GEOMETRY OF THE MAIN HIMALAYAN THRUST FAULT REVEALED BY THE GORKHA EARTHQUAKE

Large thrust faults accommodate crustal shortening caused by the collision of tectonic plates, contributing to the growth of topography over geological timescales. The Himalayan belt, which results from the collision of India into Asia, has been the locus of some of the largest earthquakes to strike the continents, including the recent 2015 magnitude 7.8 Gorkha earthquake. Competing hypotheses have been proposed to explain how topography is sustained and how the current convergence across the Himalaya is accommodated – whether this is predominately along a single thrust or is more distributed, involving out-of-sequence additional faulting. Here we use geodetically-derived surface displacements to show that whilst the Gorkha earthquake was blind, it ruptured the Main Himalayan Thrust (MHT), highlighting its ramp-and-flat geometry. Reconciling independent geological, geomorphological, geophysical and geodetic observations, we quantify the geometry of the MHT in the Kathmandu area. Present-day convergence across the Himalaya is mostly accommodated along the MHT, and no out-of-sequence thrusting is required to explain the higher uplift and incision rates at the front of the high range. In addition to the region west of the Gorkha rupture, a large portion of the MHT remains unbroken south of Kathmandu presenting a continuing seismic hazard.