Local seismic shear-wave splitting characteristics at the source region of 2015 Nepal Gorkha MW 7.8 earthquake in the middle Himalayan orogenic belt

Summary This paper has analyzed the spatial and temporal distribution of the local seismic shear-wave splitting parameters at the source region of the Nepal Gorkha MW7.8 earthquake in 2015. There are significant spatial variations of valid results within the aftershock zone. The fast directions change abruptly from west to east, revealing heterogeneity in the physical property or geometry of the upper crustal medium along the strike of the seismogenic fault. Combined with seismic tomography and magnetotelluric sounding, it is inferred that the variation of the upper crust from west to east in this region may represent a response to the tearing of the Indian lithospheric plate. The aftershock area shows large time delays, with their average results decreasing southward, indicating that a large amount of stress accumulated in the upper crust along the seismogenic area during the seismogenic process of the Gorkha earthquake, with the stress gradually weakening from north to south. Furthermore, time delays in the dense aftershock zone remained highly unstable for nearly one year after the Gorkha earthquake, reflecting that the stress accumulated during the seismogenic process of this earthquake failed to fully release with the mainshock and strong aftershocks, and the stress still adjusted violently.