SUMATRAN SEGMENT OF THE INDONESIAN SUBDUCTION ZONE - MORPHOLOGY OF THE WADATI-BENIOFF ZONE AND SEISMOTECTONIC PATTERN OF THE CONTINENTAL WEDGE

The results of a detailed study on the geometrical distribution of ear thquake foci in the Sumatran region allowed the authors to distinguish the foci belonging to the recent Wadati-Benioff zone from those occur ring in the continental wedge. The morphology of the Wadati-Benioff zo ne, its main geometrical parameters and the variable depth of its pene tration into the upper mantle were established. The existence of an in termediate-depth aseismic gap in the Wadati-Benioff zone was discovere d and its spatial relationship with young calc-alkaline volcanism was confirmed. The subduction process was correlated with the stratigraphy and geology of the Sumatran region. The duration of the present cycle of subduction was estimated to be 6-8 Ma. The occurrence of Oligocene volcanism and the locations of deep earthquakes point to the existenc e of a Tertiary subduction zone underlying the present slab. The seism otectonic pattern of the continental wedge was described by 11 seismic ally active fracture zones, the orientation and tectonic function of w hich were checked by fault plane solutions. Copyright (C) 1996 Elsevie r Science Ltd