Upper mantle structure and geodynamics of the Sumatra subduction zone from 3-D teleseismic P-wave tomography

Abstract

Teleseismic waveforms from a passive array of 63 seismic stations are applied to image the 3-D P-wave velocity structure of the mantle in the Sumatra subduction zone. In this study, 102 teleseisms with good signal-to-noise ratios were selected. In total, 1627 P-wave relative arrivals have been extracted with high accuracy based on multi-channel cross-correlation and adaptive stacking methods. With this dataset, we conducted the 3-D tomographic inversions for the 3-D P-wave velocity structure of the upper mantle in the Sumatra subduction zone. The tomographic results suggest significant lateral variations of P-wave velocity in the mantle. The trench-parallel low-velocity features in the upper mantle reveal that there may be a trench-parallel flow associated with 3-D mantle flow beneath the subducting slab. The subducting Indo-Australian Plate is also clearly imaged as a continuous high-velocity anomalous zone in the mantle along the Sumatra Island. Beneath southern Sumatra, there could have a lateral detachment of high-velocity anomaly from ∼560 km to the deep part of the slab, which may imply the slab tearing. If it is true, we could infer that this tear in the slab may be terminated around 17.5 Ma. The advancing speed of the Indo-Australian Plate increasing from North Sumatra to South Sumatra plays a key role in the slab tear, which suggests likely that the northeastward subducting of the Indo-Australian Plate along the Sunda trench and the southeastward clockwise motion of the Sunda Shelf may form a torque tracing the Sumatra Island that facilitated the clockwise rotation of the fast-moving plate. With the oblique subduction of the Indo-Australian Plate, the asymmetric distribution of stresses along this torque on either side may induce the slab tearing.