Radiation efficiency and apparent stress of small earthquakes in a South African gold mine

[1] Nine triaxial borehole accelerometers were installed within 200 m along a 2650-m-deep haulage tunnel in the Mponeng gold mine in South Africa. We analyzed the high sample rate recordings ( 15 kHz) to determine source parameters of small earthquakes in the mine. To study the source processes, we carried out multiple time window waveform inversions for five larger events (0.8 < M < 1.4) that occurred within 150 m of the stations. From the inversion results we could determine the fault planes for all five events and estimate the range of rupture speed. We can conclude that rupture speeds were faster than 2.5 km/s (65% of the shear wave velocity). The radiation efficiency is written as a function of the rupture speed and becomes greater with increasing rupture speed. This study indicates that radiation efficiencies of small earthquakes in the South African gold mine are almost equal to those of larger natural earthquakes. We also calculated radiated seismic energies and static stress drops of the five events to investigate their apparent stresses. Apparent stresses of the five events were from 1 to 10 MPa and static stress drops were 1 to 20 MPa. We found that the source parameters ( rupture speed, apparent stress, radiation efficiency, and static stress drop) did not largely differ from values for larger natural earthquakes. This suggests that the dynamic rupture processes of these small events were similar to those of the larger natural earthquakes.