest f these s a feature hat we call the EquatorialB furcation (Figure 1 and Plate 1). This forceps-shapedfeature is present between the equator and about 1.8Nlatitude. It is characterizedby the bifurcationof the Sumatranfault toward the southeast nto two principal active strands.The two strands are distinct from each other even at theirpoint of bifurcation about 1.8N).The greatest eparation fthese two branches is -35 km, near 0.7N. The westernbranch of the bifurcation does not rejoin the easternbranchfarther south; instead, it dies out geomorphically t about0.35N.

Other large irregularities include subparallelgeomorphically xpressed ault traces at about 5.5N, 4N,and 3.5S. The Batee fault, a right-lateral ault that may havedisplaced he island's western shelf-150 km since theOligocene Karig et al., 1980], divergessouthwardrom theSumatran ault at about4.6N. A 75-km-long old-and-thrustbelt, exhibiting clear geomorphicevidence of youthfulnesslies about 40 km west of the Sumarran fault at about 1.3N.All of these features are described in section 2.3.

2.3. Major Segments f the Sumatran FaultSuperimposed pon the broad sinusoidalgeometryof theSumarran ault are more than a dozen discontinuities,angingin width from -5 to 12 km (Plate 1). Major local changesnstrikealsooccur. Most of the discontinuities re right stepsnthe fault trace and thts representdilatational step overs.However, a few contractional bends also occur.

Theoretically, hesediscontinuities nd bends n the faultarelarge enough o influence he seismic behaviorof the fault[Harris et al., 1991;Harris and Day, 1993]. The relationshipof historicaluptureso these eometrical egment oundarieswill be the subjectof a futurepaper D. Natawidjaja ndK.S eh, manuscriptn preparation,2000).We haveused hesesecond-ordereometricrregularitiesto divide he Sumarranault nto 19 segmentsFigure andTable 1). Each segmentbears he nameof a majorriverorbay along he segment. n so naming he varioussegments,we haveabandonedheusual ractice f retaining ameshathave precedence in the scientific literature. Thenomenclatural morass inherited Iom numerous earlier studiesincludesmany fault names derived from nearbycities,districts, basins, and rivers. These include Banda Aceh Anu,LamTeubaBaro,Reuengeuetlangkejeren,la-Alas, lu-Aer, Batang-Gadis, epahiang-Makakau,etahun,MuaraLabuh, ndSemangkoe.g.,seeKatili andHehuwat,967;Cameron t al., 1983; Durham, 1940]. Sincemanyof theseoverlap ur geometricegmentoundariesr includenlyparts f oursegments,e have bandonedhem n favor famoresystematic nd precise omenclature.For he ntire roup f activeault egments,rom cehnthenorth o theSunda trait n thesouth,we have hosenhename Sumarranault," irst usedby Katili and Hehun'at[1967]. This name epresentsest he dimensionf thestructure. arliernamesor the faultare "Semangko"nd"Ulu-Aer," uggestedy VanBemmelen1949]andDurham[1940]; but these refer to local features. "Great Sumatran


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28 302 SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS

NORTHWEST

,..,, . C'

% % SOUTHEASTKUMERING SEMANGK0

INDIAN OCEAN

t Lake Volcanicrater Volcanicdifice majoralleysFigure4. Map of 20 geometricallyefined egments1:heSumatranaultsystem nd heirspatial elationshipsoactive volcanoes,major graben,and akes.

1908, may have involved rupture of all or most of thissegment Berlage, 1934].2.3.3. Kumering segment 5.3S to 4.35S). This 150-km-long segment uns between the dilatationalstep over atSuoh Valley to a contractionalog at 4.35S. Near the centerof this segment, the waters of Lake Ranau occupy a latePleistocenecaldera and conceal about 9 km of the trace (Platei and Figure 4). The southern art of the Kumeringsegmenttraverses he drainagesof the Werkuk and upper Semangkorivers. A less active southeastward continuation of thissegmentmay form the northeasternlank of the SemangkoValley [Pramumijoyo nd Sebrier, 1991],but we did nothaveadequatematerials o determinets activity here.North of Lake Ranau, a 40-km-long reach of the faulttraverses he headwatersof the Kumering River. The trunkstreamof this large river doesnot cross he fault; instead, tstwo major tributaries low toward one another cross he traceof the fault and flow northeastward way from the fault fromtheir confluence.This relationshipof largestreamchannelsothe fault is common along much of the Sumarran ault; notuncommonly, he headwatersof a principal stream are nearthe fault, and none of the larger channelsof the drainagenetwork cross the fault trace. In these cases, dextral offsets ofthe stream hannels re either ambiguous r small.

The northwesternmost 5 km of the Kumering segmentdeviateswestward rom the trend of the rest of the segmentand is part of a 10-km-widecontractionalog. This portionof the segmentdisplaysa significantcomponent f reverseslip, as evidencedby a high escarpment nd a mountainousanticline north of the fault trace. Aerial photographyavailable to us did not reveal the continuation of the faulttracenorthwest f 4.35S, hroughhe restof thecontractionalbend.

High intensitiesndicate uptureof many ensof kilometersof the Kumering egment uring heMs 7.5 Liwa earthquakeof June 4, 1933 Berlage, 934]. Deadlyphreatic xplosionsoccurred weeks fter heearthquake ithin heSuohValley[Stehn, 1934].A geomorphicallyessprominent ubparalleltrand f thefault exists2.5 km to the southwest f the principal ctivetrace outh f LakeRanauNatawidjaja, 994;Widiwijayantiet al., 1996].The devastatingw 6.8 Liwa earthquakef1994 wasgenerated y this essprominentrace. The mostsev6re amagend heaftershockegion oincided itha 25-km reachof this secondaryrace.2.3.4. Manna segment4.35S o 3.8S). This85-kmsegment eviates nly a kilometeror two from beingrecfilinearbut has rather obscure erminationson bothends


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SUMA.'RA x, JavaSeaJAVA

deformationrontouter-arcidge xis

---T--orearcasinxisormalthrust faultinferred nticlinebathymetryontour(in meters)' depreasion/basin

a, active volt. dnO

%0 INDIAN OCEAN

Figure 5. Sumatran ault and relatedstructures ear the SundaStrait and bathymetricmap of the portionof theSundaStrait and surrounding eafloor.The Sunda egment f the Sumarranault formsan 1800-m-deep raben hatwidenssouthward,oward he deformationront. Northwestwardmovement f the forearcsilverplate along heSumatran ault appears o have caused hinningof the regionbetween he trenchand he strait. Bathymetry sDigital ElevationModel ETOPO02 and bathymetric urveys Smithand Sandwell, 1997].

%$

(Figure 4). The Manna segmentappearsdiscontinuous nPlate I because he trace is obscure ocally on the aerialphotographs nd topographicmaps.The southern nd of thesegmentabuts the contractionalbend mentionedabove. Thenorthernend of the segment s obscurebeyondabout3.8Sbut appears o be within a geometricallycomplex right(dilatational)step n the fault.Exceptionally clear 2.4 + 0.2 km dextral offsets of twolarge streams Air Kanan and Air Kiri) exist on the dissectedwestern lank of an extinct volcanos,.utheast f Pajarbulan(Plate 2). We encountered urprisinglywe l-preserved malltectonic landforms beneath the jungle canopy during anexcursion n 'thedrainagesof these wo streams.A destructiveearthquake occurred in the vicinity of thissegmenton June 12, 1893. The area of greatestdamagecoincidedwith the centralpart of the Manna segment Visser,1922].2.3.5. Musi segment (3.65S to 3.25S). This 70-kinsegmentof 'the Sumatran ault comprisesseveral highlydiscontinuousault segmentsFigure 4 and Plate 1). Despitegood coveragewith l:100,000-scaleaerial photography, ecouldnot dentifycleargeomorphicraces longmuchof thissegment.The longestcontinuousrace that we were able to maptraverses the southwestern lank of the large, active

stratovolcano, Bukit Kaba. Stream channels cut into theyoungest lows there are offset -700 m. We have used hesechannelso determine he slip rate of 11 mm/yr for theMusisegment (D. Natawidjaja and K. Sieh, manuscript npreparation, 000).The destructive,Ms 6.6 Kepahiang arthquake ccurredalong his segment t about3.6Son December 15, 1979. Weheardeyewitness ccounts f minor crackingalong he aultwhenwe visited n 1993, but we saw no convincing videnceof tectonic urficial upturesrom 1979.2.3.6. Ketaun segment (3.35S o 2.75S). This 85-km-long segment consists of a linear trace with severaldiscontinuitiesand stepovers of about a kilometer ndimension Plate 1 and Figure 4). The segment's outhernend is at a 6- to 8-km-widedilatational tepover onto heMusi segment.An inactiveor lessactivecontinuationf theKetaunsegment aay xtendbeneathhestratovolcanicdificeof Bukit Kaba. This possibilitys suggestedy thepresenceof a geomorphically ubdued ault, southeast f the volcanoand ---25 m eastof the centralMusi segment. he northernend of the Ketaun segment s within a 6-km-widecontractional tep over. Within this contractional tepoxerthe topography ises several hundredmetersabove hesurroundingandscape.

Twomajor ivers rossheKetaun egment,heKetaunn


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SIEHAND NATAWIDJAJA: UMATRAN AULTNEOTECTONICS 28,305theouthndheSeblatn thenorth. heSeblativer alleyappearsobeoffsetextrally17km, ndheKetaunivervalleyay eoffset23 km. A moderatearthquakenMarch5,1952 M 6.2,U.S. GeologicalurveyUSGS)),producedighntensitieslongheKetaunegmentKraeff,1952].2.3.7. Dikit segment 2.75S o 2.3S). This is apredominantlyinear,0-km-longegmentith everalhort,obscureectionslong ts northernew kilometersPlate1and igure). It shares contractionaltepoverwith theKetaunegmentn its southeasternnd. Its northwesternterminations at oneof the arger ilatationaltep vers longthe umarranault. On the southwesternlankof this11-km-widetep ver, heDikit segmentisappearsnto heedificeof he mall tratovolcanounyit.This s oneof the ew clearassociationsf a di atational tepoveranda volcano long heSumatranault.The malliamond-hapedalderafDipatiampatsoffset~500mby the ault. Justnorthof the smallcalderaake,atabout.65S, he main trace appears o form an enigmaticd0gleg.heDikitRiverValley ollowshe ault or 20 km.Wearenot convinced hat this represents dextraloffset of20kin,becausehe constructionf two largevolcanic dificeshasundoubtedly bscuredolder drainageson the blocknortheastf the fault.23.8. Siulak segment (2.25S to 1.7S). Cleardilatationaltepoversdemarcatehe terminationsf this 70-kin-longegmentFigure4 and Plate 1). The 11-kmwidestepovert thesoutheasternnd s thewidestdilatationaltepover longhe Sumatranault,but our aerialphotographyidnot eveal its structural details. The northern terminus of theSiulakegments a 4-km-widestepoveron the westernlankof the great active stratovolcano erinci. West dippingnormalaults cut lavas of Melenggokvolcano here, andappearo transfer slip from the Siulak segment o itsnorthwesterneighbor.Along the Siulak segment'ssoutheasterneach, LakeKerinci nd he alluvium of a broad valley obscure he faulttraceor-30 km. Two largeearthquakesavecaused everedamagelong he Siulak segment f the Sumatranault. OnJune, 1909,mostof the region raversed y this segmentwas evastatedy an earthquakeudged o havea magnitudeofaboutMs 7.7 [Abe, 1981]. The zone of greatest amageduringheM 7.0 earthquakef October , 1995,waswithinthebroadvalley northwestof Lake Kerinci (Indonesiannewspaperompas,October7, 1995).2.3.9. Suliti segment1.75S o 1.0S). his95-km-longsegmentasa comparativelytraightault trace,whichterminatesn both the northwest and southeast at di atationalstep verswithin volcanicedifices Figure 4 and Plate 1).Thenorthwesterntepover,at Lake Diatasand Talangvolcano,s 4 km wide. The details'Of'the central reachesofthe egmentreobscureecausehe ault raverseshenarrowvalley f the Suliti River headwaters or more than 50 km.Howmuch f thiscoursef the aultalongheSulitiRivervalleyepresents dexraloffset s unknownbecausehetrunktreamoes otcrosshe ault. Along hesouthernmostpart f thissegment,ributaries f the Liki River are offsetseveral undred meters.The irstof two arge arthquakesf June , 943 (Ms7.1[PachecondSykes,992]),may avenvolvedupturef henorthernart of the Su itisegment,udging rom seriousdamageo Muaralabuhillage,25 km northwestf

southeasternerminus f the segmentNatawidjaja t aI.,1995].2.3.10. Sumani segment 1.0S o 0.5S). This 60-kin-longsegmentunsnorthwestwardrom the volcanic erraneofLake Diatas o the southwesternlank of Lake Singkarak,which occupies structural raben, ather than a volcaniccalderaFigure andPlate1). Two opposingrcuate ormaloblique aults orm opographiccarpshatrise400 m abovethe surfaceof the lake (Plate 3). Ancient uplandsurfaces,with drainages lowing away from the lake, are clearlytruncated y the steep carps oundinghe ake basinand husappear o have been faulted down below the waters of thelake.Failureof the Sumani egment roducedhe second f twolarge earthquakesMs 7.4 [Pachecoand Sykes, 1992]) onJune9, 1943 [Natawidjajaet al., 1995]. Shaking ntensitiesindicate hat the northwesternnd of the fault rupturewasbeneath he lake. Eyewitnessaccounts ed Untung et al.[1985] to conclude hat right-lateraloffsets of up to 2 moccurred ear he town of Solok,but Natawidjaja et aI. [1995]couldonly verify offsets f ~1 m. Analysisof geodetic ata

supports meter or so of dextralslip [Prawirodirdjoet al.,this issue].High intensities n the vicinity of Lakes Dibawah andDiatassuggesthat he entiresoutheasternart of the segmentalso uptured, ndperhaps ven he northwesternart of theSuliti segment.The first of two largeearthquakesn August4, 1926 wasmost severe n the narrow zone along the Sumani segment.Anotherearthquake, n October1, 1822, was most severebetween he Marapi andTalangvolcanoesWichman, scitedby Visser [1927]). Thus this earthquakemay well haveinvolved uptureof the Sumani egment.Genrichet al. [thisissue]show hat strainaccumulation uring he early to mid-1990s s consistent ith 23 + 5 mngyr of dextralslip on thissegment.2.3.11. Sianok segment (0.7S to about 0.1N). Thispredominantly traight nd continuous egment uns 90 kmfrom the northeast hore of Lake Singkarak,along thesouthwestlank of the greatstratovolcanoarapi o a 10-km-wide right stepover at the equator Plate 1 and Figure4). Itssouthern 8 km, on the flank of Lake Singkarak, s arcuateand must have a significant omponent f normal aultingdown oward he ake. Geomorphic xpression f the ault isparticularly nteresting long the Sianok segmentbecausettraverseshe flank of Marapi volcanoand the young,200-m-thick pyroclastic low depositof Maninjou volcano. Streamchannelslowing off Marapi displayclear dextra offsets hatrange rom -120 to 600 m. The trunk channelof the SianokRiver is incised nto the ManinjouTuff and displayoffsetsof~700 m (Figure 3). We havebeenable to use heseoffsets odetermine dextral ate of slip of- 11 mm/yr (D. NatawidjajaandK. Sieh,manuscriptn preparation,2000).The secondof two large earthquakes n August4, 1926,wasmostsevere long he southeasternortionof the Sianoksegment.This is consistent ith Visser's 1927] observationof fault rupturebetweenBukittinggiand Singkarak. Genrichet aI. [this issue] show that strain accumulation across hissegmentn the early to mid-1990s s consistent ith dextralslip of 23 + 3 mngyr.2.3.12. Sumpur segment equator o 0.3N). Dataalongthis 30-km-long segment nd its northwestern eighborarescant.Our map s based redominantlypon1:250,000-scale


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28,306 SIEH AND NATAWIDJAJA' SUMATRAN FAULT NEOTECTONICS


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SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS 28,307

geologicapsRocktal.,1983,spdental., 982] ndpoorlyeproduced:50,000-scaleopographicaps.Both ermini of the Sumpursegmentare at largedilatationalteps.Thus, etweenheSianok ndBarumunsegments,he faultexperiences35-km-wide,ouble-dilatationaltepover. The northwesterntep s associatedwith highwest acing scarpmentnd he adjoining idevalleyf heSumpur-Rokaniver.2.3.13.Barumunsegment 0.3N o 1.2N). This 15-Ian-longegments broadly oncaveoward he southwestandormsmostof the northeasterneg of the EquatorialBifurcationFigure andPlate1). Thesoutheastern0 kmoftheBarumunegmentorms he boundary etween highwestacingescarpmentnd the broaddepressionf theSumpuriver.We interprethisescarpmentndadjacentdepressiono be evidencef a significantomponentfextensionalip slip on thisportionof the Barumun egment.Weplacehe northwesternnd of the Barumun egmentsomewhatrbitrarily t an abrupt15 bend n the traceof thefault,earheheadwatersf theBarumun iver.0nlyalongts northernmost5 km havewe beenable oinspect:100,000-scaleerial hotography.herehe aulttracesisplay leargeomorphicvidence f strikeslip.Thechannelf the BarumunRiver may be offsetabout20 km, butthis ffsets not compelling ecausehe trunk stream oesnotcrosshe fault.2.3.14. Angkola segment (0.3N to 1.8N). Thesouthwesternranchof the EquatorialBifurcation onsists facontinuousault with an abrupt30 bend at about0.65N(FigureandPlate1). Geomorphicxpressions particularlyclearbetween about 0.8N and 0.5N. Katili and Hehuwat[1967] sedoffsetsof tributaries o the Angkola River atabout.55N o demonstrateight-lateral ffsets anging rom200o 1200m along his segment.The northern 0 km of theAngkolaegment onsists f a set of discontinuousaultsonthe outhwesternlank of the Sarullagraben. Although arge-scale erial photographs o show minor, discontinuousfaulting t about 0.35N, the lack of through goinggeomorphicxpression f the westernbranchsouthof 0.5Nshowshat he fault is significantlyess active there. Thewesternegment oesnot rejoin the northeasterntrandustnorth f the Equator. Geologic mapping supports hisinterpretation,nd suggestshat total slip on the westernbranchannotbe large [Rock et al., 1983]. Geodeticmeasurementspanninghe early to mid-1990ssuggesthatmodemtrain atesare higher n the vicinityof theAngkolasegmenthanon themainsegmentarthereast Genrich t al.,thisssue]. ombinedlipat depth t a rateof 23 _+ mm/yrsatisfieshegeodetic easurements.TheAngkola egmentf the Sumarranaultproducedhefamousarthquakef 1892,duringheestablishmentf thefirstrimaryriangulationetworkn the egion.Differencesinangles easuredust before nd after the earthquakeenablediIler 1895] o calculatehatcoseismicight-lateraldislocationsf at least m hadoccurredlong northwesttrendinginecoincidentith hatportionf the ault racebetween.45ONnd1.2N.These eodeticata, long iththoserom he 1906SanFranciscoarthquakend 1891Mino-Owariarthquakenspired,eid1913]o ormulatehetheoryf elastic ebound Yeatset al., 1997, Chapter ].Prawirodirdjot al. [this ssue] ave eanalyzedheDutchdatand oncludehat hedextral lipwas4.5 + 0.6m. Themosteriousamageeportedn 1892wasalonghe ault n

the valleys of the Gadis and Angkola Rivers, betweenMalintangandLubukRaya volcanoesVisser,1922].2.3.15. Toru segment 1.2N o 2.0N). Major bends nthe fault trace delimit this segmentof the Sumatran ault(Figure4 andPlate ). We define he southernerminuso beat a regional endof 15 at .2N. The topographicigheastof the bendsuggestshat his s a contractional end.The northwesternermination f the Tom segment ccurs t a15 regionalbend n the fault, which s coincidentwith a 2.5-km dilatational tepover.We canbe confident hat hisbendis dilatational ecausehe segmento the northwest oesnotdisplaynet verticaldeformation crosshe fault and he bendcoincides ith theTamtungdepression.Northwest of Sibual-buali volcano, a 30-kin-wide calderanortheast f the fault is truncated y the fault. The otherhalfof the caldera, southwest of the fault, must be concealedbeneath oungvolcanicdeposits. he geomorphic xpressionof the fault in the vicinity of the runcated aldera s unusuallycomplex. Significant omponentsf dip slip occuron faultsthat splay northward rom the main trace into the caldera.The Tom segment has not produced a major historicalearthquake, ut right-lateral lip near the northern nd of thissegment id generate he Ms 6.4 PahaeJahe earthquake f1984.

2.3.16. Renun segment 2.0N o 3.55N). This longestsegment f the Sumarranault traverseshe westernlank ofthe 80-km-long Toba caldera, alleged to be the largestQuaternary aldera n Earth [Chesner t al., 1991 . Much oftheRenunsegmentraverseshe hickpyroclasticlow depositof that 73,000-year-old ruption. The regional xpressionfthis225-km-longsegments linear,except or a dogleg longits northwesternmost0 km, where the segment orms thesouthwesternlank of the Alas Valley graben. This graben,45 km long and 9 km wide, s oneof the argest raben longthe Sumarran fault. West of Lake Toba, the fault consistsofseveral30- to 40-km-longstrands, rranged n echelon,withacross-strikeeparationsf only a kilometeror so. Althoughthe right-steppingatureof the en echelonpattern uggeststhat the fault is experiencinga minor componentoftranstensionn the uppercrust, he stepoversare associatedwith horsts,not graben.The southeasternmostartof the Renunsegment xhibitswell-defined -km offsetof the 73,000-year-old oba Tuff,whichwe haveused o determine 27 mm/yr sliprate or thefault [Sieh et al., 1991; D. Natawidjaja and K. Sieh,manuscriptn preparation,000]. GPS measurementscrossthesouthernortion f thissegmentuggestlip ates f 24 +_I mm/yrbelow -9 km. Across he northern ortionof theRenunsegment, eodeticatesappear o be 26 +_2 mm/yr[Genrich et al., this ssue].The Renun segmentwas the sourceof three majorearthquakesarly n the wentiethentury.Accountsf theseevents reverysparse,owever, nd he imitsof the upturecan only be guessedrom poorlyconstrainedsoseismalcontours. Visser [1922] reports hat shakingduring theFebruary2, 19 6, earthquakeas very strongn theTamtungalley nd hat he adius f strong haking as-200 km. TheJanuary4, 1921, arthquakeada region fseverehakingimilaro hat f theearthquakef 1916.Theradius f shakingor theearthquakef April , 1921,wastwiceas arge Visser, 922].2.3.17.Tripa segment3.2N o 4.4N). Markedirregularitynd curvature, ountainouserrain, nd


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28,308 SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICSspectaculardextral offsets of major rivers characterize his180-kin-long egment Figure 4 and Plate 1). The locationofthe main traceof the fault is well constrained y spectacularoffsetsof the Kuala Tripa and MeureuboRivers. Each ofthesedeeplyentrenchedivers displays clear offsetof-21km (Figures6 and 7 and Tables 2 and 3).The segment's southeastern erminus s the northeasternflank of the extensional las Valley graben. ts northwesternlimit is a 9-km-wide restraining end,which displays outh-side-up aults with a significantcomponent f reverseslip.Onecouldargue hatan appreciable ontractionalog at 4.0Nand a major change n strike at 3.85N ustify dividing hissegment urther.Parallel o and ---15km northeast f the centralportionofthis segment (between 4.0 and 4.25N) is another activestrike-slip fault. This 55-kin-long fault trace is also welldefinedby alignedriver valleys and streamoffsets. Streampatternssuggest hat this fault may convergewith the mainactive race t the northwesternerminus f theTripasegment.However, we could find no clear large-scale eomorphicevidence of this, nor does the l:250,000-scalegeologicmappingsuggest t [Cameron et al., 1983].

An earthquakeon September 19, 1936, occurredalong thesoutheasternmostart of the Tripa segment M., 7.2 [Newcomband McCann, 1987]). A smaller, more recent shock (rob6.0,November 15, 1990) occurred near the middle of thissegment.2.3.18. Aceh segment (4.4N to 5.4N). This 200-kin-long segmentof the Sumarran ault has a smooth sinusoidalshape nd acks major discontinuities r sharpbends Figure4

and Plate 1). The southeasternwo thirds raversemountainouserrainnd rewell xpressedyalignedajorrivercanyonsndstreamffsets.Dextral eparationsf--25and20 km on theGeumpangndWoylaRiverchannelsre.not compellingvidenceor offset,but theyaresimilarnmagnitudeo the size of clearoffsets f the TripaandMeureuboRivers farther southeastFigure 7). Thenorthwesternortionf theAceh egmentraversesregionof low relief and is obscureon l:100,000-scaleerialphotographs.Geomorphic xpressionf the lhult s subtleandstream ffsets ppearo be absenthere.Althoughomepublished aps how he Sumarranaultrunning longhesouthwesternlankof theAcehValleyandcontinuingnto hesea crosshenorthwesternoastCurray t al., 1979; ageet al., 1979], we see no geomorphicevidence of activefaultingwithin 25 km of the coastline.Therefore,we, arenotconvinced hat the fault is active northwestof about5.4N.Geomorphicevidence for inactivity is compatiblewithgeodeticobservationshat strain s accumulating t no morethan a few millimeters per year across he fault [Genrich tal., this issue].2.3.19. Seulimeum segment (5.0N to 5.9N). Thissegment epresentshe principal active trace of the Sumarranfault throughnorthernAceh province Figure 4 andPlate1).The active trace s markedby sharpescarpments nddissectedyoung volcanic deposits on the southwestern lank ofSeulawahAgam volcano. Small tributariesof the SeulimeumRiver are clearly offset a few hundredmeters. Along hecentral part of this segment,young folds appear o be offset-20 km (Figures6 and7 andTables2 and 3).

GSF - Volcano Lakes ./" River

21 23 30319 20 22 24 25 28' 26

I..,ElF_. '.;,:.'.'

'6 " 7 'k s. I 10..:. .... d. kJ.Northern part of Sumatra ..

Selectargeffts SelectmallffL 17Obmedhannelff2u(e able) t b ] (Seeable) i Linesointingohexit.......- 32

Southern part of SumatraFigure 6. Map of small and largegeomorphic ffsets long he Sumarranault. See Tables2 and 3 for moreinformation.The largestoffsets ndicate hat otalslip acrosshe ault s at least20 km.


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NORTH SUMATRA, Andaman Sea 2 -----', /

activeult one 21 offset) Anticline SynclineVleys0 20 100 km smalliverargeiver .,.,.-, rainageivide

centers and cones of active volcanoes

Figure . Two of themost ompellingargegeomorphicffsets long heSumatranault, he21-kmdextral ffsetsof theTripa and Meureubo ivers n northSumatra.The headwatersf the nearbyWoyla River and foldedQuaternaryedimentsear Nalso ppearobeoffset y his mount. hese ffsetsppearo representhe otaldextral ffset incenitialupliftof thispartof theBarisan ountainseveralmillion ears go.

Clearevidence f recentactivity along the southeastern2kmof this segment s absent rom our aerial photos,but weinferhat he fault continueshrough he long, narrowvalleyof heBaroRiver along his reach o an intersection ith theAceh egment t about 5N. Northwest from the coastline,bathymetryCurray et al., 1979; J. Curray, writtencommunication, 999], focal mechanisms (Harvard CMTcatalogue),eomorphicxpression f faultingon Weh Island,and vidence n a seismic eflectionprofile [Peter et al.,1966;Weeks t al., 1967] suggesthat the fault continuesunderwater.It is interesting hat dextral movementalong theSeulimeumegmentasproducedo deflectionf theAcehsegmentt their intersection. It is difficult to imagine howmany ilometers f dextralslip on the Seulimeum egmentcouldhave occurred without at least a broad deflection in theAceh egment.A largeearthquaken 1936 (M 7.1-7.3 [Newcomb ndMcCann,987; oetardjot al., 1985]) everelyamagedhecityof BandaAceh,but the source f theevent s unknown.An earthquaken 1964 (Ms 6.5, NationalEarthquakeInformationenterNEIC)) damagedmengRayamoreseverelyhanBandaAceh. SinceKruengRaya s closer otheSeulimeumegment,he Seulimeumegmentf theSumarranaultmay ave eneratedhis vent.n contrasto

the geomorphicvidenceor recentdextralslip along theSeulimeum egment,Gertrichet al. [this issue]show hatstrain accumulation cross his segment n the early 1990scould be nil.

2.4. Other Related Structures2.4.1. Batee fault. The Batee ault is a major right-lateralstrike-slipault hatdivergesrom heSumarranaultat about4.65N. Between ts intersectionwith the Sumarran ault and

the coastline, the fault traverses the 1000-m-highsouthwesternscarpmentf the Barisanange. Kariget al.[1980] have shown hat this structure ontinuesnto hecontinentalhelfandoffsetsheedgeof the continentalhelf-150 km and heeasterndgeof theouter-arcidge 100 km.One strandof the Batee fault terminates eforereaching henorthernartof Nias sland Plate1). Another trandunsalonghenorthernoast f Niasandappearso offset heinner rench lope ndouter-arcidge Plate1). Except erylocally,heBateeaultdoes otappearo be active n themainland f Sumatra.Although everalarge iverchannelsdisplayextraleflectionsf up o 10km,smalleridgeinesand channels xhibitno offset. We suspect hat tl,seargedeflectionsre, ndeed, extral ffsets, but the ackof clearsmall ffsetsuggestsither oactivityn thepastew ens f


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Table 2. Selected mall OffsetsAlong heSumatran ault FromNorth o South)River/Lake Name Offset, m Comments

a Aceh River 750-1000b Toru River 1700-2100

c Angkola River 1200-1400d AngkolaRiver 1000-1300(Ringkitbranch)e Sianok River 700f Anai River 600g Lake Dipatiampat 500h Musi River 700i Manna River 2400

WerkukRiver (Menjadi,Pisai, 300Rebu branches)

offsetof several treamshat ncised oungvolcanicdepositsn hesouthwestlank of Seulawah gamvolcanoexcellent ffsetof several treams eeply ut nto he 73,000year old Toba Tuffoffsets of a few streamson the northeast lank of the SorikMerapivolcanooffsets f several ributaries f the AngkolaRiverexcellentffsetsf severalrossingsf theSianok iver, eeplyincisednto he 60,000 yearold ManinjauTuffoffsetsf severalhannelsn hesouthwestlankof Merapivolcanooffset of north sidewall of the caldera lakeexcellentoffsetsof tributaries o the Musi River, on thesouthwest flank of Kaba volcanooffsetof Air Kiri andAir Kanan Plate2) whichdrainsan erodedvolcanic edificeoffsets f threechannelshataredeeply ncisednto he hick,Quaternary anauTuff

thousandsf years r activity t a ratemuchower han longthe Sumatranault. This nterpretationonflictswith he 12 +_5 mm/yr estimateof dextralslip rate of BeIlier and Sebrier[1995]. We questionhe validity of their approach, hichusesan empirical elationship f channelengthand age toderivean age or a channel.This age s thendivided nto themeasured offset to determine a rate.2.4.2. Tom fold and thrust belt. Between about 1.0 and1.5N lies a geomorphologicallyemarkable et of activefoldsand aults hat strike oughlyparallel o the Sumatranfault but lie 15 to 40 km farther southwestPlate ). The

principal manifestations f this fold-and-thrust elt are anorthweststriking anticline and sync inc. The synclineunderlies 25-by-10-kin wamp, nd heanticline ppearssa 30-by-15 km fold in Mio-Pliocene sediment. The GadisRiverand ts tributaries eander crosshesyncline nd hentraverse the anticline as an antecedent stream.In addition, several smaller northweststriking reversefaults appear o break the anticline (Plate 1). The anticlinealso s cutby smallnorthstriking trike-slipaults. However,these aultsare so smalland closelyspacedhat theydo notappearon Plate 1.

3. Discussion,nterpretations, and SpeculationsIn this paper, we have defined the geometry ndgeomorphologyf the Sumatranault. Therearenowseveralquestionshat these efinements llow us to address. heseinclude he implications f the fault's historicbehaviorndgeometry or the evaluation f future seismichazard ndquestionsbout he otal' ffsetacrosshe Sumatranault ndits role n obliqueconvergenceuring he pastmanymillionsof years. Other questionsconcern he geometric ndkinematic relationshipof the Sumatran fault to the

neighboring ubduction one and the relationship f arcvolcanismo strike-slipaulting. We address achof thesefour questionsn turn,below.3.1. Historical and Future Seismicity

In theprecedingiscussion,ehavedescribedery rieflywhat s knownabout argeearthquakeslong he Sumamnfault. Even thesehighlyabbreviatedccountsuggesthatgeometric egmentationnfluences eismic upture f theSumatran fault. In contrast to the San Andreas fault inCalifornia [Lawsonet al., 1908; Sieh, 1978], the Sumam

Table 3. Proposed argeOffsetsAcrosshe Sumarran aultFeatures Offset, m Quality Description

Quaternaryolds 20 fairlygoodMeureubo River 21 excellentTripa River 21 excellentSingkarak raben 20-22 N/ASeblat iver 17 goodKetahun iver 23 good

offsetof a few fold axeswhich deformedPliocene,Miocene, ndOligocene tratadextral ffset s clearlyndicated y thedeflection f thetrunk channeldextral ffset s clearlyndicated y thedeflection f thetrunk channelbased nan nterpretationf thegraben pening(Figure7)clearly hown y a sharp eflection f themainchannelclearly hown y a sharp eflection f themainchannel


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SIEH AND NATAWIDJAJA:SUMATRANFAULT NEOTECTONICS 28,313

^ B

0 etLake

o

o

',. Smgkarak ,'X

13 km 18 km 23 kmoffset offset offset

Figure . Hypotheticalvolutionf theSingkarakrabennd oundingormalaults howingow he ength f thenormal-obliqueaultsmight epresenthe totaloffseton theSianok ndSumani egments.rofileE showshecurrent eometry f the graben.

founderingrust but hypothesize hat the length of thefoundedngegionhas remained nchangedt --23 km sincethe aultsnitiated. This would mply that the lengthof thefounderingegionhas no bearingon the amountof totaloffset.We favorour hypothesisecauset is consistent ithother vidence for---20 km of total offset.3.2.3. Total offset. Why are the largestgeomorphicoffsetso greater han ---20km? Is it possiblehat theserepresentotalstrike-slip ffsetalong he Sumatranault? Oris here limit o thesizeof geomorphicffsetselatedo thesusceptibilityf landforms o erosionand burial? We willgiveeasonselowwhy20 km mightwell be the otaloffsetacrosshe fault, but we will also show that a total offset as

greatas -,-100 km can not be ruled out at this time.Indirectrgumentsor offsetmuchgreaterhan20 km areas ollows:One mightexpect hat he great ength f theSumarranault equiresubstantiallyreaterotaloffsetshanacoupleensof kilometers. t is certainly rue hatmanyverylongtrike-slipaults, uch s heAlpineNewZealand)ndSanAndreasCalifornia) nd manyoceanicidge-ridgetransformaultsdisplaygeologic ffsetsof hundreds fkilometersYeatstal., 1997, hapter].But his s nota strong rgumentor largeoffset,or tworeasons.irst, many other very long strike-slip aultshave

accrued nly a few tensof kilometers f offset. An exampleis Turkey's 1500-km-longNorth Anatolian ault, whichhasatotal offsetof only 85 km [Annijo et al., 1999]. Second, n astrict sense, the Sumatra fault is not one fault; rather, it is afault zone that consistsof many segments,which range inlength rom 60 to 220 km. Many strike-slip aults withlengths s shortas thesehave accrued nly a few kilometersto a few tens of kilometers of offset (for example, the SanJacinto fault in California is a zone with 24 km of dextraloffset that consistsof many disjunct segments, ens ofkilometers ong).Anotherreason o suspecthat total slip would be >20 kmis the transformation f the Sumarran ault into the spreadingcentersof the Andaman Sea [Curray et al., 1979]. Thissuggestshatoffsetcouldequal he 460 km of spreadinghathasoccurred here n the past 10 Myr. But we will seebelowthat much of this offset has been carried by faults that splayinto the forearc, west of the Sumarran ault zone.Regardless of plausible analogues and the fault'sconnection o the spreadingcentersof the Andaman Sea,direct geologicevidence or total oftket across he Sumatranfault is sparse nd equivocal. McCarthy and Elders [1997]suggest 50 km of dextralslip,on the basisof similaritiesnisolated utcrops f crystalline asement n both sidesof the


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SIEHANDNATAWIDJAJA:UMATRAN AULTNEOTECTONICS 28,315plausiblemountsfoffsetlonghe umatranault.nstead,theycceptedparsend quivocalvidenceor-100 moftotalffsetlonghe aultand ttemptedo demonstratehatthis ffsets consistentith reasonablestimatesf arc-paralleltretching.heydid not attempt rigorousassessmentf the mplicationsf the forearc eometryntotalffsetlongheSumatranault.LassaI t al. [1989] also attempted o quantify thestretchingf the forearc egionsouthof the Sunda trait.Theyhowhree eismiceflectioninesroma 80x 50 kmarean andon he lanksof thegraben t thewestern ntranceto hestrait. They annotate hesewith five stratigraphicboundaries,hose geometry and ages they defend byreferenceo unpublishedwork. They claim (withoutdiscussionr argument) hat an allegedly upper Miocenestratalackageontainseefdepositsan ndicatorf shallowwater). heyassume n uppermost iocene 5 Ma) age ortheeefs nd henuse he depthof this packetof sedimentocalculatehe "stretching actor" since 5 Ma. This factor isdescribedy Le Pichon and Sibuet [1981], who apply astretchingodelof McKenzie 1978] to passive ontinentalmargins.heuseof thismodel eems holly nappropriateoussince he parameters eeded o calculatestretching remostlynknownor the SundaStrait. LassaIet al [1989],concludey asserting, ithoutany discussionr calculation,thathisstretchingactor "probably xplains he openingofthe trait ince Ma ago,with a maximumdisplacementf 50to70 km along he centralSumatra ault." Their paper s, infact, osparse n dataand documentationhat ts conclusionsare eft undefended.

We propose simple measureof extension cross hegrabenf the Sundasegment,whichestablishes minimummountof dextralslip on the Sumatran ault. If we assumethathe aultsboundinghe graben ip 60, we cancalculatethe horizontal extension across the faults in the direction ofthe Sumatran fault. We calculate a 6.5-km lower bound onextensionf the grabenparallel o the Sumatranault if weassumehat the 2-kin heightof the scarp epresentserticalthrow crosshe faults. This assumptions manifestly nunderestimatef total vertical throw, sincehundreds f metersof deposits ithin the graben are clear on the seismicreflectionross ections.Thus 6.5 km is probably everalkilometersess than the actual amount of extension across heSundaraben.Severalmorekilometers f dextral lip couldprobablylsobe addedo totalslipalong heSumarranault fthegeometrynd imingof faulting arthereastwithin hestrait and buffed beneath >2000 m of volcanic debris(summarizedy Huchon ndLe Pichon 1984])wereknownbetter.n summary,xtensionf theSunda raben nd illedgrabenarther ast s consistentith dextral lipof the orderof10km along heSumatranault. However,moredetailedstratigraphicndstructuralatawill be necessaryo calculateextensioncrosshegrabenmoreprecisely.Letusnowattempt quantitativenalysisf stretchingftheorearcegion,o providemaximumimit odextralliponheSumatranaultduringhepastewmillion ears.Thisanalysisimplycarries he geometrical bservationsfHuchonndLe Pichon1984] o their ogical onclusion.Fromimpleolumetricalancingf the orearc edge, ecalculate00 km of stretchingf the orearc arallelo theSumatranault.Aswe discussedn section .3.1. (Figure ), the forearcbasinnd uter-arcidge reattenuatedn the egionf the

Sunda trait. These wo eatures isappearear hestrait, ndthedeformationrontbows andward.FollowingHuchon ndLe Pichon 1984],we interprethis as an indication f fault-parallelstretching nd fault-normal eckingof the forearcregion. Extensiveseismic eflectionstudiesand structural ndstratigraphicnformation rom the forearc and outer-arcregions orthof theequator how hat hepaired orearc asinandouter-arcidgedevelopedhroughouthe Miocene pochbut grew particularly apidly during the Plioceneepoch[Samuel t aI., 1997;Samuel nd Harbury, 1996]. Thusweinfer that the deformation of these features has occurredwithin ust thepast ew millionyears.We beginwith an estimate f the boundaries f the volumethat has been stretched.The concavityof the deformationfront and merging f the outer-arc idge and forearcbasinsuggesthat the current engthof the deformed egion,L, is~356 km (Figure 9). Hypocentral epthson or near thesubductionnterface onstrainhe northeast ippingbaseofthe deformed orearcwedge. The deformationront and thebaseof the continental lopedefine he seaward nd andwardboundaries f the deformed egion.Using these boundaries,we calculate hat the deformedcrustal edgeas volume, of about1.01 106 m . Weassumehat this volume s equal o the original,unreformedvolumeVo. By furtherassuminghat he cross-sectionalreasof the current southeasternnd northwestern dges of thedeformed region, A and B, have not changed sincedeformation egan,we can calculate he originalarc-parallellengthof the deformed egion. A and B are currently 870and4970 km2:

Lo = 2*V / (A+B) =258km. (1)The total amountof northwest-southeasttretchings:

A L = L - Lo= 356 km - 258 km = 98 km. (2)Since he Sumatran ault forms the northeastern oundaryofthe forearcsliverblock, we are tempted o concludehat thisestimateof stretching f the forearc equals he amountofright-laterallipalong he Sumatranault. However,n fact,this 100 km is only an upperboundon offsetof the past ewmillion yearssince here s anotherstructuren the forearcregion hat could also have accommodatedomeof thisstretching. The Mentawai fault [Diament et al., 1992],located between the forearc basin and the outer-arc ridge(Figures and8 andPlate 1), couldalsohaveaccommodatedsome of this motion. The linearity of this large structuresuggestssignificantomponentf strike-slip otion, ut hemagnitudeof strike-slipmotion, if any, has not beendocumented.3.2.5. Plausible evolution of dextral slip along theSumatranmargin. Although nowledge f the geologyofthe Sumatran fault and other faults of the Sumatran faultsystems incomplete,noughnformationxistso attemptreconstructionf the system's eformationalistory ver hepastewmillion earsFigure 0). Theprincipalonstraintson this historyare: (1) the magnitude nd timingof thediscrepancyetween preadingn the Andaman ea andstretchingear heSunda trait;2) a range f plausibleotaloffsets or the Sumatran ault; (3) the timing, style, andmagnitudef deformationn theSumatr.orearcegion;nd(4) a southeastwardecreasen thecurrentates f slipalongthe Sumatran ault. These constraintssuggest hat the


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-., , XX deformationrontUMA'I RA outer-arcidgexis. \ Javaea,, -;..-.._ ' . ___.i.__orearcasinxis ' "" Jakaru % bathymetryontour?,,.,,,y>--.-- . -.---,,-, (in meters),,r -.: g " depression/basin' * VA'---- 5 ... K X'Xa;X,, J ' activeolcano............. O-kinsoba/h

[ ..... - 56 km - - -

INDIAN OCEAN

Figure 9. Stretchingof the forearc sliver plate near the Sunda Strait, which appears o have thinned the forearcwedgeperpendicularo the deformation ront. By volumetricbalancing,we calculate hat ---I00 km of stretching fthe forearc silver hasoccurredparallel o the Sumatran ault since ormationof the outer-arc idge and forearcbasin.This would be a maximumvalue for northwestwardranslation f the part of the torearcsilver plate that is southofthe equator.

Sumarran ault systemhas evolved significantly n the pastseveral million years and that the currentconfiguration fdeformation is not representative of pre-Quaternarydeformation.One hundred kilometers of motion near the Sunda Strait

contrastsmarkedlywith the 460 km of openingsuggested yCurray et al. [1979] for the Andaman spreadingcenters(Figure 1). The contrast disappears, f one comparesAndamanextensionand Sundanoffset for similar periodsoftime. Only about 118 km of Andamanextensionmay haveaccumulated in the past 3 Myr (J. Curray, writtencommunication, 999). This does not differ greatly fi'om he100 km of stretchingof the forearc near the SundaStrait forabout he sameperiodof time (i.e. since he rapid rise of theSumatranouter-arc idge in the early Pliocene). Hence the

discrepancybetween deformation in the Andaman sea andSundaStrait during the past 3 Myr may be very smallornonexistent.Nonetheless,he current ate of slip on the Sumatranaultappears o diminishsignificantly rom northwest o southeast.Although new geodetic evidence suggests hat there s nosignificantdecreasebetween about lS and 2N [Gertrich tal., this ssue],geologicslip ratesacross his section uggestmuch argerdecreasen rate, from 27 mm/yr (near2.2N)to11 mm/yr (near 0.4S) [Sieh et al., 1991, 1994; D.Natawidjaja nd K. Sieh, manuscriptn preparation,000].Bellier and Sebrier's 1995] estimations f slip ratealonghefault, based poncorrelationsf streamengthwith age, lso

decrease from northwest to southeast.If the otaloffsetalong heSumatranault s only 20 km

Figure10. (opposite) plausiblebutnonunique)istory f deformationlong heobliquely onvergentumatranplatemargin,baseduponour work and consistent ith GPS results nd the timingof deformationn the forearcregion. (a) By about 4 Ma, the outer-arc ridge has formed. The former deformation ront and the Mentawaihomocline rovide set of referenceeaturesor assessingaterdeformations. rom4 to 2 Ma, partitioningfobliqueplate convergence ccursonly north of the equator. Dextral-slip aultson the northeastlank of the forearcsliverplate parallel he trench n northernSumatrabut swingsouthand disarticulatehe forearcbasinand outer-arcridge north of the equator. (b) Slip partitioning egins outhof the equatorabout2 Ma, with the creationof theMentawai ndSumarranaults.Transtensionontinuesn the orearc orthof theequator. c) In perhapsust hepast 100 yr, the Mentawai ault hasbecome nactive, nd the rate of slip on the Sumatranault northof 2N hasmore han doubled. This difference n slip rate may be accommodatedy a new zoneof transtensionetween heSumarranault and he deformationront n the orearc ndouter-arcegions.


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A

r m mrZ' Ma %,........_... -

(76m) ...... 40mnVyr (57 in)' '47v ' 42myr 7 m y 48mmvr

/ ..... _l . / % / z , I %

mr mr mr , -,.._ N.. Zone f transtension, 37mm/yr ' ."'"'"%'-'"-'--- -,

o 7Mapresent

(25k)

25 10 10

mr mr mr200 0 200 400 600 800 1000 ometers


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28,318 SIEH AND NATAW DJAJA: SUMATRAN FAULT NEOTECTONICS

andslip ateshavebeenconstant,hen henorthern artof thefault zonewouldbe less hana millionyearsold. Southof theEquatorial Bifurcation, where late Quaternary slip ratesappearo be --10 mrn/yr, 0 km of slipmighthaveaccruedn--2 Myr.Our calculationof-100 km of fault-parallel tretching fthe forearcnear he SundaStraitsuggestshateither he otaloffsetalong he Sumatranault is much arger han20 km orthat another structure in the Sumatran fault system hasaccommodated80 km of the stretching.The only plausibleothercandidateor dextralslip wouldbe the Mentawai ault,well constrained from seismic reflection data to run betweenthe outer-arc idge and the forearc basin [Diament et al.,1992]. The linearity of the feature suggestshat it isprincipallya strike-slip eature. Diamentet al. [1992] alsoargue hat the structure f the fault zone ndicates hat itssenses primarilystrike-slip. n our opinion, he structuralargument s a less compelling one becausewe are notconvinced that the Mentawai fault zone exhibits the "flower"structurecharacteristic f strike-slip aulting. In fact, theposition f the fault, on the northeasternlank of theouter-arcridge, s consistent ith the fault beinga backthrust,longwhich he outer-arc idgehasrisen. The existence f a largehomoclinen the sameposition elative o the forearcbasinandouter-arcidge northof the equator Karig et al., 1980](Plate 1) supports his interpretation. o it is with somereluctancehat, in the evolutionarymodelbelow, we use heMentawai ault as a strike-slip lementof the Sumarranaultsystem.A final constraint on the evolution of the Sumatran faultsystem s the Mio-Pliocenehistory of the forearc and outer-arc regions. The Andamanspreading enterswere activelyspreading t ~40 mm/yr during this period,yet we have noevidence of contemporaneous extral deformationof theforearcsliver plate southof the equator. How and where, nPliocene and late Miocene time (about 2 to 10 Ma), was thedextral componentof oblique convergence ccommodated?Matson and Moore [1992] suggest that some of thisdiscrepancycan be accommodatedby the dextral-normalfaults of the forearcregionnear Nias Island (Figure2 andPlate 1). We consider his possibilitybelow.Stratigraphic nd structural tudies y Samuelet al. [1997]and Samueland Harbury [1996] show that broadening nduplift of the outer-arcridge occurredearly in the Plioceneepoch hroughouthe Sumatranorearc egion. This is criticalto reconstructing eformationof the forearc sliver platebecause he early Pliocene growth of the outer-arcridgeproduced n elongate eature hat has beendeformed n thesubsequenteveralmillion years. The ridge s clear n thebathymetryf Plate 1. Southof about1S, t is regular nd60to 80 km wide. Its northeastern oundary s the PlioceneMentawaihomoclinal lexure. On the southwesthe ridge sbounded y a plateau hat sits at a depthof-2400 m. Wespeculate hat this plateau was formerly a part of theAustralian late and that ts northeasterndge s the formerdeformation front of the subduction zone. Similar features arealso presentbetween about 1.5N and 3N, near SimeulueIsland.

Between1.5Nand 2S, he outer-arc idge, he homocline,and the ancientdeformationront and plateauare markedlydisarticulated.Karig et al. [ 980] observedhat the Pliocenehomocline n the eastsideof Nias is dextral yoffset- 100 kmby two strands f the Batee ault. We infer from bathymetry

thathe trandf heBateeault orthwestfNias ffsetsheancienteformationront 50km, rom orthwestfNiasoa positionest f Nias Plate ). Fartherouthn hennertrenchslope,betweenNias and Siberut slands,hedeformationrontmay eoffset yaboutnadditional0 malonganother orthstriking ault.Dextralffsetf he asterndgef he orearcasinythe Batee ault s -150 km [Kariget al., 1980]. Frompaleonto ogicallyonstrainedeismic tratigraphy,atsonandMoore 1992]showhat heBatee aultwasactiveromthe ateMiocenehroughhePleistocenepochs.wentyothirty ilometersf dextral lipappearo haveoccurredn henearbySingkel ault n the late Miocene poch. Thust isreasonableo suggesthat he first few tensof kilometersfthe 150-kmdextraloffseton the northern ortions f theBatee aultaccruedn the ateMiocene.However,hebulk fthe slip must be late Plioceneand youngerbecausehePliocenehomoclineof Nias Island s offset -100 kin. Thisoffsetmusthaveaccrued verat least1.5 Myr, since shorterdurationwould require atesof dextralslip in excess f therateof relativeplatemotion.Plate1 alsoshows disruptionf the outer-arcidge ndinner trenchslopesouthof Nias Island, at the Pini basin ndbetween Tanabala and Siberut Islands. The Pini basinexperiencedapid subsidenceeginning bout4 Ma. Thissubsidences probably ontemporaneousith activity fnorth trikingaults hatboundhebasin Matson ndMoore,1992]andwithminornorth triking extral-slipaults nNias[Samuel ndHarbury,1996]. A disruptionn the nnerrenchslope arthersouth,alongstrikeof the Pini basin,mayrepresenta 40- to 50-km dextral offset of the sameancientdeformation front mentioned above.Figures 10a-10c depict a plausibleevolutionof theSumatranaultandother tructuresf theplateboundaryhatis consistentwith available geologic, geodetic,and

seismographicata. Variationsof this historyare als0possible;our principal intention is to show that the faultsystemevolvedsignificantlyn the past few millionyears.The main characteristicsf this speculative istory reasfollows: 1) thecurrent 5 mm/yrdifferencen Sumarranaultslip atenorthandsouth f theequators veryyoungperhapsonlyl00,000 years old), and (2) active normal- and dextral-slip (transtensional)aulting in the forearc and outerarcbetween S and 2N is an ancient and perhaps urrent)analogueo the stretchingt the southern ndof theSumatranfault.Figure10a shows he geometry f the regionat aboutMa. Justprior to this time, relief between he forearc asinand the outer-arc ridge increasedgreatly across hehomoclinal old between he forearc basin and outer-arc idge[Karig et al., 1980; Samuelet al., 1997;Samueland Hatbury,1996]. We speculatehat as the outer-arc idge grew, hesubductioneformationront umped outhwestwardo itspresentocation,rom a deformationront still visiblen thebathymetry,losero theouter-arcidge Plate1). From to2 Ma, dextral lipon theAcehsegmentnd heBateeaultoccurredt 37 mrn/yr, nd hehomocline,uter-arcidge,ndinner rench lopewereoffset37 km by a curved outhernextensionf theBateeault,off thenorth oast f Nias sland,and37 km moreacrosshe Pini basin. This is consistentiththe stratigraphyf Matson nd Moore 1992]. Severalkilometersf arc-parallellongationf Nias Island long

north triking extral-slipaults ndconjugateinistra -slip


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28,3

'96N

Equator

2S

4N

2N

N

6S

Plate. Geometricnd tructuraletailsf heSumatranault,he orearcasin,uter-arcidge,nd olcanicrc,suggestinghe ivisionf he umatranlate arginnto orthern,entral,nd outhernomains.he implestouterrc,orearc,nd umatranault eometriesren he outhernomain.he oincidencef his tructuraldomainithhe ourceegionf the iantMw ) subductonarthquakef 1833 uggestshat eometricalsimplicityncouragesargeuptures.he entralomainppearsohaveeenhe ourceegionf he reatMw8.4) ubductionarthquakef 1861.Fragmentationf he entralomainppearsohaveeenausedysubductionf henvestigatorractureoneuringhe astMyr.Theocusf mpingementf heractureoneon hedeformationrontwas alculatedyassuminghe urrentelativelatemotionectornd he orearcdeformationistoryfFigure0.Contoursn ed reheop f he enioff-Wadatione. athymetricontourinterval is 200 m.


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28,320 SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICSlhultsalsooccurred uring his period Samuel nd Harbury,1996]. Subruction outhof the equatorwas parallel o therelative plate motion vector and highly oblique to thedeformationront. Subructionnorth of the equatorwasmostlyor wholly dip slip becausemostor all of the dextralcomponent f plate motion was occurring long the Batee-Aceh fault.

About2 Ma (Figure 10b), both the Mentawai ault and heSumatran ault formed. From 2 Ma to 100 ka, they carried-40 mm/yrof the dextralcomponent f oblique onvergencesouth of the equator, and the subruction interfaceaccommodatednly the dip-slip component. North of theequator, 0 mm/yr of dextralslip was accommodatedy theSumatranault (10 mm/yr)andAceh-Bateeault (30 mm/yr).Figure 10c depicts our suggestion or the currentneotectonic artitioningof deformation. The Aceh-Bateefault is no longer active or is only minimally so. TheSumatran ault is slipping~15 mm/yr fasternorth of about2N than south. The massbalanceproblemcausedby thisdiscrepancys being akenup by a nascent elt of deformationthat crosses the outer-arc ridge at the equator. Thisdeformation elt is superjacento Fauzi et al. s [1996] swathof exceptionallyhigh seismic activity in the down goingoceanic lab. It alsoencompasseshe activeTom foldsof themainlandcoast, wo young aults on and southof Nias andnorth-south raben hat bathymetry uggestmay existon theinner trenchslope (Plate 1). Figure 10c is consistent ithrecentmeasurementsf geologicallymeasured umatranaultslip ratesbut is inconsistent ith the ratesof geodetic trainmeasured y GPS southof the equator.If the Sumatranault s carryingonly ~ 10 mm/yrof dextralslip southof the equator Sieh eta ., 1994; Bellier et al.,1999], the remainderof the dextralcomponent f slip mustbetaken up along either the subductionnterfaceor by a faultwithin the forearc sliver. The GPS data show no sharpgradients n shear in the forearc region, so the remainingdextralcomponents probablyaccommodatedy slip on thesubduction nterface [McCaffrey et al., this issue]. Thisportionof the dextralcomponent, , wouldbe -27 mm/yr x =58 mm/yr*sin 41 - 10 mm/yr, where 58 mm/yr is themagnitudeof relative plate motion and 41 is the anglebetween he plate motionvectorand the trenchnormaland 10mm/yr s the slip rate on the Sumatranault). Slip vectorsorearthquakes n the subduction nterfacedeviate from thetrench normal by -20 on average. These suggest hat thedextralcomponent n the interfacewould be a bit less hanourmodelpredicts, nly -16 mrn/yr.The historydepictedn Figure 10 is consistent ith thetimingof activityon faultsbothoffshore ndonshore ias[Kariget al., 1980;Matson ndMoore,1992;SamuelndHarbury, 996]. It also ncorporatesurobservationhat heBatee ault is not currentlyactive alongmostof its exposedtrace but retains clear evidence of 5-km dextral offsetsof afew of the argest hannelshatcrosst (Plate ). Restorationof ~80 km of slip on the aultsbetweenS and2N n theoffshore egioneliminateshe dimple n the subductiondeformation front west of Nias and Simeulue, just asrestorationf-80 km of slip on the combined umatranndMentawai aults nearlyeliminateshe dimplewest of theSundaStrait. Thuswe suggesthat the concavitiesf thedeformation ront west of Nias and west of the SundaStraitare features nherited rom Plio-Pleistocene extra strike-slipmotion n the forearcregion.

3.3. Tectonic odelof theSumatran lateMarginTranstensionaleckingf the orearcegion etweenOSand Nduringhepast Myr hashada profoundffectnall of themajor lementsf theplatemarginhere.The nnertrench lope,outer-arcidge,and forearcbasinhave eenfragmented y this process. Even the shapes f thesubductionnterface, he active volcanicarc, and theSumatranaultappearo havebeenaffected. n fact,wecandivide the Sumatranplate boundary nto three tectonicdomains,ased pon heir elationshipo thisPlio-PleistocenetranstensionPlate 5). The southerndomain,whichwesuggestasbeenpartof the forearc liverplateonly or hepast Myr, s themost imple eometricallynd tmcturally.Thecentralomain,hich omprisesll the ranstensionallyfragmented ieces, s the mostcomplex.Thesouthernomainas he ollowingharacteristics:1)the Sumatran ault displays a right-stepping n echelonpattern ndcourses bove he 100- to 135-km sobathsf thesubduction interface, (2) the locus of volcanism spredominantly ortheast f or near the fault, (3) the forearc

basin s remarkablyimple, 2 km deepandunbrokenymajor aults, 4) theouter-arcidge s relativelynarrow,ormsa singleantiformal igh, and s geometrically imple,5) theMentawai ault and homocline,which separatehe basin ndridge, are unbroken nd relativelystraight, nd (6) the nnertrenchslope s relativelyuniformand possessesprominentplateauabouthalf way between he active deformationrontand the outer-arc idge. The sourceof the giant Mw9)subduction arthquake f 1833 was the subductionnterfacebeneathmuch of this domain [Newcomband McCann, 1987;Zachariasenet al., 1999]. Strains measuredby GPS in theearly o mid-1990s how hat he outer-arcslands removingparallel to the relative plate motion vector and that thesubductionnterfacebeneath he southern omain s currentlyfully locked Prawirodirdjoet al., 1997;McCaffreyet aI., thisissue]. The Sumatran ault appears o be slippingat a rateofabout 10 mm/yr in the Southerndomain [Sieh et al., 1991,1994; Belllet et al., 1999].

The northern omains characterizedy theseeatures:1)a geometricallyrregularSumatran ault, with bothreleasingand restraining ends,which residesabove he 125- to 140-km subductionsobaths, 2) a volcanicarc on and northof theSumatranault, (3) a 1- to 2-km-deeporearcbasin, 4) a verybroad, structurally nd bathymetrically omplexouter-areridge, (5) a homocline long ts southernmostew hundredkilometers that is similar to the Mentawai structure of thesouthern omain,and 6) a very narrow nner rench lope.The centraldomain s distinguishedy these eatures:1)a350-km-long ectionof the Sumatranault that is markedlydiscordantwith the subductionsobaths, 2) a volcanic rcthat cuts dramaticallyacross he Sumatran ault, (3)atopographicallyhallow 0.2-0.6 km deep) orearc asin,which has been fragmentednto severalblocksduringoblique-normalaulting, 4) a fragmenteduterarc, 5)afragmentedomoclineetweenheouter-arcidge nd orearcbasin,nd 6) a fragmentednnerrenchlope. hegiantMw8.5) subductionarthquakef 1861andnumeroustherargehistoricubductionarthquakesriginatedithinhis omain[NewcombndMcCann, 987]. Strains easuredyGPSntheearly o mid-1990sndicatehat hehanging allblockacrosshecentral omains currently oving arallelo hesubductioneformationront Prawirodirdjot al., 1997;


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SIEHAND NATAWIDJAJA:UMATRAN AULTNEOTECTONICS 28,321McCaffreyt al., this ssue]. hegeologicateof slipof theSumatranault increasesmarkedly rom southeastonorthwestcrosshecentral omain,rom 11 mm/yr o -27rnm/yrSiehtal., 1991.We suspect that transtensional ragmentationhasdominatedhe central domain because he Investigatorfractureonehasbeensubductingeneathhecentral omainforhe ast everal illion ears. ts ocus f mpingementnthe eformationront has migratedrom the northerno thesouthernargin f thecentral omain uring hepast5 Myr(Plate). Thismaybesignificantecauseault ctivityn thehangingallblock f the orearcegion ppearsohave eenrestricteduringhisperiod o thecentral omainFigure10).Furthermore,he orientations f faults in the centraldomainare redominantlyorth-south,arallelo the opographicndstructuralrainof the underlying nvestigatorracture one.Wehypothesizehereforehat he topographiceterogeneityof he nvestigatorracturezone beneath he centraldomainhased to disruption f the forearcand outer-arcegions.Currently,he Investigator racturezone is also associatedwith bandof intense eismicity ithin he downgoingslabin themiddle of the central domain (Plate 5) [Fauzi et al.,1996] ndan abrupt hange n the azimuthof GPS vectors nthe uter-arcidge Prawirodirdjo t al., 1997,McCaffrey tal., his ssue].Thesubductionnterface urves roadlyacrosshe CentraldomainPlate5) [Fauzi et al., 1996]. The closeassociation fthis curve with the other elements of the central domainsuggestsauseand effect or at leasta shared ause. Couldflexuref thedowngoinglabhavebeenproducedy neckingof the hangingwall block? Or did deformationwithin thedowngoinglab lead to transtensionn the forearc sliverplate?We suggesthe former.The existenceof the 1500-km-wide boundarybetweenIndian ndAustralian latesoffshorewesternSumatra nd heAndamanslands ives eason o suspecthat he downgoingslab est f the nvestigatorracture one s deforming. hisbroad egion of deformationabuts all of the central andnorthernomains. Gordon et al. [1990] calculate hat he twooceaniclates reconverging orth-southt an angular ateof0.3/Myrbout poleof rotationn the centralndianOcean.At the Sumatran deformation front this translates into anominal3-kmnorth-southhorteningf'thedowngoinglabin the past 3 Myr. The actual nature of lithosphericdeformationestof the deformationront s quiteuncertain,however. implenorth-southucklings unlikely. Focalmechanismsndstructurendicate predominancef north-southeft-lateral lip on north-southaults [Depluset al.,1998]. To accommodate orth-southcontraction, hesestructuresouldneed o be rotating lockwise, omino-like,to enable astward xtrusion f lithosphereGordon t aI.,1990]. hepreciseociof such eformations unknown,ndsots mpact n heoverridingentral ndnorthernomainsshardoassess.onetheless,t isplausiblehat hecontrastnnature f the southern nd northern angingwall domainscould ave arisen,at least n part, from subductionfdeformingceanicithosphereeneathhenorthernomain.It ishardo magine,owever,owdextratranstensionnnorthtrikingaultswithin hecentral omain ould erelatedtosinistrallipandclockwiseotation nnorth trikingaultsin thesubjacentubductingithosphere,nlessastwardextrusionf theoceanicithosphereas ed onorthwestwardextrusionf the orearc liver late, splate ollisionas oneinTurkeynd ibet.

A more logical propositionmay be that transtensionalneckingof the central domain has led to bendingof thesubductinglab Trench-orthogonalhinningof the forearcappears to have drawn the deformation front and trenchnortheastward,ens of kilometerscloser to the mainlandcoast.If this process ad not also drawn the deeperpartsof thesubductinglabnortheastward,he dip of the interfacen theforearc nd outerarc wouldbe steeperhan n the southerndomain. The isobaths how he contrary, hat the subductionzone beneath he centraldomainhas a very similar cross-sectional rofile to that beneath he southern omain. Onetest of this hypothesis ould be to determine f the activevolcanic rc n thecentraldomains substantiallyortheastfthe ateMiocene ndPliocene rc. If so, t wouldsuggesthatthesubductionsobathsavemovednortheastwardn the pastfew million years.3.4. Relationship f the SumatranFaultto the Modern Volcanic Arc

Many havenoted heproximity f the Sumatranault o thevolcanic rc andhavesuggestedhat it formed herebecauseof the effectof magmatismn the ithospheree.g.,Fauzietal., 1996; Tikoff, 1998]. Sumatra aside for the moment,however, most trench-parallel trike-slip faults are notcoincident with their volcanic arcs. The Median TectonicLine (Japan)doesnot have an associated rc; the Denali fault(Alaska) lies much farther from the trench than the Alaskanarc volcanoes; the Atacama fault (Chile) lies between thetrenchand volcanicarc; and the Philippine ault is tens ofkilometersrom the major Philippinearc volcanoesYeatsetal., 1997]. Furthermore, ostvolcanicarcsalongobliquelyconvergentmarginsdo not sport arge strike-slip aults. Thisgeneralack of associationuggestshat the alignment f theSumatran olcanic rc and the Sumatranault is purelyacoincidence. n fact, McCaffreyet al. [this ssue]have usedfinite elementmodelingof stresses cross he obliquelyconvergent umatran late boundary o show hat formationof the trench-parallel umatranault did not require hepresencef themagmatic rc. Nonetheless,ikoff[1998]hassuggestedhat faults suchas the Sumatran ault form abovethe locus of greateststrain gradient n the lower crust ormantle,occasioned y the magmatismof the volcanicarc.BetIier and $ebrier [1994] have claimed that numeroussmalland large volcanic cones and calderasoccur at both currentandancient eleasing tepovers long he Sumarranault.We can test directly whether or not magmatismhasinfluencedhe ocationof the fault or, conversely, hetherornot faulting has influenced the location of volcanismandmagmatism. Plate 1 allows us to search or a relationshipbetween the volcanic arc and the Sumatran fault, since itdisplays ot only the mostprominent racesof the Sumatranfault but also the youngestvolcanoes. We mapped hesevolcanic eatures sing he samesourceswe used o map hefault (Figure 2). We limited our mapping o those eaturesthathavesufferedminimalerosion, incehighly eroded,oldervolcanic constructs are harder to recognizegeomorphologicallyndmappingwouldhave equired moresubstantialffort. The featureswe mappedexhibitvery littleerosional modification of their constructional landforms.Many have been active historically. Those that have beendated adiometricallyre typically


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SIEH AND NATAWIDJAJA: SUMATRAN FAULT NEOTECTONICS 28,323volcanicenters. emay ttempthis ta future ate, ut tis eyondhe copefour urrentfforts.Despitehe ackof influencef activemagmatismntectonism,ectonisms nfluencingagmatism,utonly o aminorxtent. hisconclusionontrastsith hatof Bellierand ebrier1994],whoproposedhatextensionalullapartsalongheSumatranaulthave ffectedhe ocationf thevolcanoes.n fact,ourmap howshatonly9 of the50 youngvolcanicents hown n Plate1 are ocated ithin km of amappedrace f theSumatranault Figure1). Thesere,from southeasto northwest,Suoh, Seminung,Kaba,Dipatiampat,unyit,Melenggok,alang,Sibual-buali,Seulawahgam,andPulauWeb. Kaba,Kunvit,Meleggok,Talang,ibual-buali,eulawah gam,andPulauWeb arestratovolcanoesreater han about 10 km in diameter nd,thus, mbodyhe most substantialolumes. Suoh,Kaba,Kunyit, elenggok,alang,and Sibual-bualire locatedwithin ilatational tepovers r on one of the boundingaultsof a dilatationa tep over. One of these Suoh) s a largephreaticxplosionraterhat ormed 5 days fter he argeSemangkoegmentuptureof 1933 [Stehn,1934], mostconvincinglyn associationith ectonic ctivity.BellierandSebrier1994]proposedhatTobaandRanau alderaslsoformed t extinct extensional tep overs along the Sumatranfault one,but thesehypothesesre not well founded. Theyarebased olelyon the use of SPOT imagery o map moreancientault strands n the vicinity of these two calderas.Althoughinearionsmay exist along theseallegedancientfaults,heirdocumentation f the lineations s scant,and theypresento geologicmappingo confirm heirexistencer toquantifyhestyle, ge,or amount f shear long hem.We suspecthat he associationf just 9 of the 50 youngvolcanoes ith the Sumatran fault is a random occurrence. Ifone epperedn elongateectanglewith the 1700-by-50 mdimensionsf the volcanicarc) with a randomdistribution f50points nd hen an straightines andomlyhroughts ongdimension,everal ointswould ypicallybe within2 km ofeach ine. Thus the close association of several volcanoeswith heSumatranault zone doesnot, by itself, demonstrategeneticelationship. he closeassociationf six of the eightclose ncounters ith dilatationalstep overs does,however,suggesthat ectonic tepoversare nfluencinghe ocationsof a few of the arc's volcanic centers.3.5.Relationshipf the SumatranFaultto the Subduction Zone

Thegeneral hape f the Sumatranaultmimicshatof thedeformationront offshoreso faithfully that one wondersaboutgeneticelationshipetweenhesubductionnterfaceandhestrike-slipault (Plate5). Northof theequator, othstructures re concave toward the southwest. South of theequator,oth are broadly concave oward the northeast.Alonghe entire engthof the Sumatranault on land, tshorizontalistance rom the deformation ront variesno morethan10% rom290km (Table1 andPlate ).A similarcoincidence xistsbetween he shapeof theSumatranault and hat of the subductionnterface owndipfromts race. This s clear romPlates1 and5, whichshowthe50-, 100-, and 200-km isobaths f the subductioninterface.hecontoursredrawn n he opof theWadaft-Benioffone, s defined y hypocentralocationsn theInternationaleismologicalenterISC) atalogas elocated

by Engdahl t al. [1998]) andas determined y Fauzi et al.[1996] in their ocal seismic urvey n the regionof LakeToba.From about 6S to the equator, he relationshipsparticularly egular; he subductionnterface ies 100 to 135km below he Sumarranault,except long he southemmost(Sunda) egmentPlate1 andTable 1). Between bout3.5Nand 6.0N the subduction interface is 125 to 140 km below

the Sumatranault, exceptbeneath he northernpossiblyinactive) artof theAcehsegment. hesedepthsn thenorthare, on average,~20 km greater han depthssouth of theequator. The relationship f subduction sobaths o theSumatranault s markedly berrant etween heequator ndabout 3.5N. There the traces of the Sumarran fault and thesubductionsobaths remarkedly iscordant;he depthof theinterface eneathheSumatranaultrangesrom-100 to 175km.

Because f thewell-behavedelationshipf Sumatranaultto isobathsn thenorthern ndsouthern omains, e proposethat the Sumatran ault formed first in those two domains,astwo separatestructures. As displacement n the faults hasgrown, hey have ormeda linkageacrosshe centraldomainandwill onedaybecome single tructure.4. Summary, Conclusions, ndRemaining Questions

We have used stereographic erial photographyandtopographyo map 1650 km of the Sumatran ault (Figures2and 3). The resultingmap shows hat the fault comprisesnumerous segments separated by dilatational andcontractional tepoversand abruptchanges n trend (Plate 1andFigure4). This segmentationppears o have nfluencedthe rupturedimensions f historical arge earthquakesndlimited heirmagnitudeso ~7.5.The largest geomorphically vident offsets along theSumatran ault are between17 and 23 km (Plate 3, Figures7,and 9 and Table 3). Theseare predominantly eeply ncisedfiver channels, ut one apparent ffset of a fold pair and theaccumulated ffset acrossa major step over also fall withinthis range. A lack of detailedand completemapping longthe fault precludes onfidentmatchingof geologicunitsacross he fault, but rock offsetssuggested y Katili andHehuwat [1967] and Cameron et al. [1983] support hecontentionhat the 20-km geomorphic ffsets epresenthetotal offset across the fault.The distention of forearc structures and the trench near the

SundaStraitsuggests100 km of arc-parallel tretchingf theforearc liverplatesince heearlyPlioceneFigures and8).We proposehat20 km of thiswasaccommodatedy dextralslipon theSumatranaultand hat heMentawaiault,a long,linear structurewithin the forearc egion,accommodatedheremaining extralslip.Our synthesisf data rom heSumatranault, hevolcanicarc, and the forearc egionshows hat the Sumarranorearcsliver plate consists f three tectonicdomainswith verydistinct tectonichistories Plate 5). The southerndomain(from7S o 1S) s the simplestndmayhavebeenaccretedto the forearcsliverplate only about2 Myr ago by thecreation of the Sumatran and Mentawai faults. The northerndomain northof 2N) s morecomplex, nd ts northern arthasbeenexperiencingrc-parallelranslationor at least he


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28,324 SIEH AND NATAWIDJAJA:SUMATRANFAULT NEOTECTONICSpast10 Myr. The central omains the most omplex f thethree and has been a region of transtensionetweenhenorthernand southern omainssinceat least4 Myr ago.Geodetic measurements uggest hat slip across heSumatran fault between about 0.8 S and 2.7N is nearlyuniform t about25 mm/yr Genrich t aI., this ssue].Theserates re ncompatible ith the 27 andl1 mm/yrgeologic liprates that we have determinedat 2.2N and 0.3S [Sieh et al.,1991, 1994; D. Natawidjajaand K. Sieh, manuscriptnpreparation,000). We proposehat hegeologic ifferencein rateshas arisen n just the past 100 ka or so, becausestructural evidence for accommodationof the 15 mrn/yrdifferences obscure. We suggesthat a belt of auxiliary,transtensional deformation between the Sumatran fault andthe trench is the nascentmanifestation f this rate change(Figure10). This belt ncludeshe westernAngkola) ranchof the EquatorialBifurcation,he Tom fold-and-thrusteltalong he mainland oast, ndsubmarineaults n the orearcbasin,outer-arc idge,and nner renchslope.Although he Sumatranault and he Sumarranolcanic rcsharehe sameungle,neither ppearso have undamentallyaffected the location of the other. Rather than beingcoincident,he fault and the arc intertwine Figure 11). Theaveragedenterine of thevolcanic rc s distinctly ortheastof, not at, the Sumatran ault. Nevertheless,he few volcaniccenters hat are on or very near the Sumatran ault arepredominantlyt major extensional tepovers,whichmaywell haveattracted smallpercent f the arcvolcanism. hedramatic bend in the modem volcanic arc between 0.7N and2.5N s mostprobably heresultof transtensionalhinning fthe forearcsliverplate n the past4 Myr. We can not rule outthe possiblility hat the Pliocene and Miocene volcanicarcwere lesssinuous ndcloser o the locusof later strike-slipfaulting.The broad similarity n shapeof the Sumatran ault andsubductionnterfacesuggests genetic elationship.Thebroad, low-amplitudesinusoidalshape of the subductioninterfaces mimicked y the Sumatranault,andalongmostof its trace the Sumatran fault lies above the 110- to 140-kmisobathsf the subductionnterface.These elationshipsreparticularlyegularnorthof 3.5Nandsouth f theequator,nthe northern nd southern omains. We suggesthat theSumatranault first formedas two separateaults n these wodomains,ndare n theprocessf linking ogetherhroughthe central domain and across the volcanic arc. We ascribethe disruptedatureof the centraldomain's uter-arcidgeand forearcbasins o its locationabove he Investigatorfracture one hroughouthepast5 Myr.

Not unexpectedly, his work has generatedas manyquestions s answers:What are the detailsof the creation ndevolution of the three tectonic domains of the forearc sliverplate? How, for example, did deformation in thetranstensionalentraldomainevolve hrough he pastseveralmillionyears?Why did the Sumarranault form where t did,290 km from the subduction deformation front and 100 to 150km above the subduction nterface? Would careful, detailedmapping onfirm otalSumatranaultoffsets f only 20 km?When did the contrast n slip ratesalong he Sumatran aultbegin? Why is this gradient n rates not apparentn thegeodetic ata? Is it plausiblehat the Mentawai ault hasastrike-slip omponent s arge as 80 km? Did the two faultsoriginate mere2 Myr ago?

Ourmap f theSumatranault an erve sa umping-offpointorcareful nalysisf theseismicazardosedy hismajor tructure.o whatdegree oeshehistoricalecordflargeearthquakeslong he Sumarranaultdemonstratehatlarge structuralrregularitiesonstrainuptureengths?Would primitive nstrumentalecordshelp constrainhesource arametersf theseargeevents f the irsthalfof hetwentiethcentury? Whetheror not segmentationf theSumatranault asmarkedlynfluenceduptures,nsweringthese questions ould profoundly ffect our generalunderstandingf the mportancef structuraleometrynseismic uptureprocesses.Acknowledgments.hisworkbegan ithan invitationyYehuda Bock and his colleaguesat the IndonesianNationalCoordinationAgency for Surveying and Mapping(BAKOSURTANAL) to visit Sumatra. They were interestednestablishingbetter eologicontextor theirGPSgeodetictudies,supportednderNSF grantsEAR-8817067and EAR-9004376.Ourinitial reconnaissance,n 1991, supportedby donations rom heCaltech Associates, onvincedus that geologicalwork aimed tunderstandinghe active ectonics f the aultcouldbe very ruitful.In 1992, we initiated our own NSF sponsored roject EAR-

9205591) to map and characterize the Sumatran fault.BAKOSURTANAL, through Bock, supplied most of ourtopographicoverage f the fault. We are grateful o SuparkandHery Harjono at the Indonesian nstitute of Sciences LIPI)forhelping sacquiremany mportanterialphotographsndarrangingfieldwork. Rudy Bachruddin t the Volcanological urvey fIndonesia VSI) and GunawanBurhan at the GeologicalResearchand Development enters GRDC) also helpedus by providingessential erial photographslongportions f the Sumarranault.Various private companies lso allowed us access o topographicmaps ndaerialphotographs. aulTapponnierIPG Paris) haredtopographic aps rom the Singkarakegion. We alsobenefitedfrom conversations ith Rob McCaffrey and Bob Kieckhefer ndfrom an initial GIS compilation f our data by CarolynWhite.Reviews f anearlymanuscripty Yehuda ock, oseph urray,ndRobMcCaffreywereveryhelpful.Finally,we greatly ppreciatehethorough,houghtful,nd careful eviews f Jeff Freymuller,liSilver, and Paul Tapponnier. This is CaltechSeismologicalLaboratory ontribution625.ReferencesAbe, K., Magnitudes f large shallow arthquakesrom 1904 o980: Phys.Earth Planet. nter. 27, 72-92, 981.Allen, C., A. Gillespie,H. Yuan, K. Sieh,Z. Buchun, ndZ.Chengnan,ed River andassociatedaults,Yunnan rovince,China: Quaternaryeology, liprateandseismic azard, eol.Soc.Am. Bull., 95, 686-700, 1984.Allen,C.R., SanAndreasaultzone n SanGorgonio ass, outhernCalifornia, Geol. Soc.Am. Bull., 68, 315-350, 1957.Armijo, ., B. Meyer, ndA. Hubert,Westwardropagationf heNorth Anatolian ault into the northern egean: iming ndkinematics, eology, 7, 267-270, 1999.Aspden, .A., W. Kartawa, .T. Aldiss,A. Djunuddin,.Whandoyo,. Diatma,M.C.G. Clarke, ndH. Harahap,hegeology f the Padangsidempuannd SibolgaQuadrangle,Sumatra,eport eol. es. ndDev.Centr., andung,ndonesia,1982.Barka, ., TheNorthAnatolianault one, nn.Tectonicae,, 164-195, 1992.Bellier,O., andM. Sebrier, elationshipetweenectonismndvolcanismlongheGreatumatranault one educedySPOTimage nalyses,ectonophysics,33,215-231, 994.Bellier, ., andM. Sebrier,s theslip atevariationn heGreatSumatranault ccommodatedy fore-arctretching?:eophys.Res.

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Neotectonics of the Sumatran Fault_ Indonesia