Stress drops and radiated seismic energies of microearthquakes in a South African gold mine

Abstract

We estimate stress drops and radiated seismic energies of 20 microearthquakes (0.0 < MW < 1.3) in a South African gold mine to investigate their rupture characteristics and scaling relationships to large earthquakes. We analyze seismograms of borehole accelerometers recorded with high sampling rate (15 kHz) within 200 m of the hypocenters at the depth of 2650 m. The waveform data have very high signal‐to‐noise ratio and no significant later phases are observed at all stations. Corner frequencies and quality factors of the anelastic attenuation Q are estimated from spectra of velocity seismograms by assuming the omega squared model of Boatwright (1978). We also investigate moment tensors for double couple solutions and volumetric components from the waveform inversion. Static stress drops of the 20 earthquakes calculated from the model of Madariaga (1976) are from 3.2 to 88 MPa and scaled energies (= ER/Mo; the ratio of the radiated energy ER to the seismic moment Mo) are from 4.2 × 10−6 to 1.1 × 10−4. We find that both the static stress drops and the scaled energies of the analyzed earthquakes are comparable to those values of larger earthquakes. Our results indicate that the dynamic rupture processes of these microearthquakes are similar to those of larger earthquakes.