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SEISMIC EVENTS WITH IMPLOSIONAL FOCAL MECHANISMS IN THE COEUR D'ALENE MINING DISTRICT, NORTHERN IDAHO

Rockbursts are a major hazard in the Coeur d'Alene mining district of northern Idaho. To study the source mechanisms of mining-related seismicity, we operated 15 portable seismographs in the Coeur d'Alene mining district for one week in September 1990. We found the velocity model of the district to consist of a halfspace with a P wave velocity of 5.74 km/s and an S wave velocity of 3.47 km/s. In-mine seismic monitoring systems provided excellent control on the velocity model and most hypocenter locations. Twenty-one events, with sufficient focal sphere coverage for fault plane solutions, were recorded. Eleven of the events produced all-dilatational first motions; for all events, 90% of first motions observed were dilatational. Ten events produced unsolvable first-motion data in terms of the usual double-couple mechanism, but seven of these unsolvable events were found consistent with an implosional source and two others were consistent with a shear-implosional source. Our results provide firm observational evidence that dilatational first motions are predominant in this deep metal-mining district, probably the result of implosional and shear-implosional source mechanisms.