Mining-induced earthquakes pose a risk to workers in deep mines, while large earthquakes that occur near plate boundaries (and occasionally in stable continental regions) pose a risk to the public. A five-year Japanese–South African collaborative project ‘Observational studies in South African mines to mitigate seismic risks’ was concluded in 2015. Acoustic emission sensors, accelerometers, strain meters, and controlled seismic sources were installed in three deep Witwatersrand gold mines to monitor the deformation of the rock mass, the accumulation of damage during the earthquake preparation phase, and the propagation of the rupture front. A surface array of accelerometers was installed in the Far West Rand mining district. This data was integrated with measurements of stress, in-stope closure, and strong motion, as well as data recorded by the mine-wide seismic networks. New insights into the physics of earthquakes were gained, and technologies developed or adapted to assess seismic hazard and mitigate rockburst risks.

INTRODUCTION

Earthquakes pose a significant risk to workers in deep and overstressed mines and people residing near to these mines (such as gold mines in South Africa), and inhabitants of regions close to plate boundaries (such as Japan). While the earthquake hazard is less in stable continental regions, rare large earthquakes can occur and cause considerable damage if buildings are not designed to withstand shaking. The risk posed by seismicity in South African mining districts is exemplified by the M5.5 seismic event that struck the North West Province at 12:22 p.m. (local time) on 5 August 2014. It was the largest event ever to occur in a South African mining district. Dwellings and other buildings in the nearby towns of Khuma, Stilfontein, and Orkney suffered extensive damage (Figure 1). Strong shaking was experienced in Johannesburg and Pretoria, and was felt as far away as Durban and Cape Town. One person died as a result of the collapse of a garden wall, and many others suffered minor injuries. The cost of damage was estimated to be R130 million (B. Manzunzu, Council for Geoscience, personal communication, 2015).

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