Imaging the fault ruptures of the great 2012 indian ocean intraplate earthquakes from back-projection of teleseismic p-waves

P-wave observations from seismic stations in Europe and Japan were used to track the short-period energy release of the April 11, 2012 Mw 8.7 mainshock and Mw 8.2 aftershock that occurred in the Indian Ocean. Both were intraplate strike-slip events that ruptured in the region of a diffuse plate boundary within the Indo-Australian plate. We performed back-projection analyses using 85 vertical component broadband stations in Europe and 72 F-net stations and ~760 Hi-net stations in Japan. The high-passed back-projection images from the different arrays show similar trends in the short-period energy release. The locations of short-period rupture coincided with the complex aftershock distribution which suggests rupture on multiple conjugate fault planes. Back-projection results for the Mw 8.2 aftershock suggest bilateral rupture on an NNE-SSW fault plane with dominant energy located to the NNE. Seven large aftershocks were used to retrieve travel-time correction values to be interpolated over the grid region. The new back-projection results of the European array using the empirical aftershock corrections aligned the mainshock subevents and projected into linear fault trace features. We further investigate the mainshock fault geometry and rupture properties by back-projecting synthetic seismograms modeled from four finite fault slip models. The geometry of the fault models differ between the primary fault being conjugate or orthogonal or sub-parallel to the secondary fault. One of the fault models included a super-shear rupture parameter. Out of the four synthetic derived images, the orthogonal fault model, is the most similar to the data-derived back-projection images. We conclude that the mainshock ruptured four conjugate faults; two WNW-ESE subparallel faults, one fault orthogonal to the main fault, and a possible WNW-ESE fault to the far west.