The filling of the second reservoir, LG3, in the James Bay hydroelectric development has resulted in a number of small induced earthquakes up to magnitude 3.7. The activity has occurred in two main areas, one of which can be associated with the previously mapped LG3 fault and the other with the intersection of two dominant regional structures. The significant induced activity started 7 months after filling commenced, when the water depth had reached 64 m, and continued for another 11 months. After a lull of about 6 months, a short burst of events, much more energetic than that during filling, occurred probably on the LG3 fault after a 2 m lowering of the water level. A further episode of seismicity occurred in the spring of 1984, also during the lowering of the water level. Based on the data available, it is suggested that postfilling seismicity permits one to isolate two classes of reservoirs from the general population of reservoirs, those with seismicity associated with refilling and those with seismicity associated with lowering water levels. LG3 belongs to the latter class where seismicity recurrs during lowering of a few meters of water.

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