An attempt to directly monitor stress buildup and triggered earthquakes within one fault distance in South African deep gold mines
Abstract
Mining takes place at depths of 2-3km, thereby frequently inducing events with M>2 in the close vicinity of stopes. So, with sensors installed ahead of time in seismogenic areas, we can directly observe temporal change in stress, deformation and material (e.g. Seismogenic Process Monitoring, 2002, Balkema). At Bambanani mine, we could successfully record strain change up to 10-4 with 25 Hz 24 bit sampling associated with M ∼2 events at a ∼100m distance with an Ishii strainmeter (see Takeuchi's poster). However, our previous experiments had a lot to be desired: only single strainmeter or only remote (>200m) geophones with less dynamic range available, no in-situ stress measurement, no a priori or afterward material investigation and so on. To solve these, we deployed new experimental sites at Tau Tona and Mponeng with significant contrast in geology and current stress, where we carry out in situ stress measurement, install two Ishii strainmeters, two strong ground motion meters and multiple thermometers within 10 - 20 m from faults. The site at Tau Tona is within a pillar with a fault 2.9km deep, where an M>2 event is anticipated with mining advance. Geology is simply Quartzite, and the fault consists of intensely fractured quartzite ∼30cm thick with 0.5-1cm thick clay-like gouge at both boundaries. The fault is well exposed at adjacent stopes and tunnels, enabling investigation of fault material property. In four boreholes already drilled 7-15m long, significant borehole breakout is seen associated with active mining near the site. A direction of maximum stress has changed since June as mining has advanced, currently being 20-30 degrees oblique to the fault. The site at Mponeng mine (3.1km deep, ( ∼200m)3 in volume) has a fault zone several tens of meters thick in a planned pillar. The fault zone was originated by ancient, large lateral fault movement, and now is characterized by complex geology consisting of chopped various-sized surrounding country rock (Basalt and Quartzite), filled with solidified fault gouge and accompanying some weak planes. In five holes 18-30m long already drilled, we don't see so significant borehole breakout as those at Tau Tona site because mining is not yet active near the site. Regardless of complex geology, weak planes were well located by logging with a borehole camera. In addition to the mine's seismic network ( ∼500m spacing), adjacent high-resolution temporary geophone network ( ∼200m spacing) and our own additional wide-dynamic-range network ( ∼200m spacing) covers the fault zone 3-dimensionally.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2003
- Bibcode:
- 2003AGUFM.S41C0102O
- Keywords:
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- 1208 Crustal movements: intraplate (8110);
- 1242 Seismic deformations (7205);
- 7209 Earthquake dynamics and mechanics