HR: 0830h
AN: S41C-0103    [PDF]
TI: Coseismic strain changes of 10$^{-4}$ and 7$\times$10$^{-5}$ recorded with Ishii strainmeter by continuous 24bit 25Hz monitoring on seismogenic fault of M~3 in deep gold mine, South Africa
AU: * Takeuchi, J
EM: rp010999@se.ritsumei.ac.jp
AF: Ritsumeikan University, 1-1-1, Nojihigashi, Kusatsu, Shi 525-0058 Japan
AU: Ogasawara, H
EM: ogasawar@se.ritsumei.ac.jp
AF: Ritsumeikan University, 1-1-1, Nojihigashi, Kusatsu, Shi 525-0058 Japan
AU: Ishii, H
AF: Tono Research Institute of Earthquake Science, 1-63, Yamanouchi, Arino-cho, Mizuho, Gif 509-6132 Japan
AU: Nakao, S
AF: The University of Tokyo, 1-1-1 Yayoi, Bunkyou, Tok 113-0032 Japan
AU: Ando, M
AF: Nagoya University, Furouchou, Chikusa, Aic 464-8602 Japan
AU: Aswegen, G V
AF: ISS International Ltd, 23 Electron Street, Technopark, Ste 12063 South Africa
AU: Mendcki, A
AF: ISS International Ltd, 23 Electron Street, Technopark, Ste 12063 South Africa
AU: Cichowicz, A
AF: ISS International Ltd, 23 Electron Street, Technopark, Ste 12063 South Africa
AU: Mountfort, P
AF: ISS International Ltd, 23 Electron Street, Technopark, Ste 12063 South Africa
AU: Ide, S
AF: The University of Tokyo, 1-1-1 Yayoi, Bunkyou, Tok 113-0032 Japan
AU: Kawakata, H
AF: Nagoya University, Furouchou, Chikusa, Aic 464-8602 Japan
AU: Kawakata, H
AF: Kyoto University, Gokajou, Ujishi, Kyo 611-0011 Japan
AB: We will report on coseismic strain step ranging 10$^{-5}$ to 10$^{-4}$ recorded with 24bit 25Hz continuous monitoring, associated with seismic events with M~2 at a distance about 100m at Bambanani mine, Welkom, South Africa. We have been observing mine tremors of -1.2$<$M$<$3 at a hypocentral distance of near dozens of meters since September 2000 at the mine. We installed an Ishii multi-component borehole strainmeter several meters from a potential seismogenic fault to monitor normal and slip-driving shear strains on it. It accommodates as large strain as 10$^{-4}$, and detects as subtle strain as 10$^{-9}$. To record rock mass behavior just before and after a tremor, data are continuously recorded with a 24bit 25Hz monitoring system. Recently, the strainmeter recorded two big coseismic steps up to 10$^{-4}$, which can be recorded only within a seismogenic volume. On 2003 February 14, an event of magnitude 2.5 at a distance of about 70m from the strainmeter and it produced maximum coseismic strain change of $\pm$10$^{-5}$. Its foreshocks were recorded by the strainmeter from 36.5 seconds before the main shock. The last big foreshock was M1.4 just 2 seconds before the main shock, which was followed by a significant strain change. At this moment, we don't know if that is just a postseismic change of the M1.4 foreshock or preseismic change of the main shock. On 2003 April 12, the biggest coseismic strain change of 10$^{-4}$, amoung those we have ever recorded by the strain meter, occurred by an M2.5 event at a distance of about 100m from strainmeter. No foreshocks were recorded with the event of 2003 April 12. Although the non-causal effects of a recording system are successfully removed with minimum phase filter, a precursor strain change larger than 10$^{-8}$ was not seen until the onset (0.1 s just before the event). During a period with significant postseismic change, the directions of principal strain-change axes were stable, suggesting that each channel worked consistently. However, it should be noted that duration of coseismic step was much longer than that is derived from seismograms. We have been investigating whether that is caused by strainmeter's instrumental response or a real rock mass behavior.
DE: 7209 Earthquake dynamics and mechanics
SC: Seismology [S]
MN: 2003 Fall Meeting