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The massive M_w = 7.8 earthquake which rocked the Nepal Himalaya on 25 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 main shock ruptured the frictionally locked segment of the MHT, initiating near the locking line and rupturing all the way updip close to its surface expression near the foothills of the Himalaya. The main shock was followed by 41 aftershocks within 26 h, among which a couple were larger than magnitude (M_w) 6.5. These two large aftershocks occurred on fault(s) which had similar orientation as the one that caused the main shock, contributing to strain release along the MHT. The rupture area of the main shock overlaps the meisoseismal zone of the 1833 Nepal earthquake and is immediately to the west of the 1934 Bihar-Nepal earthquake. This region had accumulated ~3 m of slip in the past 182 years, converging at a rate of ~18 mm/yr. The close match of the accumulated slip with the coseismic slip of the main event confirms that majority of the convergence between India and Tibet is stored as elastic strain energy and is released by brittle failure in earthquakes. This Nepal earthquake has highlighted that other segments of the Himalaya too have significant unrelieved elastic strain and may also rupture in similar or greater earthquakes in the future.

Keywords

Earthquake, Rupture Parameters, Source Mechanism, Seismotectonics.

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