Review of semi-controlled earthquake-generation experiments in South African deep gold mines (1992-2001)

摘要

Mining takes place at depths of 2000-3000 m in South Africa, thereby inducing events with M equal to or larger than 3 in the close vicinity of stopes, with the largest events so far recorded being M > 5. As a result, seis-mogenic 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 yielded abundant, fundamentally important results in this area. In more recent times, wide-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 three experimental fields in South African mines: near a strong dike 1700-m deep in a mine, a homogeneous area 2700-m deep without existing faults or dikes in another mine. In 2000 we began to continuously monitor normal and shear strains on a fault with 25-Hz and 24-bit sampling, where an M～3 event is expected. In this paper, we review our activities to date and present future prospects.