Paleoseismicity and seismic hazard along the Great Sumatran Fault (Indonesia)

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 Sap (between 3 degrees N and 5 degrees 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. (C) 1997 Elsevier Science Ltd.