Satellite radar interferometry reveals mining induced seismic deformation in South Africa

Shortly after midnight on April 23rd 1999, the South African mining town of Welkom was shaken by a magnitude (local) 4.5 earthquake. Because of resulting power failures and substantial damage to the Eland shaft of Matjhabeng mine, two mine-workers were killed and hundreds of others were temporarily trapped more than a kilometre underground. Although seismicity due to rockbursts in the deep (>2 km) South African gold mines is relatively common, the consequences are not usually as serious as those resulting from the April 1999 event. By means of ERS SAR interferometry, we have successfully derived a map of the surface deformation resulting from the April 1999 event, and our result is being used in rockburst characterisation models. The location and magnitude of the deformation coincides well with that expected, given the epicentre location and the subsurface geological structure. The deformation feature is a 5 km long elliptical depression centred on the Eland Shaft of Matjhabeng mine. Its depth at the centre is 9 cm. Our result is significant not only because of our success in imaging the effects of a seismic event two orders of magnitude smaller than most of those previously imaged by interferometry, but also because it demonstrates the potential that the interferometric technique offers in providing critical information for rockburst amelioration studies