UNZIPPING OF LOCKED MHT BY THE 2015, MW 7.8 GORKHA EARTHQUAKE, NEPAL

We use high-rate GPS, seismological and Synthetic Aperture Radar imagery (SAR) measurements to produce a detailed image of the seismic rupture during the 2015 Mw 7.8 Gorkha earthquake, Nepal. The earthquake ruptured a 150x50km elliptical patch of the Main Himalayan Thrust (MHT) along which the Himalaya is thrust over India. The MHT was known from interseismic GPS measurements to be locked from the surface to beneath the front of the High Himalaya. The earthquake initiated 75km northwest of Kathmandu at the front of the High Himalaya and the seismic rupture propagated eastwards at ~2.8 km/s unzipping the lower edge of the locked zone over ~150km. High frequency seismic waves (~ 1 Hz) were radiated continuously in this process from this zone of presumably high and heterogeneous pre-seismic stress. Most of the moment was actually released updip of the sources of high frequency seismic waves on northern portion of the 12-15km deep decollement between Kathmnadu and the High Himalaya. The GPS record reveal a slip pulse of ~20 km width, ~6 s duration with peak sliding velocity of ~1 m/s.The slip pulse shows a remarkable smooth onset indicating a large effective slip-weakening distance of several meters. This smooth onset can explain the moderate ground shaking at high frequencies (>1Hz) and the limited damage to regular dwellings within Kathmandu basin. By contrast, the entire basin resonated at ~4-5 s resulting in the collapse of some tall buildings. The study suggests a deterministic control, of probably structural origin, of the source characteristics and induced ground shaking. The Gorkha earthquake ruptured only a small fraction of the Locked portion of the MHT so that larger earthquakes are needed to transfer interseismic strain to the sub-Himalaya where most of the 2cm/yr convergence across the range is accommodated over a millenaries time scale.