Stress triggering of the great Indian Ocean strike‐slip earthquakes in a diffuse plate boundary zone

Abstract

On April 11, 2012, two great magnitude 8+ earthquakes occurred within a two‐hour period off the west coast of northern Sumatra, Indonesia, in the broadly distributed India‐Australia plate boundary zone. The magnitude 8.6 mainshock holds the distinction of being both the largest instrumentally recorded strike‐slip earthquake and the largest earthquake away from a recognized plate boundary fault. The mainshock involved sequential ruptures of multiple fault planes oriented nearly perpendicular to each other. The adjacent 2004 megathrust earthquake statically loaded the northern Wharton Basin oceanic lithosphere on both of the 2012 mainshock fault plane orientations, and greatly enhanced the rate of earthquake activity in the region for a year. Viscoelastic relaxation of the asthenosphere following the 2004 and 2005 megathrust earthquakes continued to positively stress the offshore region, correlating with the locations of later strike‐slip earthquakes, including two magnitude 7+ and the 2012 magnitude 8+ earthquakes.