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Abstract:
Triggered seismicity in the Koyna region, starting soon after the impoundment of Shivajisagar water reservoir in 1962, has continued to this day. The activity, confined to 20 km x 30 km area and 10 km depth, is modulated by the annual loading and unloading cycles of the Shivajisagar and nearby Warna reservoirs. The 1967 M6.3 Koyna earthquake caused a NNE-trending surface rupture zone near Donichawadi, along which seismic activity in the past few decades is aligned.To investigate the recurrent seismicity, a 3 km deep scientific borehole was drilled in the vicinity of the Donichawadi fault. Downhole measurements of physical and mechanical properties, borehole images, and composition of formation gases in the deep crystalline basement, combined with seismological data and laboratory geological and rock mechanical datasets provide new insights on the underlying conditions favouring recurrent seismicity: (i) Helium concentrations in the borehole suggest that the subsurface fault damage zones potentially associated with the Donichawadi fault. (ii) Drill core samples show evidences of repeated brittle deformations (iii) Water bearing zones are identified at multiple depths, providing strong evidence for percolation of water to deeper levels. (iv) Stress regime and earthquake focal mechanisms support transitional strike-slip to normal faulting environment. (v) The presence of optimally oriented, critically stressed and hydraulically conductive fractures creates conducive conditions for reactivation of faults with small changes in fluid pressure and/or frictional strength.(vi) Strain budget estimates from seismological and GPS data suggest that release of strain energy in terms of earthquake activity may continue for a longer time.
