Seismological evidence and source characteristics of 28 April 2021 Assam valley earthquake sequence in Northeast India: Kopili fault activation and seismic hazard implications

A damaging earthquake (Mw6.4) occurred in the Assam valley on April 28, 2021, followed by about 60 earthquakes (M ≤ 4.6). The mainshock and its aftershocks occurred within the area of ~ 20 × 20 km2 that is oriented in the NW–SE direction, indicating the rupture area of the mainshock. Using full-waveform inversion, we determined the focal mechanisms of the mainshock and its aftershocks, which revealed predominantly strike-slip faulting on a gently northeast-dipping plane extending to 50 km depth. The seismic activity aligns with the NW-SE dipping nodal plane, akin to the Kopili Fault (KF) zone. The alignment of P-axis trends, oriented NE-SW to NNE-SSW, correlates with the movement of the Indian Plate. Strong motion records from various sites in NE India suggest rupture directivity in the NW–SE direction, consistent with the orientation of the KF. The mainshock exhibited a high-stress drop of ~ 80 bars with a significant variation in PGA value from 0.7 to 265 cm/s2. Coulomb failure stress changes (ΔCFS) associated with the mainshock, corresponding to the causative KF, clearly demonstrate that KF is a seismically activated fault in which almost all aftershocks are located in a zone of compression, corroborating the NW-SE dipping source. We observed that the locations of aftershocks were directed towards the main boundary thrust (MBT), suggesting the transfer of stress towards the MBT from the KF. The strong correlation persists between the static stress changes and the aftershocks, indicating KF as the causative fault, which becomes activated due to appreciable structural heterogeneities associated with the KF. Peak Ground Acceleration, variability with its maximum 265 gals, shows varying seismogenic strengths beneath the earthquake sources in and around the KF. The rupture propagation showed diversity in source parameters, reflected through different particle motions and Coulomb stress perturbations towards the MBT. The observed seismicity patterns and source parameters offer valuable insights into earthquake behavior in and around KF, cautioning us to adopt comprehensive seismic hazard reassessment by adopting risk mitigation strategies for the region with the activated KF in Assam, India.