Seismic velocity imaging of the Kumaon–Garhwal Himalaya, India

Since the initial collision at 55 Ma, rocks of the Indian crust below the Himalayas have undergone modification chemically and compositionally due to the ongoing India–Asia convergence. The local earthquake tomography images a shallow (~ 1–2°) north-easterly dipping low-velocity layer (10–20% drop in Vp and Vs, 10–15% increase in Vp/Vs) beneath the region between 10 and 20 km depth, which is inferred as the main Himalayan thrust (MHT). The presence of this low-velocity layer may be attributed to the presence of aqueous/metamorphic fluids or high fluid pressure, which may trigger crustal earthquakes by lowering the frictional coefficient (~ 0.01–0.08) on the MHT. The 1803 Mw8.2 Garhwal, 1991 Mw6.8 Uttarkashi and 1999 Mw6.4 Chamoli earthquakes have also been modelled to be triggered on the MHT, by the presence of aqueous/metamorphic fluids or high pore-fluid pressure. Besides, our modelling predicts three un-ruptured similar low-velocity zones on the MHT for generating future moderate to large fluid-triggered earthquakes in the region. The mapped low-velocity anomalies at 25–35 km depths further support the idea of the presence of a relatively higher temperature due to the hotter mantle below, which induces ductile rheology that prevents the lower crustal seismicity.