The Uttarakhand Himalaya: An image of the main Himalayan thrust, Moho, and lithosphere-asthenosphere boundary

Abstract

We use seismic velocity tomography and Common Conversion Point (CCP) stacked radial PRFs to model the main Himalayan thrust (MHT), Moho, and Lithosphere-Asthenosphere boundary (LAB) beneath the Uttarakhand Himalaya using broadband data from 55 three-component seismographs. The Main Himalayan Thrust (MHT) between 8 and 20 km depth, which our modelling identifies as the source of the majority of the recent micro-earthquake activity, is characterised by larger negative Ps/P amplitudes, a 10-20% drop in Vp and Vs, and a 10-15% increase in Vp/Vs. We simulate a strongly undulating northeast-dipping crust-mantle interface between 30 and 55 km depth (with larger positive Ps/P amplitudes). In addition, we establish a lithosphere-asthenosphere boundary with a northeast-dipping depth between 130 and 180 km (with larger negative Ps/P amplitudes). Both the presence of metamorphic/aqueous fluids and the predicted double Moho structure are hypothesised to result from the continued northward convergence of the Indian plate beneath the most active rupture zone in the Garhwal - Kumaon Himalaya. One possible explanation for the double Moho structure is that the Indian crust and upper mantle became entangled during the collision of the Indian and Eurasian plates. Above the aforementioned rupture zone, where all Mw6.5 events have occurred so far, the upper crust is likely accumulating the strain energy needed to generate moderate to large earthquakes on the MHT.