The profound reach of the 11 april 2012 m 8.6 Indian ocean earthquake: Short-term global triggering followed by a longer-term global shadow

Abstract

The 11 April 2012 M 8.6 Indian Ocean earthquake was an unusually large intraoceanic strike-slip event. For several days, the global M ≥4:5 and M ≥6:5 seismicity rate at remote distances (i.e., thousands of kilometers from the mainshock) was elevated. The strike-slip mainshock appears through its Love waves to have triggered a global burst of strike-slip aftershocks over several days. But the M ≥6:5 rate subsequently dropped to zero for the succeeding 95 days, although theM ≤6:0 global rate was close to background during this period. Such an extended period without an M ≥6:5 event has happened rarely over the past century, and never after a large mainshock. Quiescent periods following previous large (M ≥8) mainshocks over the past century are either much shorter or begin so long after a given mainshock that no physical interpretation is warranted. The 2012 mainshock is unique in terms of both the short-lived global increase and subsequent long quiescent period. We believe that the two components are linked and interpret this pattern as the product of dynamic stressing of a global system of faults. Transient dynamic stresses can encourage shortterm triggering, but, paradoxically, it can also inhibit rupture temporarily until background tectonic loading restores the system to its premainshock stress levels.