Experimental and in-situ physical properties measurements from a seismogenic zone in a deep South African mine

Abstract

Since the 1990s, South Africa has experienced several sizeable earthquakes occurring in mining regions at depths ranging between 2 km and 4 km below the surface. These consist of the Welkom (ML 4.2) and Stilfontein (ML 5.3) earthquakes, which occurred in 1990 and 2005, respectively. The occurrence of these seismic events resulted in the destruction of mine infrastructure and some cases the death of miners. More recently, the 2014 ML 5.5 earthquake, which occurred in the Orkney mining district, in the Witwatersrand Basin, is the largest event to occur in a mining region in South Africa (Figure 1). Although mining-related seismic events are common in South Africa (ML<2), the large magnitude and shallow hypocentral depth (~5 km) of this earthquake raise concern for future large events occurring at active mining levels and their potential to negatively affect mining operations. Additionally, the physical properties of the rocks around the active rupture zone are not well understood because no boreholes had been drilled at depths in and around the aftershock zone. Therefore, no meaningful correlations of the lithological units with physical properties obtained from experimental and in-situ measurements have been done.