ABSTRACT
Changes in local stress state due to mining perturb the stability of rock mass surrounding the excavations. The subsequent readjustment of rock towards a new equilibrium causes mining-induced seismic events. At a gold mine in South Africa, mining has reached down to depths of 3 to 4 km, which is the deepest in the world. A temporary, small array of seismometers and crustal deformation instruments (30 m wide × 200 m long) were deployed along a horizontal tunnel 2650 m deep of the mine to record the mining-induced events. In the gold mine, S-wave splitting with differences in arrival time (Δt) within a few ms was very clearly observed. This study purposes to monitor stress changes due to mining activity through shear wave splitting by analyzing recordings of events at the depths of ~2600 m and ~ 3600 m. We tried to determine the location of an anisotropic area causing shear wave splitting, and to detect temporal changes in shear wave splitting. It was found that anisotropy was not homogeneously distributed over ray paths, but limited near seismic sources. This was consistent with the fact that both stress and seismicity were concentrated only near mining area. For deeper seismic events, Δt was larger and b-value was conversely smaller. This was consistent with the fact that the larger the depth, the larger was the stress. For events at deeper mining area, a significant temporal change in Δt (from 3 to 1 ms) was detected during the study period of February to October 1996.
