Semi-controlled Earthquake-generation Experiments to Monitor the Entire Life Span of an Earthquake in South African Deep Gold Mines

Mining takes place at depths of 2-3 km in South Africa, thereby inducing events with M \textgreater 3 in the close vicinity of stopes, with the largest events so far recorded being M \textgreater 5. As a result, seismogenic processes can be monitored at very short distances with sensors installed ahead of time in seismogenic areas. We refer to this process as a semi-controlled earthquake-generation experiment, which cannot be done with natural earthquakes monitored from the Earth's surface. In the 1970s-1980s, pioneering work (e.g. McGarr et al. 1975) yielded abundant, fundamentally important results in this area. In more recent times, broad-band and wide-dynamic-range monitoring has enabled us to study additional details of the seismogenic process. Therefore, we have attempted to monitor the entire life span of an earthquake within a hypocentral distance of a few hundred meters. To date, we have monitored in six experimental fields in South African mines, the pilot field being near a strong dike 1700-m deep in a mine. The second was a homogeneous area 2700-m deep without existing faults or dikes in another mine. From 2000 we began to continuously monitor normal and shear strains on faults with 25-Hz and 24-bit sampling, where an event with M about 3 is expected at 2400-2600 m deep. Then, we have successfully monitored the entire strain history \textgreater 1E-4 in a fault loss associated with a few seismic events with M \textgreater 2. However, there were no close strong motion meter available to locate asperities; only a single strainmeter was available, not enabling us to locate strain change source; no in-situ stress measurements were carried out at the site; no information available to constrain strength. In order to solve the problems, from 2003 to 2004, we deployed new experimental fields at fault bracket/stabilizing pillars in South African deep gold mines 2900-m deep. We installed multiple strainmeters, arrays of strong ground motion meters, sensitive thermometer array to monitor seismic heat generation, and fault displacement meters. At the field, dense thermometer array to monitor strength of a fault was successfully deployed. In this paper, we review our activities to date and future prospects.