J. Geodyaamics Vol. 24, Nos l-4, pp. 169-183,1997 8 1997 Eisevier Science Ud All rights reserved. Printed in Great Britain PIk so264-37@7(%poo51-8 0264-3707/97 $17.00+0.00 PALEOSEISMICITY AND SEISMIC HAZARD ALONG THE GREAT SUMATRAN FAULT (INDONESIA) 0. Bellier,’ M. !%brier,’ S. Pramumijoyo? Th. Beaudouin,’ H. Harjono,3 I. Bahar4 and 0. Fom? ‘URA-CNRS-GCophysique et Geodynamique Inteme, B&t.509, Universid Paris-Sud, 91405 Orsay Cedex, France *Universitas Gadjah Mada, Teknik Geologi FrI, Yogyakarta,Indonesia ‘LIP1 Geoteknology (Indonesian Institute of Sciences), Bandung, Indonesia ‘DGGMR/PPPG (Geological Research and Development Centre), Bandung, Indonesia ‘lnstitut Astrophysique Spatial, Universite Paris-Sud, 91405 Orsay Cedex, France (Received 6 November 199s: accepted in revised form I7 October 1996) Abstract-The Great Sumatran Fault (GSF) is a 1650~km-long dextral strike-slip fault zone which accommodates part of the oblique convergence of the subduction between the Indo- Australian and Eurasian plates. To define the seismic hazard along this fault, we used paleoseismology and neotectonics. To characterise the seismic history of the southern GSF we excavated four trenches. Within these trenches, the occurrence of only one paleosol related to a seismic event indicates that in a wet, tropical region, the degradation rate of organic matter could be faster than seismic recurrence. The trenching method permitted us to identify only one recent earthquake, reactivating the southern GSF. As the trenching method does not seem efficient to constrain knowledge of seismicity in this region, we have developed an active tectonic study to characterise the seismic hazard along the GSF. We created a large-scale segmentation map which allows 18 major fault segments with lengths ranging between 45 and 200 km to be recognised. We complemented the segmentation map reporting major earthquake ruptures on the basis of the historical seismicity which recorded 17 earthquakes since 1835. The segmentation map indicates a northward increase of segment lengths which parallels the GSF slip-rate increase. This observation suggests a northward increase of seismic hazard along the GSF. Segmentation and historical seismicity provide evidence of a 300~km-long seismic gap (between 3”N and 5’N) around a locked restraining bend which can be considered as having high potential for seismic hazard in Sumatra. The magnitude of the maximum expected earthquake for each segment was estimated through two empirical methods. These estimates give higher maximum magnitude and shorter seismic recurrence intervals for segments in northern Sumatra, confirming a northward increase of seismic hazard. 0 1997 Elsevier Science Ltd INTRODUCTION The 1650~km-long Great Sumatran Fault (GSF) is a NW-trending right-lateral fault that parallels the western Sunda Trench and follows the magmatic arc southward from the Andaman Sea back- arc basin to the Sunda Strait (Fig. 1). Off Sumatra, the convergence direction is oblique to the 169 170 0. Bellier er al. plate boundary. Assuming that strike-slip motion along the GSF could accommodate the trench- parallel component of the oblique convergence, the maximum slip-rate of the GSF estimated by global modelling should be in the range 40-60mm/yr (Jarrard, 1986; McCaffrey, 1992). Consequently, the GSF is one of the fastest strike-slip faults in the world. However, the GSF segmentation characteristics and seismic hazard parameters are very poorly constrained. Because only 17 seismic events (M>6 or MMI>VIII) during the last two centuries have been lE SUNDA STRAIT Fig. 1. Regional geodynamic framework of Sumatra showing slip-rates along the major faults of Sumatra (after Bellier and SCbrier, 1995). ‘GSF’ represents the Great Sumatran Fault and ‘MF’ the Mentawai’ Fault. Paleoseismicity along the Great SumatranFault 171 reported (e.g. Beaudouin er al., 1995) and there is no evidence for significant fault creep (e.g., Duquesnoy er al., 1996) along the GSF, the fault seems to be characterised by a stick-slip behaviour, appearing to remain locked between earthquakes. In order to evaluate whether the GSF has a high seismic potential we conducted trench observations across the southernmost GSF segment to constrain its surface source parameters such as fault geometry, slip per event, and recurrence interval between major events. To improve the GSF seismic hazard assessment, we compiled a large-scale segmentation map based on topographic characteristics. We complemented this map showing the earthquake ruptures of the last century on the basis of a historical seismicity study, and we calculated the moment magnitudes for the maximum earthquake expected on each segment. TECTONIC SETTING: CONVERGENCE OBLIQUITY VERSUS SUMATRAN DEFORMATION In subduction areas associated with oblique convergence, relative plate motion may be partitioned between displacements along the subduction plane and deformation within the overriding plate (Fitch, 1972). Some of this overriding plate deformation corresponds to a shear component localized on a margin-parallel, strike-slip fault zone. The roughly NNE-trending convergence (DeMets et al., 1990; McCaffrey, 1992; Tregoning et al., 1994) at the Sunda trench is oblique off Sumatra. It has to be accommodated by both subduction and strike-slip deformation in the overriding plate; that is, mostly along the GSF (Bellier and Sebrier, 1995). To estimate slip-rate, an along-strike survey of the GSF was conducted using geomorphic feature offsets observed on SPOT images and topographic maps (Bellier and Stbrier, 1995). These estimates show a southward slip-rate decrease along the GSF from about 23+2 mm/yr at 2”N, to 6*4 mm/yr at S’S, near Ranau lake (Fig. 1 and Table 1). SPOT image analysis and structural field studies suggest that the GSF slip-rate variation is chiefly accommodated by transpressional deformation in the back-arc (Bellier and Sebrier, 1995; Detourbet, 1995). Several dextral-reverse oblique-slip fault zones splay southeastward from the GSF to reach the Sumatran back-arc basins. The horizontal slip-rate of these faults appears to be about 1 mm/yr. The fore-arc is to the west, bounded by another fault parallel to the GSF major fault zone: the Mentawai’ Fault (MF on Fig. 1). Most of the fore-arc sliver, between the GSF and the MF, should be under compression as indicated by the MF geometry which suggests dextral movement (Diament et al., 1992) with the western block upthrust (Karig et al., 1980). This idea is supported by field observations that showed a roughly N40°E-trending shortening (Detourbet, 1995) similar to the compression in the back-arc (Mount and Suppe, 1992). These relationships suggest that even if the active deformation of Sumatra is mostly localized along the GSF, a combination of different modes of deformation throughout the fore-arc platelet to the back-arc domain contribute to accommodate the oblique convergence. SUMATRADEFORMATION AND SEISMIC HAZARD Bellier and SCbrier (1995) show a northward increase of the dextral slip-rate along the GSF. Assuming that seismic hazard is roughly proportional to slip-rate, the difference in slip-rate suggests that seismic hazard had to be higher along the northern rather than the southern GSF; that is, for a similar sized earthquake the recurrence interval should be shorter in the north than in the south. In addition, the slip-rate study suggested that deformation throughout the fore-arc platelet to the back-arc domain contribute to accommodate the convergence (Bellier and Sebrier, 1995). This implies that although most of the convergence is accommodated along the GSF, 172 0. Bellier er al. active deformation and thus seismic hazard is distributed within a 500-km-broad zone from the fore-arc to the back-arc. PALEOSEIMOLOGY OF THE SEMANGKA SEGMENT: TRENCH OBSERVATIONS Paleoseismology and seismic hazard studies are very difficult to perform with significant results under adverse wet tropical conditions. By way of experiment, we excavated a set of four trenches along the Semangka segment of the south GSF, where the strike-slip movement changes to oblique-normal faulting (Fig. 2). The trenches were located at the foot of the 800-m- high east-facing escarpment that limits the Semangka Valley to the West. All these trenches were excavated at the foot of faceted spurs (Fig. 3), not affected by landslides, and away from major alluvial fans. In order to minimise water-table problems, excavation was carried out at the end of the dry season. The trenches were 20-39 m-long, and up to 4.6-m-deep (Fig. 4). North Way Kerap trench (trench Tl) The N55’E-trending trench Tl was dug at the foot of a 35-m-high faceted spur where the fault trace is markedly linear. The mean angle of the scarp slope is steep (about 30”) and is interrupted by two slope breaks inclined at 35”. Tl walls exposed a Present-day black organic-rich soil Table 1. GSF segments, moment magnitudes and recurrence intervals for maximum expected earthquakes along the GSF. Numbers of the segments Sl to S18 refer to numbers shown on Fig. 5a. Location gives approximately the segment extremities. Moment magnitudes calculated by seismic moment (e.g. Kanamori, 1983) and statistical (Wells and Coppersmith, 1994) methods. Seismic recurrence intervals for the expected maximum magnitude are provided where slip-rates are available (Bellier and Stbrier, 1995) Statistical Seismic moment method method N” Location Length D=l m D=4 m Slip rate Recurrrence (km) MO MO (mmhd (yr) lOI Nm) ( lOI Nm) M, Mw M, Sl Semangka bay/Suwoh graben 70 4.2 7.0