Near-field monitoring of seismic source behavior at South African deep gold mines

HR: 13:40h
AN: S33D-01 [Abstracts]
TI: Near-field monitoring of seismic source behavior at South African deep gold mines
AU: * Ogasawara, H
EM: ogasawar@se.ritsumei.ac.jp
AF: Ritsumeikan U, Noji, Kusatsu, 525, Japan
AU: Nakatani, M
AF: U Tokyo, Bunkyo, Tokyo, 113, Japan
AU: Iio, Y
AF: DPRI Kyoto U, Gokasho, Uji, 611, Japan
AU: Ishii, H
AF: TRIES, Akeyo, Mizunami, 509, Japan
AU: Yamada, T
AF: U Tokyo, Bunkyo, Tokyo, 113, Japan
AU: Naoi, M
AF: U Tokyo, Bunkyo, Tokyo, 113, Japan
AU: Yasutake, G
AF: Ritsumeikan U, Noji, Kusatsu, 525, Japan
AU: Kawakata, H
AF: Ritsumeikan U, Noji, Kusatsu, 525, Japan
AU: Yamamoto, A
AF: Ritsumeikan U, Noji, Kusatsu, 525, Japan
AU: Yamauchi, T
AF: Nagoya U, Chikusa, Nagoya, 464, Japan
AU: Nakao, S
AF: Kagoshima U, Korimoto, Kagoshima, 890, Japan
AU: Yabe, Y
AF: Tohoku U, Aoba, Sendai, 980, Japan
AU: Otsuki, K
AF: Tohoku U, Aoba, Sendai, 980, Japan
AU: Otsuki, K
AF: AIST, Higashi, Tsukuba, 305, Japan
AU: Satoh, T
AF: U Tokyo, Bunkyo, Tokyo, 113, Japan
AU: Satoh, T
AF: AIST, Higashi, Tsukuba, 305, Japan
AU: Kato, A
AF: U Tokyo, Bunkyo, Tokyo, 113, Japan
AU: Shinya, Y
AF: U Tokyo, Bunkyo, Tokyo, 113, Japan
AU: Nagata, K
AF: U Tokyo, Bunkyo, Tokyo, 113, Japan
AU: Kuwano, O
AF: U Tokyo, Bunkyo, Tokyo, 113, Japan
AU: Igarashi, T
AF: U Tokyo, Bunkyo, Tokyo, 113, Japan
AU: Miyake, H
AF: U Tokyo, Bunkyo, Tokyo, 113, Japan
AU: Ide, S
AF: ISS International Ltd, Technopark, Stellenbosch, 7613, South Africa
AU: van Aswegen, G
AF: ISS International Ltd, Technopark, Stellenbosch, 7613, South Africa
AU: Mendecki, A
AF: Seismogen CC, Carleton Johns, Carletonville, 2500, South Africa
AU: Ward, T
AF: Seismogen CC, Carleton Johns, Carletonville, 2500, South Africa
AU: Research Group, S
AF: SeeSA, Semi-controlled Earthquake-generation Experiments in South African gold mines, Kusatsu, JPN 525,
AB: We introduce our SeeSA projects, as important as dense array monitoring According to a mining plan and a geological map detailing locations of faults or weakness, we can anticipate potential M > 2 seismic sources at depths of 2.0 - 3.6 km at South African gold mines. At such potential sources, we have installed instruments prior to an onset of irreversible process to monitor earthquake generation process. From the previous projects for periods of from a year to a few years, the possible widest dynamic range and resolution have revealed the finest detail of the process since 1995 in cooperation with ISS International Ltd and South African gold mines (Mponeng, Bambanani, Tau Tona, Buffelsfontein GM, and ERPM), Wits Univ., Geohydroseis CC., Seismogen CC., OHMS CC., GFZ, GMuG, CSIR. The talk summarizes examples of our successful monitoring and introduces some on-going projects. Highlighted are the following. Yamada et al. [05, 07] demonstrated that mine tremors have rupture process as complex as natural larger earthquakes and the scale dependency of rupture parameters is similar to that for natural larger earthquakes. We successfully recorded strain accumulations larger than 100 micro strain, followed by several hundreds of seismic events (-1 < M < 3; distance < ~ 250 m). The seismicity within about 100m from strainmeters caused frequent, seismic strain-steps; the largest recorded was greater than 100 micro strain by an M2.5 earthquake at a distance within ~100 m. One of the most important results were that no detectable accelerating precursors preceded strain-steps associated with several hundreds of the earthquakes (- 1 < M < 3) catalogued by mine’s seismic networks (hereinafter Catalogued E/Q; Takeuchi 05), while significant post-seismic drifts followed some strain-steps by Catalogued E/Qs. Frequently observed were episodic strain changes with durations of much slower than strain-steps associated with the Catalogued E/Qs [Naoi et al. 06]. Striking were some examples of slowest strain-steps (with minutes durations), preceded by significant forerunners [Naoi et al. 06; Yasutake et al. 06]. With a clearest recording, in which the strain change was as large as 2 micro strains, a forerunner undergoes a linear change [Yasutake et al. 07AGU], not in a continuously accelerating manner that is predicted, e.g., by a typical rate-and-state-dependent friction-law. At a site in a ~ 50m-wide fault zone with multiple gouge zones ( < ~ 1 m thick) at 104 Level at Mponeng mine, Yasutake et al. [07AGU] found several examples of slow strain steps detected by a pair of strainmeters that was ideally spaced. With the recordings, we could constrain locations and magnitudes of the slower strain- steps, Mw being from -1.5 to -0.2. A seismic moment was proportional to a cube of a duration period, much as regular natural larger earthquake. The tendency was different from those proposed by Ide et al. [07] for slow events, much larger than those we found at Mponeng.
DE: 1209 Tectonic deformation (6924)
DE: 7215 Earthquake source observations (1240)
DE: 7270 Tomography (6982, 8180)
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Seismology [S]
MN: 2007 Fall Meeting