Geodetically constrained slip on the Main Himalayan Thrust fault from the 2015 Gorkha earthquake

Abstract:

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. Whilst the vast majority of slip is buried at depth, triggered near surface slip was imaged in the Sentinel-1 coseismic interferograms along a 26 km long discontinuity, 10 km north of the Main Frontal Thrust. This surface break follows the trace of the Main Dun Thrust (MDT), a relatively minor splay. This displacement is seen to grow in the central portion of the splay in the proceeding week. Slip from the largest (Mw 7.3) aftershock that occurred 17 days later fills in most of the eastern gap in the slip contours of the mainshock at the lower edge of the fault rupture. 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. At the shallow end of the rupture, slip tapers off sharply and is markedly uniform along strike for the 140 km length rupture, and at a near constant depth of 11 km. Such a sharp limit on slip could result from the soleing out of other thrusts such as the MBT onto the MHT, resulting in structural complexity on the fault interface and a wide damage zone impeding up-dip rupture propagation, leaving a wide locked width at shallower depth.