Shear‐Wave Velocity Reveals Heterogeneous Geometry of the Main Himalayan Thrust System and Deep Structure Beneath the Nepal Himalayas

Abstract

AbstractThe Himalayas is currently rising due to the collision of the Indian and Asian plates and hosts frequent earthquakes, some of which are devastating, such as the 2015 Mw7.8 Gorkha earthquake. Despite the importance of deep dynamic processes to understand the uplift of the Himalayas and the occurrence of large earthquakes, it remains limitedly constrained due to the lack of a detailed three‐dimensional subsurface image under this region. Here, we construct new models of shear‐wave velocity and radial anisotropy down to the 150 km depth from Rayleigh‐ and Love‐wave tomography in the Nepal Himalayas. We find that the 2015 Gorkha earthquake and its main aftershock occurred in a velocity contrast that is presumably interpreted as Main Himalayan Thrust (MHT). A duplex structure, imaged as relatively high velocities, is inferred to exist above MHT under the Lesser Himalayas. This duplex shows heterogeneous features along the strike of the Himalayas that may control the rupture behavior during the occurrence of a large earthquake. Additionally, a low‐velocity anomaly is observed at depths from Moho to 100 km under the Lhasa Terrane and north of the Himalayan Terrane between 85° and 88°E. We interpret this low‐velocity anomaly to be likely caused by mantle upwelling resulting from either possible Indian slab tearing, or northward subduction of the Indian plate. If this is the case, the north‐south trending rifts that situate within the dispersal of the low‐velocity anomaly are probably associated with the mantle upwelling. This study provides a new independent constraint on the geometry of the MHT system and deep dynamic processes occurring in the Nepal Himalaya.