HR: 0800h
AN: T21A-0370    [Abstracts]
TI: Slow strain steps observed by two Ishii strainmeters within an M3 source area at a 2.9km depth, Mponeng gold mine, South Africa
AU: * Yasutake, G
EM: rp014036@se.ritsumei.ac.jp
AF: Fac Sci Engr, Ritsumeikan U, Kusatsu, Japan, 525, Japan
AU: Ogawasara, H
EM: ogasawar@se.ritsumei.ac.jp
AF: Fac Sci Engr, Ritsumeikan U, Kusatsu, Japan, 525, Japan
AU: Kawakata, H
AF: Fac Sci Engr, Ritsumeikan U, Kusatsu, Japan, 525, Japan
AU: Morishita, K
AF: Fac Sci Engr, Ritsumeikan U, Kusatsu, Japan, 525, Japan
AU: Yamamoto, A
AF: Fac Sci Engr, Ritsumeikan U, Kusatsu, Japan, 525, Japan
AU: Takeuchi, J
AF: Fac Sci Engr, Ritsumeikan U, Kusatsu, Japan, 525, Japan
AU: Shimoda, N
AF: Fac Sci Engr, Ritsumeikan U, Kusatsu, Japan, 525, Japan
AU: Naoi, M
AF: U Tokyo, Tokyo, Japan, 113, Japan
AU: Nakatani, M
AF: U Tokyo, Tokyo, Japan, 113, Japan
AU: Kato, A
AF: U Tokyo, Tokyo, Japan, 113, Japan
AU: Ishii, H
AF: Tono Res Inst Eq Sci, Mizunami, Japan, 509, Japan
AU: Nakao, S
AF: Kagoshima U, Kagoshima, Japan, 890, Japan
AU: Otsuki, K
AF: Tohoku U, Sendai, Japan, 980, Japan
AU: Yamauchi, T
AF: Nagoya U, Nagoya, Japan, 464, Japan
AU: Iio, Y
AF: Kyoto U, Uji, Japan, 611, Japan
AU: Carlsten, R
AF: Mponeng mine, Western Levels, South Africa, 2501, South Africa
AU: McGill, R
AF: Anglogold Ashanti Ltd., Jo'burg, South Africa, 2001, South Africa
AU: Tony, T
AF: Seismogen CC., Carletonville, South Africa, 2500, South Africa
AU: Aswegen, G v
AF: ISS International, Stellenbosch, South Africa, 7613, South Africa
AU: Mendecki, A J
AF: ISS International, Stellenbosch, South Africa, 7613, South Africa
AU: Lenegan, P
AF: ISS International, Stellenbosch, South Africa, 7613, South Africa
AU: Reserch Group SeeSA
AB: The Research Group for Semi-controlled Earthquake-generation Experiments in South African gold mines (SeeSA) has attempted to observe the details of the source fault behavior near the Mw2-3 earthquake source area [e.g., Iio and Fukao, 1992]. At a depth of 2.9km in Mponeng gold mine, one of our observational sites, two Ishii strainmeters were installed near the fault where M3 earthquakes were anticipated [e.g., Ogasawara et al., 2005]. We analyzed the ~8-month strainmeter recordings. During this period, a high seismicity was induced by active mining, more than 5000 earthquakes (-3<Mw<3) being catalogued using mine seismic array within a 500³m³ volume around the strainmeters. In the Mponeng recordings, we found 6 steps with much longer durations than seen in normal earthquakes. In a preceding study, Naoi et al. [2006] found similar steps (slow-strain step) at the Bambanani mine site. The slow- steps found at both sites are possibly aseismic events because they had no corresponding catalogued earthquakes. 5 of 6 slow-steps at Mponeng were recorded with both strainmeters and they are synchronized with each other. Hereafter, we refer to the 5 slow-steps as SstM. The strain changes for one of SstM was on the order of 1E-6 and with a duration of ~8-minute. This is the largest, clearest slow-step ever recorded in SeeSA projects. Hereafter, we refer to this as SstML. The remaining 4 of 5 SstMs was on the order of 1E-7 and with duration of from 10-minutes to 1-hour. We successfully constrained the source distances and Mw of SstM, taking into accounts the ratio of magnitudes of strain changes and geometrical settings of strainmeters. In the result, the source of SstML was within ~3m from one strainmeter and Mw<-1.5. Mws for the other 4 SstMs were also constrained to be between -1.2 and 0.7. There have never been observed these small slow events. Ide et al. [2007] proposed a new scaling law to unify slow events that were recently found at subducting plate boundaries, including ETS, LFE, VFL, SSE and silent EQ. This relationship, that a seismic moment is proportional to their duration, clearly differs from that of regular earthquakes. On the other hand, SstMs follow another scaling relationship that a seismic moment is proportional to the cube of their duration. SstML were preceded by a clear forerunner linearly increasing with time, not accelerating with time. Hagiwara and Otsuki [2007] found similar phenomena in laboratory experiments, and the phenomena are thought to be seen during Riedel shear process. After the forerunner, SstML undergoes the fastest change followed by a logarithmic decay, with a sequence of smaller, but clear, shorter-steps.
DE: 7200 SEISMOLOGY
DE: 7215 Earthquake source observations (1240)
DE: 9305 Africa
SC: Tectonophysics [T]
MN: 2007 Fall Meeting