Evolution of the 2015 Mw 7.9 Gorkha earthquake in Nepal Himalaya: Insights from local and teleseismic earthquake analysis

A portion of the central segment of the Himalayan orogen was partially ruptured across ~150 km during the 2015 Mw 7.9 Gorkha earthquake, which caused an overwhelming fatality of ~9000 lives. Study of the immediate aftershocks reported an eastward migration of the seismic front. Whereas, spatial average provided hints at fluid migration along the Main Himalayan Thrust (MHT) owing to the presence of a prominent low-velocity layer at MHT with fluctuating anisotropic directions. In this study, we employ the local and teleseismic earthquakes recorded at the year-long (2015-2016) deployment of seismic stations at the NAMASTE (Nepal Array Measuring Aftershock Seismicity Trailing Earthquake) network to investigate the temporal evolution of the aftershock sequence. The seismic front is observed to migrate eastwards immediately after the mainshock as reported, but analysis of the prolonged activity over the year reveals that the locus of seismicity migrates back from east to west towards the mainshock hypocenter. Shear wave splitting measurements extracted from local earthquakes indicate E-W realignment of the fast polarization direction of the aftershocks, as opposed to the initial NNW-SSE direction of the mainshock. A prominent low-velocity layer, discerned from receiver functions computed using teleseismic earthquakes, is observed to migrate along the rupture zone of the aftershock sequence. The changing direction of fluid migration along the low-velocity rupture zone at MHT could possibly be inciting the oscillation of anisotropic characteristic of the crust as the aftershock sequence evolves. We intend to further investigate and validate the observations through finite element modelling to constrain the variability of stress redistribution following a major earthquake, and gain insights on evolution of earthquake cycles in an orogenic setting.