1D Velocity Structure and Characteristics of Contemporary Local Seismicity around the Tehri Region, Garhwal Himalaya

Abstract

Abstract A 1D velocity model of the Tehri region in the Garhwal Himalaya is estimated from the travel‐time inversion of 145 well‐located local events having 1177 P and 1090 S arrivals. The velocity model consists of six layers up to 24 km depth, with P ‐ and S ‐wave velocities ranging from 4.42 to 6.78 km/s and 2.41 to 3.71 km/s, respectively. The depth of the Moho, estimated using travel‐time curves of crustal phases, is about 46 km. A low‐velocity layer deciphered between 12 and 14 km depths is ascribed to fractured basement thrust representing the upper surface of the Indian plate. Using the proposed velocity model, 1457 events are relocated. About 70% of the locatable events occur in the Inner Lesser Himalaya between the Main Central thrust (MCT) and the Srinagar thrust. The postulated depth of the basement thrust in the vicinity of the MCT is about 10–12 km. The depth distribution of events delineates the geometry of the seismically active Main Himalayan thrust (MHT) below a 300‐km‐long segment of the MCT. The MHT is composed of two shallow‐dipping fracture zones that seem to represent seismically active thrust zones dipping in opposite directions. Two seismicity zones, at 10 and 15 km depths with a 5 km vertical separation, define a flat‐ramp‐flat type structure of the MHT in the vicinity of the MCT. The postulated front of the underthrusting Indian plate is at a depth of about 15–18 km. The lower‐flat seismicity zone bifurcates into two, indicating further slicing of the lower‐flat zone. The postulated thickness of the brittle part of the underthrusting Indian crust is about 20 km in the vicinity of the MCT.