Determination of focal depths of moderate earthquakes in Northeast Indian region using depth phase sPn

The precise focal depth of earthquakes is a crucial parameter for better characterising of complex tectonic zones, especially where seismogenic faults occur at different depths. Knowing accurate focal depth also allows recalculation of the equivalent magnitude for the reassessment of the mapping of the slip rate deficit and seismic hazard in the complex tectonic region. In the present study, we determined the focal depths using depth phases for earthquakes with magnitude 3.5 <= M <= 5.0 (except a few earthquakes with magnitude > 5) from 2011 to 2018 in the entire Northeast India region. We also validate the focal depths determined from the depth phases using point source waveform inversion. Our study area includes the NE-syntaxis arc to the eastern subduction belt, northern collision thrust belt and its foredeep/foreland area in the southern Brahmaputra plain and prominent Shillong plateau. For computing the focal depth, based on the tectonic setting and existing faults, we divided the region into five significant blocks, i.e. Himalaya collision zone, Assam Valley, Shillong Plateau, Eastern Himalaya Syntaxis, and Indo-Burmese Wedge. The focal depth variations in the five tectonic provinces suggested that the seismogenic depth in all the tectonic zones are shallow, i.e. 10-20 km. The shallow seismogenic zone is further supported by high seismic velocity. We also demarcated the geometry of mapped faults in Assam Valley and Shillong Plateau regions using cross-sectional seismicity plots. It is however noted that more data are needed for the Himalayan collision zone and Northeast syntaxis where multiple faults are involved.