| Seismogenic responses of a large reservoir depend on the site conditions as well as the characteristics of the reservoir. Observed seismicity at Monticello and Jocassee, South Carolina, Oroville, California, and Koyna, India, were examined to understand the factors that can influence the mechanism of Reservoir Induced Seismicity (RIS) and test the applicability of the 2-D elastic models postulated to explain the mechanism of RIS.;Monticello Reservoir is located in a region with lateral variations in lithological units and in situ stress conditions. Failure at this reservoir occurred by reverse faulting on steeply dipping faults. The initial seismicity occurred in response to elastic stress changes and undrained pore pressure changes in contrast to the subsequent seismicity which was dominated by pore pressure diffusion. At Lake Jocassee, located in a region of relatively homogeneous lithology, most of the seismicity occurs below the lake and follows a N-S trend. The regions below Lake Jocassee are sensitive to lake level fluctuations, whereas no such association was observed at Monticello Reservoir.;Monsoonal refilling at Koyna Lake results in large annual changes in water levels. Continuing seismicity decades after filling and its systematic correlation with the seasonal changes make Koyna Lake different from any other known case of RIS. The temporal pattern of seismicity suggests weakening during unloading phases and strengthening during loading phases. The mechanism of increasing stability during the filling period may be due to the increase in stability due to the elastic response. The most active periods at Koyna generally occurring 2-3 months after the onset of filling (September-December) are probably related to increased pore pressure due to diffusion. At Oroville reservoir, California, a much debated case of RIS, the major event occurred by normal faulting in the Sierra foothills. The hypocentral distribution at Oroville is controlled by a large N-S trending fault. The seismicity at Oroville is an accelerated response of a tectonically weak area.;The study concludes that the mechanical effects due to reservoir impoundment can destabilize regions that are critically stressed. Although site heterogeneities help to localize seismicity, regions of failure depend primarily on stress regime, orientation of the fault and location of the reservoir with respect to the fault. |
