Seismic imaging of forearc backthrusts at northern Sumatra subduction zone

Abstract

Forearc tectonics at accretionary convergent margins has variously been studied using analogue and numerical modelling techniques. Numerous geophysical investigations have targeted the subsurface structure of active forearc settings at convergent margins. However, several critical details of the structure, mode of tectonic evolution and the role forearcs play in the subduction seismic cycle remain to be further understood, especially for large accretionary margins. In this study, we present a high-resolution deep seismic reflection image of the northern Sumatran subduction forearc, near the 2004 December 26 Sumatra earthquake epicentral region. The profile clearly demarcates the backthrust branches at the seaward edge of the Aceh forearc basin, along which the inner forearc continues to evolve. Sharp bathymetric features at the seafloor suggest that the imaged backthrusts are active. Coincident wide-angle seismic tomographic image of the Sumatra forearc allows us to image the geometry of the seaward dipping backstop buttress, with which the imaged backthrust branches are associated. The presence of forearc backthrusting confirms model predictions for the development of backthrusting over seaward dipping backstops. The West Andaman fault at the seaward edge of Aceh basin appears to be a shallow tributary of the backthrust and sheds light on the complex deformation of the forearc. Uplifting along the backthrust branches may explain the presence of forearc islands observed all along Sumatran margin and help further constrain the tectonic models for their evolution. Moreover, if these backthrusts slip coseismically, they would contribute to tsunamigenesis and seismic risk in the region.