Remote Triggering of Large Seismic Events in Mines: Evidence Based on the Spatial and Temporal Migration of Microseismic Events Along Increasing Stress Gradients
We examine the distribution of microseismicity associated with a sequence of 10 large events (2.1 ≤ M ≤ 3.0) that followed minor development blasts over a 75-hour period in three widely-separated (200–300 m) regions of Strathcona mine, Sudbury, Ontario in June 1988. The microseismicity was observed to migrate between these regions during two intervals without large event occurrences (24 and 38 hours, respectively). The conditions under which remote mobilization of pre-existing structures can occur was investigated by including underground fracture mapping and stress data, fault-plane solutions, and three-dimensional distributions of P-wave velocities. A good agreement was found between dominant nodal plane orientations and lineations in microseismicity with identified fractures. Within the three regions, also corresponding to zones of high P-wave velocity, failure was associated with different fracture sets. The distributions of P and T axes and damage related to the large events, suggested that these events were directly influenced by the regional stress field, whereas the microseismic events appeared to have been influenced by local stress conditions. Based on our results, we propose that the migration in microseismicity can be explained by a transfer of stress along a network of connected fractures that were close to failure. Moreover, the migration followed a path of increasing velocity, suggesting that failure, under the constant lithologic and discontinuity distribution at our site, proceeded along a stress gradient to areas with stresses at or near failure and resulted in the remote triggering of large events on a mine-wide scale. Using these arguments, it can be expected that a highly stressed region could fail as a result of event triggers originating in moderately stressed regions through a quasi-static transfer of stress/strain in the rock mass.
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