Abstract

Fault-plane solutions for seismic events in the deep underground mines of the Coeur d'Alene Mining District in northern Idaho have been difficult to reconcile with actual observations of mine damage. Examination of rockburst damage in stopes as well as measurements of slip movements on bedding planes suggests that rockbursts in the Lucky Friday Mine are closely associated with gradual bedding-plane slip. This progressive slip reduces the physical dimensions of stopes and increases compressive stress resulting in sudden implosive failure of the stope margins. Thus, much of the difficulty in evaluating seismological data arises from the implosional nature of these rockbursts, making the first motion of the events difficult to interpret. In the summer of 1998, we acquired several months of high-quality seismic data from five stations in the vicinity of the Lucky Friday Mine. This is the first time that a network of high dynamic-range, three-component instruments has been deployed in this mining district. On 29 August, 1988 at 6:09 UTC, a ML 2.9 event occurred at the Lucky Friday Mine. This event resulted from bedding-plane slip and caused extensive damage in the mine that was carefully documented. Using a least-squares procedure, we were able to find a well-constrained moment tensor solution that matches the P and S displacement amplitudes measured at the five temporary stations. We derived a model of the mechanism to compare with the underground observations of bedding-plane slip and damage. We conclude that high-quality seismological instrumentation is required if meaningful results are to be obtained for understanding the source mechanisms of shear-implosional rockbursts.

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