Stress reconstruction and lithosphere dynamics along the Sumatra subduction margin

Abstract

Gravity modeling and stress reconstructions over the Sumatra subduction margin between Indian-Australian and Asian plates provide insights into the geodynamic setting and kinematics of the area. Based on ridge-trench interaction, plate obliquity, plate age and seismicity, the area was divided into two sectors (northern and southern) for gravity modeling along two profiles. The stress data with magnitude Mw ≥ 5.0 and focal depth ≤ 100 km for the subducting plate were compiled for six zones (I–VI), and were isolated under different time-domains for high-resolution study. The reconstructed gravity models were used to isolate the events for subducting (oceanic/Indian-Australian) and overriding (continental/Asian) plates. The data sets for the subducting lithosphere were used for stress inversion to determine the predominant stress regimes for pre-earthquake and post-earthquake phases of the Sumatra earthquake of December 2004. The results were interpreted in terms of operative intra-plate stress fields, and for reconstruction of kinematics and dynamics along the Sumatra subduction margin. Analysis shows both sparse and intense seismic activities identified over different time-segments in the six zones under pre- and post-December 2004 phases with respect to the incidence of Mw 9.2 off Sumatra mega-earthquake of 2004. The stress regime of the 2004 mega-event and subsequent release of strain energy through cyclic changes of the stress obliquities in the northern part of the study area account for heterogeneous coupling caused by rough topography of the subducting Indian-Australian plate. Reduction of compressive stress regime in the deeper segment of the subducting plate in the northern sector triggered twin great strike-slip dominated earthquakes in 2012 in the floor of the northeastern Indian Ocean. The weak zones of a spreading ridge subducting below the Asian plate possibly suffered strain localization and weakening. The less transmission of co- and post-seismic strain energy from northern to southern part of the margin was possibly prohibited near the zone of ridge trench interaction. The trench-orthogonal tearing (slab window) at the edge of the subducting plate in this area cannot be ruled out. The slab window, caused by the subduction of the young buoyant ridge and the trench-ward movement of the upper plate, has transformed the central segment into compressional regime.