The Richter or local magnitude scale (ML) has been used, with some modifications, to measure the size of earthquakes since 1935. It has long been recognised that this single-number representation of a multi-dimensional phenomenon is inadequate and cannot fully describe the energy released by an earthquake or the displacement caused by it. The high stresses encountered in deep South African gold mines induce numerous mining-related seismic events. Large databases of good quality data make it possible to identify and quantify the effect of various factors on estimates of the size of the events, principally source effects (various failure mechanisms), path effects (especially geological inhomogeneity), and the unequal radiation of energy with azimuth. We propose that three parameters that give insight into the damage potential should be routinely reported: (i) Area of the source (m2), which is easily derived from the relatively stable Moment parameter, (ii) Energy per unit area of the source (KJ/m2), derived from model-based source parameters, and (iii) Azimuth of strongest shaking (degrees).


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