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 earthquake foci in the Sumatran region allowed the authors to distinguish the foci belonging to the recent Wadati-Benioff zone from those occurring in the continental wedge. The morphology of the Wadati-Benioff zone, its main geometrical parameters and the variable depth of its penetration into the upper mantle were established. The existence of an intermediate-depth aseismic gap in the Wadati-Benioff zone was discovered 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 existence of a Tertiary subduction zone underlying the present slab. The seismotectonic pattern of the continental wedge was described by 11 seismically active fracture zones, the orientation and tectonic function of which were checked by fault plane solutions.