The 2005 and 2010 Earthquakes on the Sumatra-Andaman Trench: Evidence for Post-2004 Megathrust Intraplate Rejuvenation

Great earthquakes on oblique subduction zones accommodate slip on strike-slip faults on the overriding and subducting plates, leading to increased intraplate seismicity. After the 2004 Sumatra earthquake, there was an increase in activity in the Wharton basin (WB), which is considered to be part of a diffuse plate boundary. Here, the intraplate deformation occurs in response to northwest-southeast directed compression as opposed to the north-northeast-south-southwest directed plate motion along the Sumatra-Andaman subduction zone. Earthquakes here mostly occur by left-lateral strike slip on north-south (N-S) trending fossil fractures, although ruptures on east-west planes are not uncommon. Here, we discuss the 24 July 2005 M-w 7.2 and the 12 June 2010 M-w 7.5 earthquakes, both of which were left-lateral strike-slip events sourced in the northern WB. Our source models for these events, based on teleseismic waveform inversion, favor slip on the N-S fossil fracture systems, consistent with previous studies in the WB. Although the former sourced at similar to 16 km depth was not tsunamigenic, the latter (similar to 34 km) triggered a small tsunami, possibly due to its oblique slip at similar to 5 km depth. We use conceptual models of plate deformation to suggest that both these events have resulted from post-2004 redistribution of stresses and rejuvenation of intraslab fracture systems within the WB, an actively deforming oceanic intraplate region