Earthquake source parameters and scaling relationships from microseismicity at TauTona Gold Mine, South Africa

The issue of earthquake source scaling continues to draw considerable debate within the seismological community. Findings that both support and refute the claim that systematic differences between the source processes of small and large earthquakes may exist, motivate the study of how source parameters, such as seismic moment, corner frequency, radiated seismic energy, and apparent stress, scale over a wide range of magnitudes. To address this question, we are conducting a compressive examination of earthquake source parameters from microseismicity recorded at the TauTona gold mine in South Africa. At the TauTona gold mine, hundreds to thousands of earthquakes are recorded everyday within a few meters to kilometers of seismometers installed at depth throughout the mine. This high-rate of seismicity and close proximity to the recording instruments provides the ideal location and dataset to investigate source parameters and scaling relationships for earthquakes with a wide magnitude range of -4 < Mw < 4. We focus our investigation on earthquakes recorded during mining quiet hours to minimize blasts and rockburts in our catalog, and focus on earthquakes that occurred along the Pretorious Fault, the largest fault system running through the mine, to evaluate source parameters of fault zone earthquakes. The mine seismic network operated by the Institute of Mine Seismology (IMS) with a sample rate range of 3 - 2000 Hz has been 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 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. We find EGF pairs with correlation coefficients > 0.6 to ensure the comparison of earthquakes with similar mechanisms and only model spectra with a log of signal to noise ratio greater than 3. We stack high-quality spectral ratios with low residuals compared with a Brune model to determine corner frequencies and use well-constrained seismic moments from the IMS catalog to obtain radiated seismic energy and apparent stress. Our initial results for 35 earthquakes with a magnitude range of -1 < Mw < 2 find log of energy to moment ratios between -3.9 and -5.5 and apparent stress values between 0.1 and 5.1 MPa using a shear modulus of 41.7 GPa. These results are consistent with other mining seismicity studies and show no change in source parameter scaling for earthquakes in this magnitude range.