3D structural modelling of the Kopili fault zone in North-East India: Seismotectonic analysis utilising focal mechanism solutions of small-to-moderate earthquakes

This work presents new findings from recent seismic data which have never been exploited so far for the weak-to-moderately energetic (M_L 2.3–6.4) earthquakes and to depict a 3D scheme for the Kopili Fault (KF) Zone. For the first time, we figure out different fault kinematics in the upper crust such as reverse and strike-slip mechanisms located along the KF zone. Moment Tensor (MT) solutions are successfully obtained through full-waveform inversions for 60 seismic events. The stress inversions are estimated based on the MT results as well as previous findings from 16 small to moderate earthquakes (M_W 3.6–6.3). The MT solutions demonstrate that strike-slip kinematics is the most prevalent while a few thrust/normal earthquake events are observed in the area. The seismic cross-sections display that these earthquakes took place at a depth of 40 km and are mostly generated on the eastern side of the fault. According to the 3D spatial and temporal distribution of relocated events, the KF zone is not a completely vertical fault but rather one with a slight eastward dip. Our stress inversion results also indicate that this fault is equivalent to a fault plane with a dip angle of around 80˚, which is compatible with the previously estimated dip angle (75˚). We interpret that the research region is under a transpressional stress regime, which is likely to be the reason for the current activity of the KF zone. We postulate that our findings can be useful to enhance seismic hazard assessment in the densely populated region, which is also covered by vital infrastructure.