Complex seismic image of the Main Himalayan Thrust in Garhwal and implication to earthquake hazard

Abstract

The largest Himalayan earthquakes generally occur on the Main Himalayan Thrust (MHT), a shallow dipping decollement along which the Indian crust thrusts under the Himalayas, and propagate up to the southern front of the Himalayas where the MHT emerges at the surface. Accurate imaging of MHT is critical to quantify the earthquake hazard potential as well as understand the forces that control mountain heights. We present a high-resolution image of the MHT across the strike of Garhwal Himalaya using ambient noise tomography from waveforms recorded on a closely spaced linear profile of 26 broadband seismographs. Two important findings emerge from this study. First, evidence for two distinct ramps on the MHT – a gentle dipping (~9°) at shallow depth (~7–12 km) located 40 km north of the Main Boundary Thrust, and other a steeply dipping (~30–35°) at ~15–25 km depth beneath the higher Himalayan front. Second, the MHT in Garhwal Himalaya extends beyond the Main Frontal Thrust into the Ganga basin possibly as a blind thrust. Both the inferences, double ramp on the MHT, and possible extension of MHT in the Ganga basin have significant implications for the evolution of Himalayan topography and earthquake hazard potential in the Garhwal region and cities in the Ganga basin.