Modelling of Moment Tensors and Source Parameters of the 25 July 2021 Rare Lower Crustal Hyderabad (India) Earthquake of Mw 3.9

A seismic network of ten three-component broadband seismographs was deployed in and around Hyderabad city during September–October 2020, which enabled us to record a rare lower-crustal cratonic earthquake of M w 3.9 that occurred on 25 July 2021 (23:31:01.1) at 35 km depth below the region about 140 km south of Hyderabad city. Utilizing broadband waveforms from this network, we model source parameters and moment tensor solution of this rare event using simultaneous inversion of S-wave spectra and ISOLA software, respectively. The average corner frequency, seismic moment, moment magnitude, stress drop, and source radius are 3.87 Hz, 7.14E + 14 N-m, 3.8, 3.92 MPa, and 229 m, respectively. The modelled mean crustal Q varies from 506 (at VKB) to 4136 (at NLG), with an average of 2182 ± 1178, suggesting lower crustal attenuation below the Hyderabad region. We perform a deviatoric moment tensor inversion of multiple point sources on the band-passed (0.04–0.1 Hz) displacement traces of the Hyderabad event. The best fit is obtained at 35 km centroid depth, with a moment magnitude of 3.9, and a normal dip-slip mechanism with a minor strike-slip component with strike of 169°, dip of 65°, and rake of −113°. The P-axis orients N42°E, which is parallel to the direction of the absolute plate motion direction of the Indian Plate, while the T-axis orients E–W. The occurrence of this earthquake could be attributed to the sudden movement on the almost-vertical plane in the viscous lower crust due to high fluid pore pressure resulting from the presence of CO 2 -rich mantle fluids.