INVESTIGATING SOURCE PROPERTIES OF MINING-INDUCED EARTHQUAKES AND THE RESULTING SEISMIC HAZARD IN TAUTONA GOLD MINE, SOUTH AFRICA

Hundreds of small earthquakes (-4 ≤ M_w ≤ 4) occur each day in TauTona Gold Mine, South Africa, resulting in high levels of seismic hazard throughout the working areas. Here we present an examination of earthquake source parameters, including corner frequency, radiated seismic energy, and apparent stress, from the mining-induced microseismicity to determine if systematic differences exist between the source processes of small and large earthquakes. The high-rate of seismicity and close proximity to the recording instruments (meters to kilometers) provides an ideal location and dataset to investigate source parameters and scaling relationships for earthquakes with a wide magnitude range of -4 ≤ M_w ≤ 4. We focus our investigation on earthquakes recorded during mining quiet hours to minimize blasts and rockburts in our catalog and analyze earthquakes in the vicinity of the ancient Pretorious Fault, the largest natural fault system running through the mine that has been reactivated by the recent mining.

The mine seismic network operated by the Institute of Mine Seismology (IMS) was enhanced by a tight array of high-quality instruments deployed in the Pretorious Fault Zone at the deepest part of the mine (~3.6 km depth) as part of the Natural Laboratory in South African Mines (NELSAM). The NELSAM network (e.g. Reches, 2006) includes 3 strong-motion accelerometers, 5 weak-motion accelerometers, and 3 geophones with a combined sample rate range of 6 – 12 kHz that allows us to reliably constrain corner frequencies of very small earthquakes. We use spectral analysis techniques and an omega-squared source model determined by an Empirical Green’s Function (EGF) method to obtain earthquake corner frequencies and use well-constrained seismic moments from the IMS catalog to obtain radiated seismic energy and apparent stress. Our initial results for 125 earthquakes with a magnitude range of -1.7 ≤ M_w ≤ 2.1 in the vicinity of the Pretorious Fault find apparent stress values between 0.12 and 29 MPa. These results are ~5 times the values found in other studies at both small and large magnitudes but are consistent with other mining-induced seismicity studies that show no change in source parameter scaling with magnitude.