Revisiting the reservoir-triggered seismicity of Koyna India using near-source seismological observations and relationship with reservoir water level changes

The Koyna-Warna region in western Maharashtra, India, continues to experience seismic activity, even six decades after the impoundment of Koyna Reservoir. The seismicity of Koyna region is primarily linked to the Donichawadi fault zone, which ruptured the surface during the 1967 M6.3 Koyna earthquake. In 2017, a 3 km-deep scientific borehole (Koyna Pilot Borehole, KFD1) was drilled, revealing subsurface fault characteristics through downhole geophysical logs. The present study utilizes data acquired from a near-field, 5-station broadband seismological network surrounding KFD1, which has been operational since January 2022. The analysis identifies a zone of recurrent earthquakes within a 1–2 km radius of KFD1, where 13 M > 2 earthquakes occurred over 21 months, recurring approximately every four months. Precise earthquake locations, obtained using an updated velocity model of the Koyna area, delineate four distinct sub-zones within the Koyna fault zone. The northernmost sub-zone produces stronger, shallower earthquakes (4–8 km deep) with a strike-slip faulting mechanism. The second sub-zone generates deeper earthquakes (7–10 km deep) with a combination of normal and strike-slip faulting mechanisms. The third sub-zone experiences recurrent seismic activity at 4–8 km depth, while the southernmost sub-zone produces smaller, shallower earthquakes (2–4 km deep). Identifying these sub-zones helps in understanding the underlying tectonic processes of the Koyna region.

Further analysis reveals a positive correlation between the water level fluctuation of Koyna reservoir and earthquake occurrence within the region. A statistical analysis of earthquake records from 2005 to 2017 yielded a correlation coefficient of 0.15. However, analysis focusing on recent, near-source seismic data of 2022 demonstrates an enhanced correlation of 0.25. These findings underscore the critical role of continued near-source monitoring in refining our understanding of reservoir-triggered seismicity in the Koyna region.