Coseismic and aseismic deformations associated with mining-induced seismic events located in deep level mines in South Africa

Abstract

Two underground sites in a deep level gold mine in South Africa were instrumented by the Council for Scientific and Industrial Research (CSIR) with tilt meters and seismic monitors. One of the sites was also instrumented by Japanese-German Underground Acoustic emission Research in South Africa (JAGUARS) with a small network, approx. 40 m span, of eight Acoustic Emission (AE) sensors. The rate of tilt and the seismic ground motion were analysed in order to understand the coseismic and aseismic deformation of the rocks. A good correspondence between the coseismic and the aseismic deformations was found. The rate of coseismic and aseismic tilt, as well as seismicity recorded by the mine seismic network, are approximately constant until the daily blasting time, which takes place from about 19:30 until shortly before 21:00. During the blasting time and the subsequent seismic events, the coseismic and aseismic tilt shows a rapid increase. Much of the aseismic deformation, however, occurs independently of the seismic events and blasting and was described as ‘slow’ or aseismic events. During the monitoring period a seismic event with MW 2.1 occurred in the vicinity of the instrumented site. This event was recorded by both the CSIR integrated monitoring system and JAGUARS acoustic emission network. More than 21,000 AE aftershocks were located in the first 150 hours after the main event. Using the distribution of the AE events the position of the rupture area was successfully delineated. The tilt changes associated with this event showed a well pronounced after-tilt. The distribution of the AE events following the main shock was related to after-tilt in order to quantify post-slip behaviour of the source. No evidence was found for coseismic expansion of the source after the main slip. Therefore the hypothesis of the post-seismic creep type behaviour of the source was proposed to explain the large amount of tilt following the main shock. Previous studies using numerical modelling and analytical tools show that for this specific event and rupture area, the amount of measured tilt is highly sensitive to seismic activity in specific regions, in particular, to the bottom corner of the source area. Thus, it is concluded that the post-seismic creep behaviour, if present, most probably occurred in the bottom corner of the rupture area, a region also characterised by large amounts of aftershock activity.