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 mining-induced 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 reduction-ΔV together with normal faulting entailing shear deformation ΣAD, where the summation is over fault planes of areaA and average slipD. For these events the ratio-ΔV/ΣAD ranges from 0.58 to 0.92, with an average value of 0.71. For the remaining three events ΔV is not significantly different from zero; these events are largely double-couple 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 ΔV=0 to-ΔV≈ΣAD. 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.
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McGarr, A. Moment tensors of ten witwatersrand mine tremors. PAGEOPH 139, 781–800 (1992). https://doi.org/10.1007/BF00879963
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DOI: https://doi.org/10.1007/BF00879963