Three-dimensional velocity structure around Tehri region of the Garhwal Lesser Himalaya: constraints on geometry of the underthrusting Indian plate

We investigate the upper crustal velocity structure beneath the Tehri region of the Garhwal Himalaya. The investigated region is situated within the 700-km-long central seismic gap of the Himalaya that has experienced three gap-filling earthquakes since 1991 including the recent 2015 Nepal earthquake (M-w 7.8). The local tomographic inversion is based on a data set of 1365 events collected from 2008 January to 2012 December by a 12-station local network that covers an area of about 100 x 80 km around Tehri Dam. We perform a simultaneous inversion for P-and S-wave velocity anomalies. Tomograms are interpreted in the backdrop of the regional geological and tectonic framework of the region. The spatial distribution of relocated events from the 3-D velocity model has shed new light on the pattern of seismicity in the vicinity of the Main Central thrust (MCT), and has elucidated the structure of the underthrusting Indian plate. Our model exhibits a significant negative velocity anomaly up to similar to 5 per cent beneath the central part of the Garhwal Inner Lesser Himalaya, and a P-wave low velocity anomaly near the Chamoli region. The seismicity zone around the Chamoli region may be attributed to the presence of fluid-filled rocks. Furthermore, an area with similar to 3-4 per cent positive velocity anomaly is delineated to the northwest of the Uttarkashi thrust in the vicinity of the MCT. Significant findings of the study include: a flat-ramp-flat-type subsurface geometry of the underthrusting Indian plate below the Garhwal Himalaya, high-velocity images representing the trend and configuration of Delhi-Haridwar ridge below the Sub Himalaya and Lesser Himalaya and a seismically active zone representing geometrical asperity on the basement thrust in the vicinity of the MCT.