The geometry of the subducted slab beneath Sumatra revealed by regional and teleseismic traveltime tomography

Abstract

The geometry of the subducted Indo-Australian plate beneath Sumatra is still controversial because of the historical lack of a dense seismic network. Since 2005, Indonesia has been establishing a relatively dense seismic network for real time earthquake monitoring and tsunami warning. The seismic data accumulated by this network make it possible to achieve high-resolution tomographic images of the velocity structure beneath Sumatra by using eikonal equation-based seismic tomography method. Our P-wave tomographic images derived from regional seismic and teleseismic traveltime data demonstrate that the slab in the upper mantle generally follows the strike of the trench and the orientation of the volcanic arc. Additionally, the slab exhibits a sinusoidal shape with a low degree of curvature. Our tomographic results also reveal that the maximum penetration depth of the subducted slab increases from the north to south. The subducted slab beneath the northern tip of Sumatra barely arrives at the 410-km discontinuity, while the slab in the south penetrates to a depth of at least 660 km. Our inversion further indicates that the subducted slab is characterized by a tear at a depth of 120 km between 0° N and 2° N, which may be closely related to the supervolcanic eruption of the Toba caldera in northern Sumatra during the Pleistocene. Moreover, we report that the dip of the subducted slab varies significantly (i.e., dramatically decreases) across the Sunda Strait; therefore, we infer that another subvertical slab tear exists beneath the Sunda Strait.