ABSTRACT
We review our endeavor to probe spatial variation in stress in seismogenic zones in deep hard-rock gold mines in South Africa. Highly stressed ground, especially in remnants or pillars, often increases the risks posed by normal-faulting earthquakes to mining operations. Some of these seismogenic zones were identified by AE monitoring, allowing us to explore them with drill holes shorter than several tens of meters in length. A M5.5 strike-slip earthquake, not a usual mining-induced earthquake, took place in 2014 on an unknown nearly-vertical geological structure, with the upper fringe of the aftershock zone several hundreds of meters below the mining horizons. The aftershock zone was elucidated by in-mine dense geophone network and surface strong motion network. This allowed us to drill two holes of 817 and 700 m length from 2.9 km depth. Integration of in-situ stress measurements by both conventional and new methods allows us to constrain the stress field accurately.
