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Category: Project

Description: Four large, dominantly strike-slip, earthquakes have occurred in recent years in the Wharton Basin off the coast of Sumatra. The southernmost was the Mw 7.9 event on 2000 June 18, followed by the largest known intraplate earthquake – the Mw 8.6 event on 2012 April 11, with an Mw 8.2 ‘aftershock’ within a few hours. The most recent Mw 7.8 event on 2016 March 2 happened in an area with no previously recorded large earthquake. For each event we have undertaken analysis of the high-frequency energy radiation as a function of time, using a suite of global stations with good azimuthal control, so that we can map out source evolution in three-dimensions. Each of the events involves a component of conjugate faulting. The largest energy emission links to near north-south features reflecting seafloor topography, with linking east-west rupture. High-frequency radiation is emitted through the full thickness of the lithosphere in the main part of the event. As frequency is reduced there is a distinct change in radiation characteristics, though migration of energy emission to depth is common. Significant energy release occurs at depths where deformation would be expected to be ductile, suggesting differences between instantaneous and long-term rheology. The complex behaviour recognised in the 2012 great earthquakes looks to be a feature of strike-slip events in this zone off the Sumatran subduction zone. Each of the events occurs in a region where surface wave tomography models show slightly lower shear wavespeed than their surroundings. These contrasts suggest changes in the physical parameters of the Indo-Australian plate in the area of breakup. The conjugate ruptures in response to an oblique stress-field form part of a diffuse zone of deformation associated with the impending divorce between the Indian and Australian components of the plate.