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Abstract:
The Koyna-Warna seismic zone, located in peninsular India is characterized by persistent seismic activity within an area of 20 km x 30 km area since the impoundment of the Shivajisagar water reservoir in 1962. In general, the hypocenters lie in the depth range 2-10 km, in the ~2.4-2.7 Ga granitic basement that underlies ~1 km - thick Deccan trap cover. Fresh core samples of the basement granitoids recovered from recent scientific drilling in the area have been studied. Physical studies suggest a brittle deformation sequence from cataclasite to fault breccia and fault gouge materials. These rocks contain meso- to microscopic scale fractures which are mostly inter- or trans-granular in nature. Widths of fractures vary from micrometer to centimeter scale, and the anastomosing nature facilitates fluid channelization potentially down to the hypocentral depths. Microstructural and geochemical studies suggest most of those are partially or completely sealed by ferruginous, siliceous and carbonate materials. Such secondary materials present in both crystal and amorphous forms imply a conducive setting for mineralization. The fractures dominantly control deformations nearby the hypocentral depths and secondary mineralizations. They also can be correlated with related changes in pore pressure and temperature during earthquakes.
