Moment tensors of ten witwatersrand mine tremors
Abstract
Ground motions, recorded both underground and on the surface in two of the South African Gold mining districts, were inverted to determine complete moment tensors for 10 mininginduced tremors in the magnitude range 1.9 to 3.3. The resulting moment tensors fall into two separate categories. Seven of the events involve substantial coseismic volumetric reductionΔ V together with normal faulting entailing shear deformation Σ AD, where the summation is over fault planes of area A and average slip D. For these events the ratioΔ V/Σ AD ranges from 0.58 to 0.92, with an average value of 0.71. For the remaining three events Δ V is not significantly different from zero; these events are largely doublecouple sources involving normal faulting. Surprisingly, the two types of source mechanism appear to be very distinct in that there is not a continuous distribution of the source mix from Δ V=0 toΔ V≈Σ AD. Presumably, the coseismic closure indicates substantial interaction between a mine stope and adjacent shear failure in the surrounding rock, under the influence of an ambient stress for which the maximum principal stress is oriented vertically.
 Publication:

Pure and Applied Geophysics
 Pub Date:
 September 1992
 DOI:
 10.1007/BF00879963
 Bibcode:
 1992PApGe.139..781M
 Keywords:

 Induced seismicity;
 earthquake source mechanism;
 implosive moment tensor component