The Pretorious fault, Mponeng mine, South Africa: A site selected for the establishment of a natural earthquake laboratory

One major obstacle in earthquake investigations is the lack of direct and near-field observations. To reduce this limitation, we initiated the NELSAM project (Natural Earthquake Laboratory in South African Mines) for in-situ seismic observations at 3.5 km depth (session S11 this meeting). The project includes instrumentation of the complete frequency range (creepmeters, seismometers, strain meters, accelerometers, temperature sensors, acoustic emission transducers). This dense 3D array is designed to monitor fault activity before, during, and after M = 2.5-3.5 earthquakes at distances of 1-100 m from their anticipated hypocenters. The site characterization will include 3D mapping, in-situ stress measurements on local and regional scales, and rock mechanics analysis. We describe here the structural and seismic features of the first site where monitoring is planned to start circa April, 2005.We screened and ranked 12 active faults in South African mines according to dimensions, internal structure, accessibility, and likelihood of M 3.0 seismic events. The selected site is on the Pretorius fault, one of the largest faults in the Western Deep Levels gold mines, WSW of Johannesburg. This fault-zone is at least 10 km long; it is very steep and inclined toward the SSE with throw up to 70 m. It contains many individual fault surfaces that vary in inclination from 40 deg to vertical; some have thin breccia zones with quartz cementation, and a few display a fine-grain gouge, but there is no major, well-developed gouge zone. The Pretorius fault accommodated some amount of strike-slip displacement as indicated by frequent inversion of the sense of fault throw, the occurrence of horizontal slickenside striations, and its steepness. The Pretorius fault-zone was active in Archaean times, and it has been locally reactivated by the mining operations in Mponeng and Tautona mines, which provide access to depths of 3.0-3.5 km. Our monitoring site will be established where both sides of the Pretorius fault will be mined during the next few years; the practice in the deep mines and numerical modeling predict profound increase of the seismic activity at the site during the next 2-4 years. The associated increase of shear stresses on the fault is expected to generate a few earthquakes of M>3.0 along surfaces of hundreds to thousands of sq. meters within the fault zone. Monitoring the fault zone activity and particularly the larger events is the main objective of NELSAM.