Case histories of some artificial crustal disturbances

Abstract The Shivajisagar lake area of the Koyna Hydroelectric Project, situated in the Deccan Trap region of the aseismic Indian Shield began to exhibit mild seismicity immediately after the impounding of the reservoir in 1963. This mild seismicity culminated in a major earthquake of magnitude 7.0 on December 10, 1967, followed by numerous aftershocks of diminishing magnitude and frequency, obeying a relationship of the type t − b . It has been observed that a significant part of these earthquakes, especially the aftershocks, was found to be correlated with reservoir water load. In the case of a few new water reservoirs in India, situated mostly in marginal areas of the aseismic Indian Shield, very minor seismic activity, though of an isolated nature, was recently noticed after impounding. On the other hand, very large reservoirs situated well inside the stable Indian Shield, did not exhibit any noticeable seismic activity after impounding. Studies with an ultra-high-sensitivity seismograph (magnification∼ 10 6 ) near a newly impounded reservoir in the Deccan Trap reveal evidence of the occurrence of minute tremors of magnitude below zero. Thus, the geographical distribution of the above water reservoirs exhibiting seismic activity implies the probable influence of a geotectonic pattern in the process.