Seismicity anomaly prior to a moderate rock burst: A case study

Abstract A moderate rock burst whose Richter magnitude was lessthan M1.5 occurred on the 7500 level of the Star mine, Burke, Idaho, at 10.09 a.m. (18:09, UTC) on 3 September 1975. The burst was preceded by a rapid increase of seismic activity that was followed by a distinct decrease prior to the burst. Miners were evacuated from 7500-9 West stope, located in the immediate vicinity of the seismicity build-up, prior to the burst. Examination of the pre- and post-burst seismic data shows that the time duration, τ d , during which the seismicity increased was 188 s. This increase was associated with the formation of an elliptically shaped zone of area, A i , equal to approx 7.86 × 10 6 cmtcm 2 in the hypocentral region of the burst. This increase was then followed by a seismicity decrease lasting approx 65 m . During this time interval, seismicity was concentrated outside this zone and primarily outside the aftershock region of the burst. A slight increase of seismicity developed in the hypocentral region approx 10 m prior to the burst. The time interval, τ, between the seismicity increase and the burst was 68.5 m . The burst was followed by 22 aftershocks that defined an elliptically shaped zone of area, A , equal to approx 1.74 × 10 7 cm 2 . Previous theoretical and experimental studies by the Bureau of Mines have shown that the functional relationship of τ d and τ to the areas A i and A are τ d = 2.43 × 10 −4 A i and τ = 2.43 × 10 −4 A , respectively, where both τ d and τ are measured in seconds and no changes are assumed to occur in the far-field boundary conditions during the time duration of τ d and τ. Studies by the Bureau of Mines at the Galena mine, Wallace, Idaho, have shown that a lower limit to the ratio τ/τ d is 12.5. Consequently, once τ d is known, a minimum predicted time to the burst is possible. The predicted time to the 3 September burst is 40 m , as compared with the observed value of 68.5 m . However, based upon the observed areas A i and A 0 , the “predicted” times τ d0 and τ 0 are 191 s and 70.3 m , respectively, in good agreement with the observed times of 188 s and 68.5 m . Thus, not only was this burst predicted, but the analysis admits a more realistic estimate of the ratio τ/τ d (=21.8) required for future accurate prediction times.