Seismic risks posed by mine flooding

Abstract

Many South African gold mines will flood when they close, as the groundwater will gradually fill the mining voids. Preliminary investigations have shown that flooding of mines can generate increased levels of seismicity. Examples are given that illustrate some of the characteristic differences between fluid-induced seismic events and mining induced events. This paper aims to create awareness that fluid-induced seismicity will become increasingly important in South Africa when closed mines are allowed to flood. Such flooding-induced seismicity can have significant environmental, social and economic consequences, and may endanger neighbouring mines and surface communities. While fluid-induced seismicity has been observed in other settings (e.g. filling of dams, oil-well stimulation and hydrothermal fields), no detailed study of seismicity associated with flooding of deep mines has ever been conducted anywhere. It is possible that mine flooding could lead to potentially disastrous seismicity, which may result in high continuous pumping costs by the state to prevent or to contain flooding. Research needs to be conducted, which establishes the potential relationships between flooding and the magnitude and frequency of triggered and induced seismicity resulting from mine flooding. A thorough understanding of the interaction between flooding and seismicity will allow the impact of mine flooding on safety to be determined. In particular, the maximum credible earthquake size resulting from the flooding of deep gold mines in South Africa needs to be determined. The identified risks will in turn allow appropriate mitigating strategies to be developed. Such strategies will influence South African mine closure policies