Moment tensors of ten witwatersrand mine tremors |
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Authors: | A McGarr |
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Institution: | (1) U.S. Geological Survey, MS 977, 94025 Menlo Park, CA, USA |
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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- V together with normal faulting entailing shear deformation AD, where the summation is over fault planes of areaA and average slipD. For these events the ratio- V/ AD ranges from 0.58 to 0.92, with an average value of 0.71. For the remaining three events V 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 V=0 to- V![ap](/content/r517121485m53124/xxlarge8776.gif) 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. |
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Keywords: | Induced seismicity earthquake source mechanism implosive moment tensor component |
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