The April 2012 Indian Ocean earthquakes: Seismotectonic context and implications for their mechanisms

•The April 2012 earthquakes occurred in the Northern Wharton Basin (nWB).•The nWB is part of the seismically active diffuse Indo-Australian plate boundary.•Regional geology and tectonic history favor failure on pre-existing structures.•April 2012 slip models based on seismological evidence suggest a complex mechanism.•We examine the background seismicity and tectonics to better evaluate these models.

The 11 April 2012 earthquakes (Mw 8.6 and Mw 8.2) were sourced within the Northern Wharton Basin in the northeastern part of the Indo-Australian diffuse plate boundary. This unusually active oceanic intraplate region has generated many large earthquakes in the past, most of which are believed to have occurred by strike-slip motion, triggered by the NW-SE oriented compressional stresses acting across the Indian and Australian plates. In the aftermath of the 2004 megathrust earthquake along the nearby Sunda Trench, increased seismicity in the Northern Wharton Basin is attributed to the stress transfer from the Sumatra-Andaman plate boundary. Models proposed for the April 2012 earthquakes differ somewhat in details but partly attribute their complex rupture to the reactivation of pre-existing structures. These structures include previously mapped N-S trending fracture zones within the Northern Wharton Basin and E-W lineations across the Ninetyeast Ridge. In this paper, we review the regional tectonics and past seismicity on the Indo-Australian Plate in order to understand the seismotectonic setting of the April 2012 Indian Ocean earthquakes.