Seismicity Distribution and Focal Mechanisms of Gorkha Earthquake and its Aftershock

Nepal is located in a seismically active region, prone to high magnitude events. It has experience of major earthquakes in its history due to ongoing collision of India with Asia which is accommodated by slip on the Main Himalayan Thrust (MHT). On the 2015 April 25, at 11.56am , Gorkha earthquake of Mw 7.8 occurred at about 77km NW of Kathmandu (capital of Nepal) with the epicentre located near Barpak village. It is followed by the successive aftershocks towards the east of the main shock, causing great economic loss and casualties. This earthquake is presumed to be a repeat of 1833 earthquake indicating an average interseismic period of 200 years. This study shows result of focal mechanisms from 12 local station strong motion data of main event and its aftershocks.In this study aftershocks were used to understand the rupture extent, static stress changes with large earthquake and fault plane parameters. The focal mechanism solution of 34 significant earthquakes in this area with magnitude ML 4 to 7.6 (including main event) covering the area of 150-km-long rupture of the Himalayan décollement, were analyzed along with the stress changes with respect to the tectonic structures of this area. The focal depth of these event are in the range of 2 to 40km. The focal solution towards the northern part of the slip area shows the normal to oblique normal strike slip whereas the southern part north of the MHT shows almost all are thrust fault with higher depth than the former.