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Semi-controlled seismogenic experiments in South African deep gold mines
To study the preparation process of an earthquake, we should get closer to the hypocentre. In co-operation with ISS International, we had an experimental field ((similar to)200 m x (similar to)200 m) without dykes and faults in a South African deep gold mine. We have monitored more than 20,000 seismic events (M<2) with borehole tri-axial accelerometers. We used a data acquisition system with 15 kHz sampling and 120-dB dynamic range. In a remnant area at a distance of 100-200 m, an earthquake sequence associated with an M (similar to) 2 event occurred. Significant changes in seismic parameters such as stress drop, b-value, energy index were observed associated with the sequence. We installed Ishii's borehole strainmeters and could monitor as large a strain change as 10-4 associated with mining. However, sampling was only 4 times per hour and 12 bit A/D. A distance from the M2 sequence was (similar to) 100 m, being not close enough. Therefore, we found another field with several crosscut tunnels excavated across a fault where an M=2-3 event is expected. We installed the strainmeter within several metres of the fault to monitor both shear and normal strains on the fault. The strains are continuously digitized with a 24-bit 25-Hz A/D conversion. The Earth Tide (10-8) is clearly detected. Seismicity induced by longwall mining can be compared to earthquake swarm induced by fluid intrusion to vertical dykes. We also found a decrease in energy index in the source area of M>5 events a few days before the major events.
Semi-controlled seismogenic experiments in South African deep gold mines
To study the preparation process of an earthquake, we should get closer to the hypocentre. In co-operation with ISS International, we had an experimental field ((similar to)200 m x (similar to)200 m) without dykes and faults in a South African deep gold mine. We have monitored more than 20,000 seismic events (M<2) with borehole tri-axial accelerometers. We used a data acquisition system with 15 kHz sampling and 120-dB dynamic range. In a remnant area at a distance of 100-200 m, an earthquake sequence associated with an M (similar to) 2 event occurred. Significant changes in seismic parameters such as stress drop, b-value, energy index were observed associated with the sequence. We installed Ishii's borehole strainmeters and could monitor as large a strain change as 10-4 associated with mining. However, sampling was only 4 times per hour and 12 bit A/D. A distance from the M2 sequence was (similar to) 100 m, being not close enough. Therefore, we found another field with several crosscut tunnels excavated across a fault where an M=2-3 event is expected. We installed the strainmeter within several metres of the fault to monitor both shear and normal strains on the fault. The strains are continuously digitized with a 24-bit 25-Hz A/D conversion. The Earth Tide (10-8) is clearly detected. Seismicity induced by longwall mining can be compared to earthquake swarm induced by fluid intrusion to vertical dykes. We also found a decrease in energy index in the source area of M>5 events a few days before the major events.
Semi-controlled seismogenic experiments in South African deep gold mines
Ogasawara, H. (author) / Sato, S. (author) / Nishii, S. (author) / Sumitomo, N. (author) / Ishii, H. (author) / Iio, Y. (author) / Nakao, S. (author) / Ando, M. (author) / Takano, M. (author) / Nagai, N. (author)
Journal of the South African Institute of Mining and Metallurgy ; 102 ; 243-250
2002
8 Seiten, 26 Quellen
Article (Journal)
English
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