The 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 M(w)7.8
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 degrees and 88 degrees
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.