Fault interactions and triggering during the 10 January 2012 Mw 7.2 Sumatra earthquake

The 10 January 2012 M-w 7.2 Sumatra earthquake in the Wharton basin occurred 3months before the great M-w 8.6 and M-w 8.2 earthquakes in the same region, which had complex ruptures and are the largest strike-slip earthquakes ever recorded. Teleseismic P wave back projection of the M-w 7.2 earthquake images a unilateral rupture lasting approximate to 40s without observable frequency dependency (low frequency, 0.05-0.3Hz, high frequency, 0.3-1Hz). In addition to radiation bursts during the M-w 7.2 main shock, coherent energy releases from 50 to 75s and from 100 to 125s are observed about 143km northeast of the main shock rupture and landward of the trench. Analysis of globally recorded P waves, in both 0.02-0.05Hz velocity records and 1-5Hz stacked envelope functions, confirms the presence of coherent sources during the time windows. The observed energy bursts are likely to be large early aftershocks occurring on or near the subduction interface. Both dynamic and static triggering could have induced these early aftershocks, as they initiated after the surface wave passed by, and the Coulomb stress perturbations from the M-w 7.2 main shock promote earthquakes in the observed locations. The earthquake sequence is a clear example of a seaward-intraplate strike-slip earthquake triggering landward-intraplate earthquakes in the same region, in contrast to previously reported normal-reverse or reverse-normal interactions at subduction zones.