ABSTRACT
b-value, stress drop and energy index for an M w~2 event and smaller events before and after the event are investigated using near-source, redundant, wide-dynamic-range recordings. They took place in a remnant area about 250 m2 wide without a working stope 2650 m deep in Western Deep Levels gold mine in South Africa. All events were monitored at a distance between 100 and 200 m with nine tri-axial borehole accelerometers with a 15-kHz 120-dB digitization. Lower b-value and higher stress drop were observed for a moving 50-event window for the events before the M w~2 event, while higher b-value and lower stress drop after the M w~2 event. However, inverse correlation between b- value and stress drop were caused by significant dependence of stress drop on magnitude, a smaller event having smaller stress drop. We introduced the energy index (EI) to cancel out the event-size-dependency. We successfully monitored mining induced loading subjected to the seismogenic area through the strain recordings in the adjacent mining area. Then, we successfully detected sudden increase in EI associated with sudden increase in loading, suggesting elastic strengthening of rock mass. Thereafter, a few days prior to the M w~2 events, EI turned to decrease regardless of monotonic increase of loading, suggesting an onset of weakening of rock mass after a peak stress.
