Source-Characterisation of two moderate-sized earthquakes in Nepal employing detailed Moment Tensor Analysis

Two moderate-sized earthquakes occurred recently in Nepal on 8th November 2022 (Mw=6.1) and on 12th November 2022 (Mw=6.0). The location of these earthquakes falls at 384km and 329km respectively NW of the 25th April 2015 Gorkha earthquake. Both these earthquakes have been studied to understand the source rupture process employing Moment Tensor analysis by estimating the focal mechanism, and source parameters and relocating their hypocentral parameters using their regional waveforms recorded by the 18-stations broadband seismograph network deployed around Tehri dam in the Garhwal Himalaya Uttarakhand.

The epicentral distance of all the stations was less than 10◦. We employ the Moment Tensor Inversion approach to invert the broadband waveforms for the mechanism and depths and assume a one-dimensional velocity model developed for the adjacent Himalayan Region. Moment tensor solutions of the events were calculated along with the simultaneous calculation of the centroid position. Joint analysis of the hypocenter position, centroid position, and nodal planes produced clear outlines of Himalayan Fault lines. The epicenter of these earthquakes is located south of the MCT.

The obtained source mechanisms are consistent with those reported in the USGS centroid moment tensor (CMT) and NEIC solutions. We evaluated the performance of waveform inversion with just two broadband stations, and the result seems extremely reliable.

Inversion results indicate that the focal mechanism of the 8th Nov 2022 earthquake is a thrust fault type, and the obtained strike (283◦), dip (43◦), and rake (83◦) from the Present study are in accordance with CMT results of the USGS (strike (286◦), dip (58◦), and rake (97◦)) and NEIC (strike (285◦), dip (37◦), and rake (82◦)). The seismic moment is 1.583e+19 and the obtained DC% is 93% hence, the mechanism is considered to be Double-Couple. The identification of the source parameters is significant to the investigation of seismic hazards in this region.