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

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.

Velocity Structure and Deep Earthquakes beneath the Kinnaur, NW Himalaya: Constraints from Relocated Seismicity and Moment Tensor Analysis

“…The crust is 46 km thick. The model also observed low velocity at 12 and 14 km depth, respectively, which it ascribed to the fractured basement thrust representing the upper surface of the Indian plate [26].…”

“…We noted that for the 1D model of Parija et al [37] yield an average rms residual error of 0.27 s in the final iteration in hypocenter locations, which was initially up to 1.04 s during the first iteration. Again similarly, the other two initial models for the Garhwal Himalaya by Kanaujia et al [26] and the Mahesh et al [41] resulted in the average rms residual error in the hypocenter locations of 0.42 s and 0.29 s, respectively, in their respective final iterations. Then lastly, the other two initial velocity models of Monsalve et al [24] for the Nepal Himalaya and the South Tibet were considered for carrying out the inversion using the same procedure in VELEST and the inversion resulted in 10 iterations for each input velocity model and ended up with a minimum average residual error of 0.66 s and 0.24 s, respectively, for the hypocenter locations.…”

“…After this, we tested with another available velocity model for the nearby Tehri region of the Garhwal section of the northwest Himalaya proposed by Kannuajia et al [26] utilizing the local seismicity. The model determined in the VELEST analysis consists of six layers between the surface and 24 km depth.…”

“…Details of the initial velocity models are provided under Table S1 to Table S5 in the supplementary materials. The initial velocity models utilized for the travel time inversion were (1) 1D velocity of Parija et al [37] for the northwest Himalaya, (2) 1D velocity of Kanaujia et al [26], (3) 1D velocity of Mahesh et al [41], (4) 1D velocity model of Monsalve et al [24] for Nepal, and (5) 1D velocity model of Monsalve et al [24] for southern Tibet. For each initial velocity model, the process of inversion was terminated when average rms did not change significantly in successive iterations.…”

“…An accurate seismic velocity model is essential for understanding the active tectonic deformations and evaluating earthquake hazard. Although the seismic structure of the Kangra and Garhwal sector of the Himalaya have been extensively studied [26][27][28], little is known of the Kinnaur sector. In this study, we utilize recordings from a 12 broadband seismographs located in the Kinnaur region to determine a one-dimensional (1D) compressional and shear wave velocity model and use this model to evaluate the dynamics of the deformation within the source zone of the 1975 Ms 6.8 Kinnaur earthquake.…”