Weak, Vertically Stronger Main Himalayan Thrust in the India-Asia Collision

Abstract

Megathrusts at convergent plate boundaries generate the largest and some of the most hazardous earthquakes on Earth. However, their physical properties, including those influencing fault slip accumulation and release and earthquake-related surface displacements, are still poorly constrained at critical depths. Here, we combine seismic imaging and geodetic modeling to investigate the structure and mechanical behavior of the Main Himalayan Thrust fault (MHT) in the center of the 2015 Mw 7.8 Gorkha rupture in Nepal. Our results from two independent observations consistently suggest the presence of a channel associated with the MHT with high compliance (shear modulus as low as ∼4 GPa) and strain anisotropy (stiffer in the vertical orientation than in the horizontal), likely arising from a weak subducting layer with north-dipping foliation. Such mechanical heterogeneity significantly influences the quantification of short-term fault kinematics and associated earthquake potential, with implications on across-scale dynamics of plate boundaries in Himalaya and elsewhere.