ABSTRACT
It is now well recognized that the earthquakes are concentrated along the margins of plates, and are caused when the plates converge, diverge or slide past each other. Such earthquakes occur on faults, some of which may be deep-seated. Analysis of damage due to earthquakes has shown that the extent of ground-shaking
and resultant damage are critically dependent upon the nature and properties of rocks and soils at or near the ground surface. Many countries prepared seismic risk maps on the basis of the past records (seismicity) in an area, and the structure and composition of rocks and soils. Earthquakes could also occur due to anthropogenic activities, such as the creation of
large water reservoirs, mining, and forceful injection of fluids into pores and cracks in crustal rocks. There is no unequivocal correlation between the height of the dam, the volume of water impounded and the magnitude of the largest earthquake. It appears that the crucial factor is the hydraulic continuity between the body of the impounded water and the deeper groundwater (facilitated by deep-seated faults?). “If there is no water in the rocks, there would be no tectonic earthquakes’’ (Bolt, 1993). The reservoir-induced Koyna earthquake of Dec. 10, 1967, killed about 200 people, and caused severe damage to Koyna Nagar Township in western India. Earthquakes of magnitude equal to or greater than 5 seem to occur when the rate of loading in the reservoir exceeded 13m per week (Gupta, 1993). Animals appear to be sense the low-magnitude precursors of the earthquakes which
the human beings cannot sense (e.g. snakes coming out of their holes). Using animal behaviour as an indicator of an imminent earthquake is, however, not easy.
