The seismicity associated with the extraction of the Scott Shaft pillar is currently being monitored using 3 surface triaxial geophone station. Due to the presence of a seismically active dyke which bisects the shaft pillar an estimation of "Seismic Potential" of the pillar was calculated to quantify the seismic hazard on a monthly basis. The seismic potential is related to the total volumetric closure (McGarr 1976) and is calculated using the mine simulation program, Minsim_D. It can be described as the amount of seismicity possible for the given mining configuration and is expressed in terms of volumetric seismic moment. The volumetric moment is then compared directly with the observed seismicity and the difference is a measure of the seismic hazard.
From 16 months of accumulated seismic data the variation in seismic potential (or volumetric seismic moment) over this time period correlates well with the observed total seismic moment, assuming a seismic deformation parameter 8 = 0.5. This implies that only 50% of the volume change has resulted in seismic failure. The current difference between theoretical and observed values, 1.3x10"Nm, is the current seismic hazard, equivalent to a single seismic event of magnitude 2.7.
The study of volumetric closure and its relevance to the estimation of seismic potential has been investigated routinely using Minsim_D on a number of Gencor mines. This work has been performed in conjunction with Excess Shear Stress analysis and the results obtained have indicated a strong correlation between estimated and observed seismicity.
The seismic problems expected during the extraction of the Scott Shaft pillar prompted an indepth investigation of the estimated seismic potential at each stage of the mining sequence. Considering the speed at which meaningful results could be obtained using volumetric closure this method Was adopted.
From earthquake studies, Brune (1968) showed that the slip rates calculated from the sums of the seismic moments correlated well with observed geodetic measurements. However, MCGarr (1976) proposed that ground deformation may be better expressed as a change in volume rather than as fault slip. McGarr showed that the sum of the seismic moments,
(Equation in full paper)
Webber (1989) studied the relationship between volumetric moment and total seismic moment in the Klerksdorp gold mining area by quantifying the seismic potential within R number of seismogenic areas in terms of 8, the seismic deformation parameter. Although the analysis was simplistic (with the assumption that total closure occurred throughout the mined out areas, resulting in underestimated values of 8), there was marked similarity between mine averages. The average value for the mining district was approximately 0.2.
