Observations from P/S Amplitude Ratios from Local-Distance Mine Related Seismic Events and Earthquakes in South Africa

Methods to discriminate between earthquakes and explosions using observations from teleseismic and regional distances have been widely investigated. These earlier studies explored discriminant performance using observations from high yield (generally greater than 1Kt) explosions that were recorded at regional and teleseismic distances. Many discriminants rely on the amplitude ratio of P and S phases, which vary systematically with source type as a result of the strong difference in shear-wave energy radiated by earthquakes and explosions. Recent studies have used a similar strategy to test P/S discriminant performance on smaller, low yield events observed at local distances (<200km). Shorter distances and higher frequency observations require more scrutiny of path effects, site responses, and source corrections. We use observations from roughly 260 mining related events (pillar collapses, rock bursts, etc. extending to depths of 4km) and aftershocks (from the August 5^th, 2014 M5.5 Orkney earthquake extending to depths of about 10km) observed at distances less than about 25km within the Klerksdorp gold mining region of South Africa. Event magnitudes range from 1 to 3.5 but include no known explosions, so our focus is on the comparison of P/S ratios for earthquakes and mining-related events. Local ground motions were recorded using surface accelerometers operating in 2013-2014 at the Kloof Gold Mine. After correcting for instrument effects, we visually pick all P- and S-wave arrival times and measure the P and S amplitudes across several frequency bands. We use the amplitude measurements to construct frequency-dependent propagation and site-amplification models and estimate P- and S-wave source amplitude/excitation values along with corresponding P/S ratios. We compare the patterns of propagation- and site-corrected ratios from earthquakes and mining-related events and explore the appropriateness of earthquake-based source corrections on local-distance P and S waves from mine-related seismicity.