Seismotectonics of a diffuse plate boundary: Observations off the Sumatra‐Andaman trench

The actively deforming Indo‐Australian intraplate region off the Sumatra‐Andaman trench hosted the largest strike‐slip earthquake recorded by modern instruments, the 2012 Mw 8.6 Wharton Basin earthquake, closely followed by a Mw 8.2 aftershock. These two large events ruptured either parallel north‐south trending faults or a series of north‐south and nearly perpendicular east‐west fault planes. No active east‐west faults had been identified in the region prior to these earthquakes, and the seismic rupture for these two earthquakes extended past the 800°C isotherm for lithosphere of this age, deep into the oceanic mantle and possibly beyond the inferred transition to ductile failure. To investigate the seismic behavior of this region, we calculate moment tensors with teleseismic body waves for 6.0 ≤ Mw ≤ 8.0 intraplate strike‐slip earthquakes. The centroid depths are located throughout the seismogenic mantle and could extend through the oceanic crust, but are generally well constrained by the 600°C isotherm and do not appear to rupture beyond the 800°C isotherm. We conclude that while many earthquakes are consistent with a thermal limit to depth, large magnitude earthquakes may be able to rupture typically aseismic zones. We also perform finite‐fault modeling for Mw ≥ 7.0 earthquakes and find a slight preference for rupture on east‐west oriented faults for the 2012 Mw 7.2 and 2005 Mw 7.2 earthquakes. This lends support for the presence of active east‐west faults in this region, consistent with the majority of previously published models of the 2012 M8+ earthquakes. Key Points Seismic slip extends from maximum expected depth to the crust in 45–75 Ma oceanic lithosphere Large (Mw > 7) strike‐slip earthquakes can rupture multiple differently oriented faults Rupture directivity modeling provides tentative support for active east‐west oriented faults