Abstract:
Seismic risk in the deep gold mines of South Africa has been studied for many decades. A clear
understanding of the effect of geotechnical conditions on the seismic hazard nevertheless remains
elusive. Certain reef types seem to be associated with a higher risk of rockbursts. The stability and
deformation behaviour of excavations on the different reef horizons are affected by the rock types and
the varying strength properties. The seismic response to mining is therefore also expected to differ
according to the geotechnical conditions. As a case study of this behaviour, the seismicity at Mponeng
mine was investigated. On the VCR (Ventersdorp Contact Reef) horizon, two areas can be delineated.
On the eastern side of the mine, the footwall is shale, and on the western side it is a strong brittle
quartzite. More large-magnitude events occur in the area with the shale footwall than the area with the
quartzite footwall. Moment tensor analyses indicated that the majority of the large-magnitude events
are not related to geological structures, but are face-related, implying that shear failure of intact rock
is occurring ahead of the mining front. Preliminary modelling indicated that the closure volume for the
shale footwall may be higher than that for the quartzite footwall, providing a possible explanation for the
observed difference in seismic response.
