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Relationship of mine tremors to induced stresses and to rock properties in the focal region

BERNARD PRICE INSTITUTE OF GEOPHYSICAL RESEARCH UNIVERSITY OF THE WITWATERSRAND, JOHANNESBURG, South Africa

Abstract

Focal coordinates for a suite of tremors associated with deep-level tabular mining were determined with uncertainties of about 35 m and the tremor positions were analyzed in terms of the stress field induced by mining in the immediate vicinity. All tremors occurred in a region of 300 m in extent and at depths near 3.1 km; their magnitudes ranged from –1.5 to 2.5. The source rock in this region has no groundwater, whatsoever. The two major factors governing the occurrence of tremors appear to be the change in the stress field from its virgin state and the strength of the rock. The positions of tremor foci coincide closely with the region where changes in the stress field due to mining are maximal. Most of the tremors were located in regions where the maximum principal stress induced by mining exceeded 1 kb or where the minimum principal induced stress was less than –160 bars, compressive stresses taken as positive. Nearly all of the tremors were located above the gold-bearing reef. Strength tests in uniaxial compression indicated that the rock above the reef is about 1.4 kb stronger than that below; Young's modulus is also higher for rock above the reef. These results suggest that tremors tend to occur in regions where the rock is capable of storing the most elastic strain energy before failure. Underground observations of shear failure indicate that tremors are the result of the rupture of previously intact rock. The fact that the majority of seismic foci occurred in regions where the stress field is calculated to be well below that required for rock fracture argues for the existence of localized residual stresses of large magnitude.

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