S51E-07
Acoustic emission monitoring in Cooke 4 gold mine in South Africa –summary of the main findings of a 5-year SATREPS project–
Friday, 18 December 2015: 09:30
307 (Moscone South)
Makoto Naoi1, Masao Nakatani2, Hirokazu Moriya3, Kenshiro Otsuki3, Thabang Kgarume4, Joachim Philipp5, Osamu Murakami6, Thabang Masakale7, Luiz Ribeiro8, Yasuo Yabe9, Hironori Kawakata10, Anthony Ward11, Ray Durrheim12 and Hiroshi Ogasawara6, (1)Kyoto University, Kyoto, Japan, (2)University of Tokyo, Bunkyo-ku, Japan, (3)Tohoku Univ, Sendai, Japan, (4)CSIR, Pretoria, South Africa, (5)GMuG Gesellschaft für Materialprüfung und Geophysik mbH, Bad Nauheim, Germany, (6)Ritsumeikan University, Kusatsu, Japan, (7)Open House Management Solutions, Potchefstroom, South Africa, (8)seismogen, Carletonville, South Africa, (9)Tohoku University, Sendai, Japan, (10)Ritsumeikan University, Kusatsu Shiga, Japan, (11)Seismogen CC, Carletonville, South Africa, (12)University of the Witwatersrand, Johannesburg, South Africa
Abstract:
We deployed a network targeting acoustic emissions (AEs) down to Mw~ –5 at 1-km depth in the Cooke 4 mine in South Africa as a part of a 5-year project called "Observational Studies in South African Mines to Mitigate Seismic Risks." This network of 30 sensors spans ~100 m, and collected waveform data for more than 4.5-million triggers in 5 years. We provide an overview of the main findings. 1) Naoi et al. (2015; Pageoph) reported that 90% of observed AEs aggregated along the advancing mining front. Their size distribution obeyed the Gutenberg-Richter law down to Mw –4, and their b-values were invariant with time from blasting, contradicting a previous study. 2) Moriya et al. (under review) showed that this AE aggregation consists of several tabular clusters, which were formed regularly as the mining front advanced. Although large earthquakes (Mw > 1) were not found, the clusters likely represent the preparation process of large shear fracture events because their geometry resembles large shear fractures, known by in-situ wall-rock observation, which formed concurrently with large seismic events. 3) Naoi et al. (2015; Tectonophysics) reported that the remaining 10% of AEs exhibit very sharp, planar distributions, with high b-values. They are likely events located on pre-existing geological faults loaded by mining-induced stresses. Such on-fault events are rare for intraplate faults, where microseismic events generally occur in the surrounding region. 4) Naoi et al. (2015; JGR) found planar clusters that newly emerged ahead of a mining front and gradually expanded to 20 m. They likely represent the evolution of a slow-slip patch on a pre-existing fault. The b-value of those AEs decreased drastically from 2.5 to 1.5, consistent with stress buildup by the approaching mining front. This project is supported by JST/JICA, SATREPS, JSPS KAKENHI Grant Numbers 21224012, 21246134, 26249137, 26887022, and MEXT's Earthquake and Volcano Hazards Observation and Research Program.
