One-Dimensional Reference Velocity Model and Precise Locations of Earthquake Hypocenters in the Kumaon-Garhwal Himalaya.

We report an optimum ID velocity model of the Kumaon-Garhwal Himalaya, based on analysis of --5000 seismograms generated at 50 digital broadband seismic stations in the region during April 2005 and June 2008. These include data from 385 local earthquakes with azimuth gaps of < 180°, each of which had a mini-mum of seven P- and five S-phase readings. This velocity model was then used to map the locations of 1150 earthquakes (1 < M < 5) that occurred in the region. A majority (~75%) of these earthquakes occurred in the upper 20 km of the crust, largely above the Main Himalayan Thrust, and forms an ~10-km-wide band along the surface trace of the Munsiari Thrust. However, we also find another parallel band of earthquakes, about 70 km to its southwest through the Tehri dam in the Lesser Himalaya, and a significant number (~15%) both in the Tethys Himalaya and within the flexed crust of the underthrusting Indian plate, beneath the Ganga basin. Furthermore, unlike the reported absence of seismicity in the lower crust of the Nepal Himalaya, and its reappearance in the shallow mantle, the Kumaon-Garhwal Himalayan crust is found to be seismically active throughout, with no event reliably recorded in the shallow mantle of this region during the epoch of our seismic recording. [ABSTRACT FROM AUTHOR]