The risks to miners, mines, and the public posed by large seismic events in the gold mining districts of South Africa

Abstract

A magnitude 5.3 seismic event occurred on 9 March 2005 in the Klerksdorp district of South Africa. The event and aftershocks shook the nearby town of Stilfontein, causing serious damage to several buildings and minor injuries to 58 people. At a nearby deep gold mine, two mineworkers lost their lives and 3200 mineworkers were evacuated under difficult circumstances. The Chief Inspector of Mines initiated an investigation into the risks to miners, mines and the public arising from seismicity in gold mining districts. It was found that the seismic event on 9 March 2005 could be ascribed to past mining, and that seismic events will continue to occur in the gold mining districts as long as deep-level mining takes place and are likely to persist for some time even after mine closure. Placement of slimes in old mining workings is unlikely to reduce risks significantly. Seismic monitoring should continue after mine closure, and the seismic hazard should be taken into account when the future use of mining land is considered. Seismic events are likely to be triggered as mines are allowed to flood. It is possible that a seismic event could cause movement on a fault transecting a water plug and/or water barrier pillar, open up a fluid pathway, and allow flow of water into populated mine workings. While it is unlikely that such an occurrence would become an uncontrollable inrush, the consequences could be disastrous and the risk must be seriously addressed. However, the risk of a seismic event on one mine causing serious damage in a neighbouring mine is considered small because major infrastructure such as shafts are usually located at least a kilometre from mine boundaries, and there is generally good cooperation between neighbouring mines with respect to mine planning and blasting schedules. The national and local monitoring networks, operated by the Council for Geoscience and mining companies, respectively, are on a par with those installed in seismically active mining districts elsewhere in the world. However, steps should be taken to improve the quality of seismic monitoring and to ensure continuity, especially as mines change hands. A range of technologies is available to mitigate the risks of underground damage resulting from large seismic events. However, if there has already been extensive mining near geological features that could host large seismic events, any further mining adjacent to the structure must be carefully planned. The Klerksdorp and Free State gold mining districts are incorporating the risks of seismicity in their disaster management plans, and Johannesburg is urged to do likewise. Some buildings are considered vulnerable to damage by large seismic events, posing safety and financial risks.