Effects of Mining-Induced Stress Perturbations on Pre-Existing Faults Near a Deep South African Gold Mine

For over a century, mining-induced earthquakes have been recorded in the deep underground mining region of Witwatersrand Basin in South Africa. The TauTona gold mine experiences an appreciable number of mining- induced earthquakes and is the site of the Natural Earthquake Laboratory in South African Mines (NELSAM) project. In this work, we constrain the virgin (i.e. unperturbed) state of stress near the TauTona gold mine using an iterative forward modeling approach that combines observations of drilling induced borehole failures in borehole images, boundary element modeling of the mining-induced stress perturbations, and forward modeling of borehole failures based on the results of the boundary element modeling. We find that the state of stress is a normal faulting regime with principal stress orientations that are slightly deviated from vertical and horizontal and therefore denoted with a (*). The maximum principal stress, Sv*, is deviated 0° to 20° from vertical towards the NNW and has a magnitude gradient of 27 ± 0.3 MPa/km. The intermediate principal stress, SHmax*, is inclined 0° to 20° from horizontal with an azimuth of 145° to 168° and has a magnitude gradient of 21.5 to 26 MPa/km. The least principal stress, Shmin*, is inclined 0° to 10° from horizontal with an azimuth of 235° to 258° and has a magnitude gradient of 13 to 15.5 MPa/km. Using this constrained stress state, we investigate the likelihood of faulting to occur both on pre-existing fault planes that are optimally-oriented to the virgin stress state and on faults affected by the mining-perturbed stress field, the latter of which is calculated with boundary element modeling. The far-field stress state indicates that the crust is in a state of frictional faulting equilibrium, such that normal faulting is likely to occur on faults striking SSE and NNW and have a dip angle of approximately 45° to 80°. The mining-induced stress perturbation affects faults relatively closer to the mining excavation. We analyze active faults observed in borehole image log data and mapped in the TauTona access tunnels. In the borehole image logs, we find evidence of recent slip on faults that have become critically stressed due to the mining-induced stress perturbation. In our investigation of the Pretorius Fault Zone (PFZ), an ancient (Pre-Cambrian) fault system, we find that stress perturbations due to recent mining advances may be responsible for reactivating segments of the PFZ that were observed to slip during a M2.2 event recorded in December 2004.