PALEOSEISMICITY AND SEISMIC HAZARD ALONG THE GREAT-SUMATRAN FAULT (INDONESIA)

The Great Sumatran Fault (GSF) is a 1650-km-long dextral strike-slip f ault zone which accommodates part of the oblique convergence of the su bduction between the Indo-Australian and Eurasian plates. To define th e seismic hazard along this fault, we used paleoseismology and neotect onics. To characterise the seismic history of the southern GSF we exca vated 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 s eismic 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 i n this region, we have developed an active tectonic study to character ise the seismic hazard along the GSF. We created a large-scale segment ation map which allows 18 major fault segments with lengths ranging be tween 45 and 200 km to be recognised, We complemented the segmentation map reporting major earthquake ruptures on the basis of the historica l seismicity which recorded 17 earthquakes since 1835. The segmentatio n map indicates a northward increase of segment lengths which parallel s the GSF slip-rate increase. This observation suggests a northward in crease of seismic hazard along the GSF. Segmentation and historical se ismicity provide evidence of a 300-km-long seismic Sap (between 3 degr ees N and 5 degrees N) around a locked restraining bend which can be c onsidered as having high potential for seismic hazard in Sumatra. The magnitude of the maximum expected earthquake for each segment was esti mated through two empirical methods. These estimates give higher maxim um magnitude and shorter seismic recurrence intervals for segments in northern Sumatra, confirming a northward increase of seismic hazard. ( C) 1997 Elsevier Science Ltd.