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

Abstract

We investigate the upper crustal velocity structure beneath the Tehri region of the GarhwalHimalaya. The investigated region is situated within the 700-km-long central seismic gap of theHimalaya that has experienced three gap-filling earthquakes since 1991 including the recent2015 Nepal earthquake (Mw 7.8). The local tomographic inversion is based on a data set of 1365events collected from 2008 January to 2012 December by a 12-station local network that coversan area of about 100 × 80 km around Tehri Dam. We perform a simultaneous inversion forP-and S-wave velocity anomalies. Tomograms are interpreted in the backdrop of the regionalgeological and tectonic framework of the region. The spatial distribution of relocated eventsfrom the 3-D velocity model has shed new light on the pattern of seismicity in the vicinity ofthe Main Central thrust (MCT), and has elucidated the structure of the underthrusting Indianplate. Our model exhibits a significant negative velocity anomaly up to ~5 per cent beneaththe central part of the Garhwal Inner Lesser Himalaya, and a P-wave low velocity anomalynear the Chamoli region. The seismicity zone around the Chamoli region may be attributed tothe presence of fluid-filled rocks. Furthermore, an area with ~3-4 per cent positive velocityanomaly 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 theunderthrusting Indian plate below the Garhwal Himalaya, high-velocity images representingthe trend and configuration of Delhi-Haridwar ridge below the Sub Himalaya and LesserHimalaya and a seismically active zone representing geometrical asperity on the basementthrust in the vicinity of the MCT.