Source Parameters of the 25th April 2015 Nepal Earthquake and its Aftershocks

The major Mw = 7.8 earthquake which rocked the Nepal Himalaya on 25th April 2015 is the largest to have occurred in this region in the past 81 years. This event occurred by slip on a ~150 km long and 55 km wide, shallow dipping (~5◦) segment of the Main Himalayan Thrust (MHT), causing the Himalaya to lurch southwestward by 4.8±1.2 m over the Indian plate. The mainshock ruptured the down-dip end of the frictionally locked segment of the MHT, initiating near the locking line and propagated up-dip. The rupture stopped ~8 km below the surface, loading the up-dip end of the MHT. The mainshock was followed by a large number of aftershocks among which four were larger than magnitude (Mw) 6.5. These strong aftershocks occurred on fault(s) with similar orientation as the mainshock, contributing to strain release on the MHT. Moderate-to-light aftershocks originated close to the up-dip edge of the MHT, close to where the mainshock rupture stopped. We use regional and local waveform data to (a) relocate these aftershocks and (b) model the source mechanism of the moderate aftershocks (Mw>5.0). This will provide valuable constraints on the up-dip geometry of the MHT, presence of ramps and associated back thrust underneath the Himalayan foothills. Our study will also highlight the adjacent segments of the Himalaya where significant unrelieved elastic strain has accumulated and may rupture in similar or greater earthquakes in the future.