Dynamic Triggering of Microseismicity inferred from Spatio/Temporal Patterns in a Mine Setting

We examine spatial and temporal patterns of microseismic events in an underground mine. Our objective is to address three key questions: Where does the seismicity occur? Why does it occur in these locations but not elsewhere? And what triggers it? We take advantage of waveform similarity by performing multiplet analysis based on the double-difference technique to obtain highly accurate relative locations. Seven vertical receiver boreholes that surround the area under investigation have recorded microseismic data during a month, thus providing a good azimuthal coverage. A minimum crosscorrelation level of 85% is used to detect 21 multiplet groups, which represents 60% of the total seismicity. The largest groups are located close to the main shaft and tunnels at orebody levels, thus we postulate seismicity is facilitated by the potential of subsidence if we assume a compacting earth and hoop stresses acting on the vertical shafts. Surprisingly, most events only occur during certain hours of the day but do not relate to blasting. They correlate with scheduled operations of rock removal. Therefore, it is likely they have been triggered by the transportation of the debris along the main shaft instead of blasting, as we initially expected. Given that seismicity is present around the main shaft but absent close to the second air shaft, we conclude that for seismicity to occur both a favourable stress state must exist, as well as additional external forces, causing dynamic triggering. This analysis provides more insight into anthropogenic processes and their roles as major initiators of seismicity during dynamic stress transfer thereby facilitating identification of hazardous areas in mine settings.