Foreshock sequence of the April 11, 2012, Mw 8.6 Indian Ocean earthquake

The 2012 off northern Sumatra Indian Ocean earthquake sequence started with a Mw 7.2 earthquake in Jan 10th. According to USGS PDE locations, this foreshock's epicenter is 21 km northeast of the epicenter of the April 11 Mw 8.6 mainshock. Four of its aftershocks subsequently were located within 10 km of the mainshock epicenter (Ji et al., 2012). However, it is of interest to notice that right before the April 11 mainshock, no earthquake within 50 km of mainshock epicenter can be found in the same catalog during a half-month period from March 26 to April 10. Unfortunately, it is not clear yet whether this represents a pre-seismic "quiescence" period in seismic activity near the mainshock epicenter, which had been reported during the studies of several previous large earthquakes (Kanamori, 1981), or is simply caused by the incompleteness of the PDE catalog in this remote area. To answer this question we use hundreds of broadband records of the Jan 10th foreshock at global seismic stations as empirical Green's functions to locate the nearby earthquakes. For each station, the correlation between this record and the waveform from Jan10th to April 11th is calculated after bandpass filtering them between 0.01 and 0.033 Hz. A nearby earthquake with similar focal mechanism to the foreshock shall be associated with a peak in the resulted correlogram, though it might be invisible due to the ambient noise. After stacking hundreds of correlograms, our preliminary analysis captures over 500 potential events, one order of magnitude more than the number of nearby events in the PDE catalog during the same period. Seismic statistic analyses will be applied to this new catalog after further improving the estimations of event locations and magnitudes. The seismic "quiescence" hypothesis will be testified. The proposed method shall be useful to study the foreshock and aftershock sequences of other oceanic large earthquakes in the remote area and earthquakes without available local network as well.