15:30 〜 16:30
[S02-P-04] Deformations in rocks in the Koyna seismogenic zone, western India obtained through scientific deep drilling
The Koyna region, located in the Deccan Flood Basalt Province of western peninsular India has been experiencing reservoir triggered seismicity since the impoundment of the Shivajisagar water reservoir in 1962. The 1967 M6.3 Koyna earthquake, 22 earthquakes of M greater than equal to 5, more than 200 earthquakes of M greater than equal to 4 and several thousand smaller earthquakes have occurred in the depth range 2-10 km and over a restricted area of approximately 20km x 30km. Scientific drilling carried out recently up to a depth of 1522 m in the vicinity of the seismogenic zone exposed the cratonic granitoids underlying the Deccan flood basalt and provided a unique opportunity to study the deformation features in an area of active seismicity. The salient results are as follows. (1) The basement rocks are dominantly composed of granite, granite-gneiss and migmatitic gneiss, typical of the Dharwar craton of peninsular India. (2) The intensity of shear fabric in the basement rocks indicate the presence of a highly strained shear zone. (3) The granitoids have been affected by later brittle deformation. (4) Occurrences of faults and fractures associated with fault breccias, fault gouge and pseudotachylite veins injected along tensile fractures are the direct evidences of seismic activity. (5) Prominent slickenlines with slickensides on fault surface, granular flow of crushed material within fault zone and intensive shattering of rocks observed without abundant alteration are related with the recent seismic activity. (6) Fracture density varies within and away from the fault zone which are frequently filled up with ferruginous secondary precipitation, signifying water channelization at depth.