Fault Zone Rocks Associated With the Reservoir‐Triggered Seismicity of the Koyna‐Warna Region (India)

Abstract In 1962 the Koyna dam was completed 220 km southeast of Mumbai (India). Since then, the area has experienced recurrent seismicity, including one of the largest human‐induced earthquakes so far (1967, M w 6.3). An extensive deep drilling project was carried out to understand this seismicity. Earthquakes are modulated by the monsoon regime and reactivate steeply dipping faults/fractures located at 3–10 km depth in the crystalline basement at ambient temperature between 70°C and 170°C. Fault zone rocks recovered from KBH1, KBH6, and KBH7 wells (∼1.5 km deep) allowed us to reconstruct the sequence of deformation events. Steeply dipping faults/fractures often filled with chlorite (lowest formation temperature 129°C ± 41°C) and calcite (precipitated from meteoric‐derived fluids) record the last brittle deformation event. We conclude that reservoir‐triggered seismicity, driven by infiltration of meteoric water at depth, pore pressure diffusion, and poroelastic response of the rock mass to reservoir operations, occurs in sub‐vertical faults/fractures often filled with calcite and chlorite.