Abstract
The 28 April 2021 earthquake Mw 6.1 (epicentre 26.69° N, 92.36° E; centroid depth ~ 40 km GCMT report,
www.globalcmt.org
) in the Sonitpur district of Assam valley, North East (NE) India occurred at the Kopili fault (KF) zone, a long transverse structure across the valley. The earthquake caused a few casualties and injuries and some damage to buildings, ground fractures, fountains and liquefactions, landslides and other such manifestations in several parts of the valley, with a maximum intensity of MMI VI. The KF is identified as a seismically most active fault that caused two (1869 and 1943) large earthquakes (Mw > 7.0) in the past and several severely felt earthquakes (Mw ≥ 5.0) in recent years. Fault plane solutions of the 28 April 2021 mainshock and six aftershocks (Mw > 3.5) are determined by waveform inversion using broadband seismograms of the nearest observatory at Tezpur. The solutions are comparable to the GCMT solution of the mainshock and correlated with the known right-lateral strike-slip motion at the KF zone. The low-stress drops of the earthquake sequence indicate relatively long source duration and small co-seismic slip of the mainshock and the aftershocks. The low-stress drop in terms of lower energy release may be due to reactivation of the KF. The low-stress drop also indicates a pre-existing fault zone at greater depth. A stress inversion analysis of the 17 available fault plane solutions indicates a NNE–SSW compressional and NNW–SSE tensional stress regime in the KF zone. The Coulomb stress change map, on the other hand, identifies the zone of triggered aftershocks.