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

Abstract

The optimum 1D velocity model is calculated for the Kinnaur sector of the NW Himalaya utilizing the arrival time information ofthe local earthquakes (137 no.) recorded with 12 broadband seismic network within the azimuthal gap of ≤180°. This optimum 1Dvelocity model is a five-layer model and ranges from the surface to 90 km in the shallow mantle. P velocity varies from 5.5 km/s to8.6 km/s in the crust and upper mantle, and S-wave velocity varies between 3.2 km/s and 4.9 km/s for the same range. When werelocated the earthquakes with the Joint Hypocenter Determination program incorporating the optimum 1D velocity model, itresulted in a lower RMS residual error of 0.23 s for the hypocenter locations compared to initial hypo71 locations. A total of1274 P and 1272 S arrival times were utilized to compute station delays. We observed positive variations in P-station delaysfrom -0.19 s below the PULG station to 0.11 s below the SRHN station. Similarly, for S-station delays, we observed negativedelays at each individual site from -0.65 s at LOSR station to -0.16 s at the SRHN station. This large variation in P-and Sstationdelays corresponds to the 3D nature of the subsurface below the Kinnaur Himalaya. The relocated seismicity isclustered along the STD fault at sub-Moho and Moho depths ranging between 40 km and 80 km. The seismicity distributionaligned across the strike of the STD and along the strike of the Kaurik-Chango fault (KCF) can be attributed to the cross-faultinteractions of the KCF and the STD fault in the area. We also observed bimodal depth distribution of seismicity in the Higherand Tethys Himalayas. The occurrence of earthquakes down to a depth of ~0-40km and 50-80 km in the study area can beinterpreted in terms of stress contribution from interseismic stress loading associated with the India-Eurasia collision tectonics.The presence of hypocentres in the shallow mantle at 120 km depth highlights the strength of the mantle, which seems to bedeforming in a brittle manner below the region. The computed focal mechanisms exhibit generally the flexing of the Indianplate below the Lesser Himalaya with shear parallel to the strike of the MCT and extension orthogonal to it. This study showsdeformation over the entire crust and shallow upper mantle levels, with differential stress conditions. Thus, we can considerthe crust and the shallow upper mantle down to depths of 120 km to be seismogenic in nature and is capable of producing themicroseismicity beneath the Kinnaur Himalaya.