ON THE NATURE OF RESERVOIR-INDUCED SEISMICITY

In most cases of reservoir-induced seismicity, seismicity follows the impoundment, large lake-level changes, or filling at a later time abov e the highest water level achieved until then. We classify this as ini tial seismicity. This ''initial seismicity'' is ascribable to the coup led poroelastic response of the reservoir to initial filling or water level changes. It is characterized by an increase in seismicity above preimpoundment levels, large event(s), general stabilization and (usua lly) a lack of seismicity beneath the deepest part of the reservoir, w idespread seismicity on the periphery, migrating outwards in one or mo re directions. With time, there is a decrease in both the number and m agnitudes of earthquakes, with the seismicity returning to preimpoundm ent levels. However, after several years some reservoirs continue to b e active; whereas, there is no seismicity at others. Preliminary resul ts of two-dimensional !similar to those by ROELOFFS, 1988) calculation s suggest that, this ''protracted seismicity'' depends on the frequenc y and amplitude of lake-level changes, reservoir dimensions and hydrom echanical properties of the substratum. Strength changes show delays w ith respect to lake-level changes. Longer period water level changes ( similar to 1 year) are more likely to cause deeper and larger earthqua kes than short period water level changes. Earthquakes occur at reserv oirs where the lake-level changes are comparable or a large fraction o f the least depth of water. The seismicity is likely to be more widesp read and deeper for a larger reservoir than for a smaller one. The ind uced seismicity is observed both beneath the deepest part of the reser voir and in the surrounding areas. The location of the seismicity is g overned by the nature of faulting below and near the reservoir.