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.

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<p>Telesesimic earthquake data recorded at eight seismograph stations across the northeast India are analysed for shear-wave splitting from core-refracted XKS phases (collectively PKS, SKS and SKKS). Shear-wave splitting parameters, derived from the analysis provide information about seismic anisotropy and deformation of the crust and upper mantle beneath each seismograph stations site. The results point towards the presence of complex and highly anisotropic crust and upper mantle beneath northeast India. Being surrounded by two seismically active plate boundaries, to the north by India-Eurasia collision plate boundary and to the east by Indo-Burman subduction plate boundary, the crust and upper mantle beneath the northeast India has been assumed to have complex deformation pattern. This present study provides an evidence for this assumption. According to station locations, we have one station BONG situated near the Main boundary thrust (at India-Eurasia collision zone), one station NAMS and eastern syntexis Himalaya, five station AZWL, SILS, DIPH and NKCR at Indo-Burman subduction plate boundary, one station SHLS and Shillong plateau bounded by Oldham Fault, Dauki Fault and Kopli fault, and one station AGAR at the boundary of Bengal basin. The direction of anisotropy is nearly E-W at BONG, NE-SW in the Indo-Burman subduction zone, nearly N-S on Shillong plateau and NW-SE at eastern syntexis of Himalaya. Source of anisotropy in the Himalaya collision boundary is result of lithospheric deformation due to finite strain induced by collision. In Shillong plateau and Indo-burman subduction boundary, source of anisotropy seems to be the asthenospheric flow-related strain which is also in harmony with the absolute plate motion (APM) of the Indian plate in a no net reference frame.</p>

High-quality data recorded by a dense network of 53 seismic stations in the Garhwal–Kumaun Himalaya between February 2017 and December 2021 is analyzed. A total of 813 local earthquakes are relocated using a newly developed regional 1D velocity model incorporating station corrections. In addition, focal mechanism solutions of M ≥ 3.8 events are estimated using waveform inversion. The relocated seismicity patterns along with the focal mechanism solutions are utilized to present a seismotectonic scenario of the region. Almost 95% of the relocated seismicity is found to be clustered along the Himalayan seismic belt (HSB), down to ∼24 km depth. Seismicity in this belt is interpreted to be caused due to interseismic stress loading associated with the ongoing India–Eurasia collision tectonics. A few scattered hypocenters in the deeper crust between 30 and 50 km depth attest the strength of the downgoing Indian plate. Focal mechanisms in the seismogenic upper crust reveal thrusting of the Indian plate beneath the Lesser Himalaya, with compression normal to the strike of the Main Central Thrust (MCT). The north-dipping thrust mechanisms can be associated with a near-horizontal Main Himalayan Thrust (MHT). In addition, more steeply dipping faults above it define the Lesser Himalayan duplex systems, similar to those in western and Nepal Himalaya. A prominent ∼50 km wide seismicity gap region observed within the HSB is probably due to (1) a locally varying locking width of the MHT; (2) an unruptured, ductile segment at the eastern end of the rupture zone of the great 1803 earthquake (Mw 7.8 ± 0.2); and (3) a slab tear in the MHT, similar to those in subduction zones.