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ABSTRACT: The variable characteristics exhibited by reservoir induced seismicity (RIS) can be adequately explained by the reduction in effective in situ stresses, brought about by impounding. Vertical reservoir loading has negligible impact and the controlling factor is the horizontal stress condition at any depth. This stress condition can be imposed on the crust by geoid changes. Analysis by Mohr circles leads to the conclusion that the crystalline basement rocks frequently exist in a state of incipient failure, often tensile.
INTRODUCTION Over seventy case histories of reservoir induced seismicity (RIS) are cited by Gupta (1992). The induced earthquakes range from shallow, less than 1 – 2 km focal depth, to moderate crustal depths of 10 – 12 km. A large majority of events appear to fall within the 4 – 6 km depth range, which would place them within the crystalline basement rocks at levels not normally considered in dam site investigations. In a majority of cases, RIS commences with impounding and reaches a maximum at times of full supply level, or shortly after, Figure 1. In other cases, no direct correlation between water level and seismic activity is obvious, although a time lag is sometimes apparent in the association. Occasionally, the seismic activity does not commence until many years after the reservoir has been in operation: at Oroville Dam after twelve years; Aswan Dam after sixteen years; Hoover Dam after forty years. In yet other cases, it is the rate of water level change which correlates with the seismic activity. A notably large number of RIS sites occur in otherwise seismically "quiet" regions: locations such as the central China shield; the Indian Deccan; the African and North American shields. By contrast, large dams in the seismic zones of the Himalayas cause no apparent increase in activity.
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