Effect of reservoir-triggered seismicity on safety of large dam projects

The first case of reservoir-triggered seismicity (RTS) was observed during impoundment of the reservoir-stored by Hoover dam in 1935. Since then, over 100 cases of RTS have been reported from large storage dams worldwide. The strongest events suspected of being triggered by reservoirs with magnitudes up to 6.3 were recorded in India, China, Greece and Zambia. However, most of them are small events. Although RTS only affects a small number of dams, it has remained controversial and has become a key safety subject for groups opposing new dams. Reservoir-triggered seismic phenomena are seismic events which needed the incremental effects of reservoir load and the build-up of pore pressure to make them happen. The effect of RTS on the seismic design of large dam projects and on buildings, infrastructure-and people living in the dam and reservoir region, are discussed. As RTS usually occurs within a few-years after impounding of the reservoir, it has an effect on the seismic hazard and design criteria for-appurtenant structures, buildings and infrastructure but not on the ground motion of the safety evaluation-earthquake, used for designing and checking the seismic safety of dams and safety-critical elements such-as spillways and low level outlets. The prediction of the largest RTS events is not possible today but by-seismic monitoring, irrational concerns about dam safety can be greatly reduced. The different aspects of RTS are discussed from the solution-oriented viewpoint of dam engineers, which may be quite different-from that of earth scientists. As dams are being built in seismic regions, there is great uncertainty, if strong earthquakes are actually triggered by reservoirs.