Located some 90 km north of the town of Sept-Îles, Quebec, the Sainte-Marguerite-3 (SM-3) hydroelectric project was one of the most ambitious in eastern North America in recent years. The project included the construction of a 171 m high dam that created a 140 km long reservoir, some 120 m deep near the dam toe. Although the reservoir is located in a weakly seismic area of the Canadian Shield, reservoir-induced earthquakes occurred. A first event (mN 1.3) was recorded in September 1999, ∼17 months after the reservoir started filling. Between October 2000 and March 2001, four additional events were detected by the Canadian National Seismograph Network, the largest being mN 1.7. During a field survey that took place between July and October 2001, 276 events of magnitude mN 1.2 were detected within 30 km of a three-component seismograph. Single-station locations, confirmed by agreement between P phase and S phase azimuths, were determined for 182 events. Most earthquakes fall in four areas with estimated focal depths mostly shallower than 2 km. Since the field survey completion, more events have taken place, but the largest one is mN 2.2 as of March 2006. This paper presents a detailed analysis of the reservoir-triggered events, together with a summary of seismic hazard evaluation of the dam site, the seismic parameters selected for dynamic analysis, and the selection of the design earthquake.

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