Duplex in the Main Himalayan Thrust illuminated by aftershocks of the 2015 Mw 7.8 Gorkha earthquake

Abstract

In April 2015, the lower locked portion of the Main Himalayan Thrust ruptured beneath Nepal, causing the disastrous Mw 7.8 Gorkha earthquake. Elucidating the enigmatic geometry of this plate boundary fault is important for understanding the nucleation and arrest of large earthquake ruptures as well as the seismic hazard, topography and tectonics of the Himalaya. Here we interpret the geometry of the Main Himalayan Thrust from the spatial distribution and rupture patterns of a dynamic sequence of aftershocks following the Gorkha earthquake, which were recorded by a rapidly deployed dense seismic network. We find that the thrust comprises two north-dipping subhorizontal planes that are connected by a system of bounded imbricate thrust faults; this structure is known as a duplex. We propose that this duplex acts as an impediment to plate convergence and accommodates tectonic stress along its complex system of faults. Such a prominent structure to the Main Himalayan Thrust is consistent with surface geological studies but challenges geophysically derived conventional models with simpler geometries. The Main Himalayan Thrust comprises two fault planes connected by imbricated faults, a structure that impedes convergence, according to an analysis of the distribution and orientation of aftershocks of the 2015 Gorkha earthquake in Nepal.