ALBERT

Description: Induced seismicity exhibits diverse source mechanisms that are often difficult to constrain for small events. Here, we use data from the in-mine seismic network, the Natural Earthquake Laboratory in South African Mines network, and a temporary Program for the Array Seismic Studies of the Continental Lithosphere deployment in TauTona Mine, South Africa, to determine full moment tensors of 100 mining-induced earthquakes in the magnitude range –2.7〈 M w 〈2.5. Ground displacement derived from velocity and acceleration data show clear near-field effects, indicating that the lowest frequencies are well resolved. Phase amplitudes of between 11 and 77 picks per event were inverted to obtain the six independent moment tensor components. The quality of each moment tensor solution is quantified using (1) the misfit between observed and synthetic waveforms, (2) bootstrap resampling to estimate uncertainties, and (3) the F -test to determine the need for including an isotropic component with an extra degree of freedom in the solution. The results indicate 82% of the events have well-constrained solutions, and 45% of the well-constrained events require an isotropic source term. Throughout the magnitude range, both deviatoric and implosive mechanisms are observed, with implosive ratios of volume change to shear deformation (V/ ) of –1.03 to –0.15. Two explosive events are observed at M w –0.5 and –0.2, withV/ =0.15 and 0.51, respectively. For the largest events, we determine maximum slip and apparent stress ( a ) and find values consistent with those of natural tectonic earthquakes, with 0.1≤ a ≤9.2 MPa. Our results support previous speculation on the nature of isotropic components of mining-induced earthquakes, in which events of all sizes begin as shear failure that may intersect a void (tunnel or stope) and cause collapse, whereas only small events result in explosive sources.