Seismicity in deep gold mines of South Africa: Implications for tectonic earthquakes

We have studied induced seismicity associated with five deep gold mines located in the Far West Rand district, Republic of South Africa, focusing on the digital data recorded from January 1994 until February 2000 by in-mine arrays of three-component geophones. The observed seismicity, which exceeds 1,000 events per day, can be divided into two kinds of events, designated as Types A and B. Type A events are tightly clustered in time and space and generally occur within 100 m of an active mining face or development tunnel; their spectra are comparatively enriched in high frequencies, and they have an upper moment-magnitude cutoff at M-max < 1. We associate these events with the "fracture-dominated" rupture of competent rock induced by dynamic stresses during blasting and quasi-static stress perturbations from the excavation and closure of individual stopes. In contrast, Type B events are temporally and spatially distributed throughout the active mining region; they represent "friction-dominated" slip in existing shear zones such as faults or dikes and have source-scaling properties that agree well with extrapolations from tectonic earthquakes. In the Far West Rand region, Type B events can have large magnitudes (M > 3), but they show a distinct lower magnitude cutoff at M-min similar to 0. We interpret this cutoff in terms of a critical-patch size for nucleation of shear failure, and we show that the data are consistent with a rate- and state-dependent friction model in which the critical slip distance D-c similar to 10(-4) m. Both the spectral predictions of this model and accelerograms of Type B events agree that f(max) similar to 200 Hz.