Microseismicity observed in an underground mine: Source mechanisms and possible causes

Microseismicity is observed during an underground mining development in Saskatchewan, Canada. The events are located near the main working level at 480 m depth and show some temporal correlation with the daily rate of rock removal. We compute the focal mechanisms of six microseismic events using the P-wave first arrival polarities and the S/P amplitude ratios. The focal mechanisms of events classified based on waveform similarity are similar to each other showing oblique faulting with dominant reverse slip components. The best-fitting plane obtained from high-resolution event locations is consistent with the calculated fault plane solutions. We, therefore, suggest that the events occur due to reactivation of unmapped faults. The microseismic event cloud is separated from the actual construction sites by a lateral distance of at least 100 m. Therefore, the recorded microseismicity cannot be triggered by the highly localized stress concentrations near the newly excavated cavities at the time of data recording. We investigate the likelihood of fault reactivation due to stress changes that might be related to a subsidence-like mechanism caused by the extensive horizontal tunnel network. Results show that such stress changes are insufficient to cause shear slippage, yet they can create a static stress state where the favorably oriented faults move closer to failure. An additional stress perturbation is required for triggering microseismicity due to fault reactivation. The peak dynamic stresses due to the vibrations caused by blasting are insufficient to cause the frictional failure of fault. However, the vibrations due to a large rock crusher placed in the 480 m level may generate sufficient peak dynamic stress to trigger microseismicity. Thus, a joint interpretation of the source mechanisms and a suitable failure criterion incorporating the static and dynamic stress modeling can produce pertinent insights into the likelihood of anthropogenic processes to yield induced seismicity. (C) 2019 Elsevier Ltd. All rights reserved.