Aftershocks of the 2012 M-w 8.6 Wharton Basin Intraplate Earthquake in the Eastern Indian Ocean Revealed by Near-Field Ocean-Bottom Seismometers

The 11 April 2012 Mw 8.6 earthquake, the largest strike-slip earthquake recorded by instrument, occurred in the eastern Indian Ocean off the Sumatra subduction zone. During March-April 2017 and March-May 2018, two arrays of broadband ocean-bottom seismometers (OBSs) were deployed, respectively, near the epicentral area of the 2012 Mw 8.6 intraplate earthquake sequence. These were the first passive-source OBS experiments near the rupture region of the mainshock and subsequent aftershocks. A total of 1888 potential local events were detected through waveform matching, from which 164 events were subsequently located using the arrival time of the first P and S waves and were analyzed for hypocentral depth. The 2017 deployment was located close to the southern end of the mainshock and its largest aftershock (Mw 8.2). The events recorded in 2017 were found to be concentrated mostly along the intersection of two subfaults (F2 and F3) of the 8.6 earthquake, the rupture plane (F4) of the Mw 8.2 event, and over a broader region of complex fault structure in the Wharton basin. In contrast, the 2018 events were concentrated at two conjugate planes at the Ninetyeast ridge near the western end of a subfault (F3) of the Mw 8.6 mainshock. These results illustrate that the aftershocks of the 2012 strike-slip earthquakes have continued in 2017 and 2018. Furthermore, the 2017 and 2018 events were found to be concentrated at depth intervals of 0-10 and 20-30 km, respectively. The relatively small number of events at the intervening depth of 10-20 km was interpreted to be the result of the presence of a serpentinized layer in the uppermost mantle in the Wharton basin, which is consistent with the results of previous multichannel seismic studies in the region.