Abstract

The Coeur d'Alene mining district lies within a minor zone of seismicity in northern Idaho along the Lewis and Clark Zone, a 400-km-long system of west-northwest-trending faults. Rockbursts (seismic events that cause damage in mines) have become increasingly common in the Coeur d'Alene district during recent years. Our epicenter map of the district shows only 52 seismic events since 1982. However, magnitude-frequency analysis suggests that regional seismograph coverage is complete only for events above magnitude 2.8, and that 90 per cent of the seismic events above magnitude 1.5 are not located by the regional seismograph network. A fault-plane solution for the 1 August 1988 magnitude 4.1 tectonic earthquake that occurred 15 km northeast of Mullan, Idaho, shows, together with previous in situ stress measurements, that the Coeur d'Alene mining district lies within a regional northeast-southwest extensional regime with the maximum compressive stress directed northwest-southeast. This orientation is deleterious for mining, as it favors right-lateral slip on the steeply dipping west-northwest-trending faults and bedding planes common in the district. Low-level seismicity in the immediate vicinity of the Lucky Friday Mine is driven in the short term by mining rate, as is evident from the decreased levels of seismic activity on weekends when mining does not take place. In the longer term, cycles of tectonic stress build-up and release may be important. Large rockbursts at the mine seem to be followed by several months of seismic quiescence.

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