Stress Drop and Radiated Seismic Energy of Microearthquakes Involving Volume Change in a South African Gold Mine

Source parameters of small earthquakes are important for clarifying the scaling relations between small and large earthquakes. However, it is often difficult to accurately determine source parameters of small earthquakes because the dominantly high-frequency seismic waves excited by small earthquakes are easily scattered and attenuated along the path. To overcome this problem we analyze the high frequency recordings of earthquakes at very close distances in the Mponeng mine in South Africa. We estimate the stress drop and radiated seismic energy of 20 microearthquakes (0.0 < MW < 1.3) that occurred in the 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 2,650 m. At all stations, the waveform data have very high signal-to-noise ratio and no significant later phases are observed. Corner frequencies and quality factors of the anelastic attenuation Q are estimated from spectra of the 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. 10 out of the 20 earthquakes have large volume changes and they range 30 through 50% of the moments for the best-fit double couples. Static stress drops of the 20 earthquakes calculated from the model of Madariaga [1976] range from 3.2 to 88 MPa. Values of the scaled energy (= ER/Mo; the ratio of the radiated energy ER to the seismic moment Mo) with corrections due to volume changes and radiation patterns range from 4.2×10-6 to 1.1×10-4. We find that both the static stress drop and the scaled energy of the analyzed earthquakes are comparable to values for larger earthquakes and no scale dependence is observed. Our results indicate that the dynamic rupture processes of these microearthquakes are similar to those of larger earthquakes, even though they include significant volume changes.