Identifying instability in the rock mass caused by water ingress into abandoned mines

Abstract

The recent increase in seismic activity in the Central Rand Basin of South Africa was investigated using two different approaches. The closure of mines throughout the basin has left a large underground void behind that has rapidly filled with water, polluted by mine workings. There is concern over the possible triggering of a large earthquake beneath the city of Johannesburg. Questions surrounding the mechanism and nature of the seismicity still persist. Two approaches were selected to uncover early clues about the seismicity and possibly shed some light on future expectations. The first approach was an analysis of the frequency-magnitude distribution (b-value) over time, while the second approach was an attempt at building a 3-dimensional geomechanical model to describe a possible driving force behind the seismicity. Results from a temporal b-value analysis show a strong correlation with a drop in the b-value and the onset of the largest events in the database. This is explained by the relationship between the b-value and physical properties of the rock mass. An overall decrease in the b-value was estimated since flooding started, indicating a shift towards a higher proportion of larger events. The 3-dimensional geomechanical model provided a measure of the stresses and shear displacements that occur where geological discontinuities intersect the mine workings. This was compared to previous estimates of stress measurements in the mines and maximum possible magnitude estimations. The spatial distribution of recent, relocated seismicity was described in terms of the results from the model, which identified unstable geological features. The distribution of these features matched the seismic clusters that were observed, which provided some insight into the current tectonic setting of the Central Rand Basin.