It is proposed here to discuss some case histories of artificial crustal disturbances in fractured rook mass. These case histories involve two oases of reservoir induced seismicity at Idukki and Kadana reservoirs, both situated in fractured rook terrain, but in different tectonic environment. The Idukki reservoir is situated in moderate seismic zone of southern India while Kadana reservoir is situated in a highly aseismic belt with no record of any significant earthquake. The Kadana dam site including reservoir area is situated in highly fractured terrain With nearby hot spring zone.
The third case history is that of rock-burst study in the deep Kolar Gold Mine area in South India. Analysis of time history of occurrence of numerous rook- bursts in the area indicate that large rook-burst up to Richter magnitude of 4.6 could occur following low b-value. indicating artificial accentuation of stress and, consequently, triggering of rock-bursts possibly due to mining activity, thereby altering the existing stress field. The Gold Mine area is situated in highly aseismic zone of granitic country rook. There is a fault closeby which exhibits intermittent and moderate seismic activity. The gold mine is the deepest in the country about 3000 m deep at deepest horizons. Fig. 1 shows an index map for the locations of the two reservoirs and the Gold mine under study with principal regional faults and seismic zones.
(Figure in full paper)
The Idukki dam (9–49'.8 N; 78–58'.2 E) is an arch dam 170 m high situated in moderately seismic zone in southernmost part of India. The area is a gravity low like other part of peninsular shield of India. Fig. 2 shows the dam, reservoir area the seismic monitoring stations and epicenters of earthquakes during post impounding period. The instruments consist of mechanical as well as very high gain electromagnetic instruments suitable for recording earthquakes down to magnitude 1.0in the Richter scale. The dam site has strong instrumentation (accelerograph). Thus the. seismic net has capability of recording macro- to microearthmovements. Reservoir induced seismicity is known to initiate at micro-level and to culminate as strong earth movements even upto magnitude 7.0 in the Richter scale as experienced at Koyna (India).
(Figure in full paper)
Flg.3 shows the time history of seismicity of the reservoir area along with lake level variations, observed tilt, and computed b-values where log N = a - bM. It is interesting to note that low b-values precede significant earthquakes - an important observation in earthquake prediction research. Similar observations on b-values have been observed. in case of other reservoirs namely Koyna, Mula and Kariba (Padale et al, 1979). Epicentres of the earthquakes during the pest impounding period cluster around the reservoir, indicating post impounding seismicity of the area.
Figs.4 and 5 show respectively distribution of epicentres and time history of seismicity in the Kadana reservoir area (23°18'.3 N; 73°491.6 E). In contrast to the Idukki reservoir, Kadana reservoir triggered fewer shocks of very small magnitude below 2.5. Water load. was similarly" smaller in the Kadana reservoir.
