MOMENT TENSORS OF 10 WITWATERSRAND MINE TREMORS

Ground motions, recorded both underground and on the surface in two of the South African Gold mining districts, were inverted to determine c omplete 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 volumetr ic reduction -DELTAV together with normal faulting entailing shear def ormation SIGMA AD, where the summation is over fault planes of area A and average slip D. For these events the ratio -DELTAV/SIGMA AD ranges from 0.58 to 0.92, with an average value of 0.71. For the remaining t hree events DELTAV is not significantly different from zero; these eve nts are largely double-couple sources involving normal faulting. Surpr isingly, the two types of source mechanism appear to be very distinct in that there is not a continuous distribution of the source mix from DELTAV = 0 to -DELTAV approximately SIGMA AD. Presumably, the coseismi c closure indicates substantial interaction between a mine stope and a djacent shear failure in the surrounding rock, under the influence of an ambient stress for which the maximum principal stress is oriented v ertically.