Japan Geoscience Union Meeting 2016

Presentation information


Symbol S (Solid Earth Sciences) » S-SS Seismology

[S-SS27] Fault Rheology and Earthquake Physics

Wed. May 25, 2016 1:45 PM - 3:10 PM Convention Hall A (2F)

Convener:*Takeshi Iinuma(National Research and Development Agency Japan Agency for Marine-Earth Science and Technology), Yuko Kase(Geological Survey of Japan, AIST), Ryosuke Ando(Graduate School of Science, University of Tokyo), Wataru Tanikawa(Japan Agency for Marine-Earth Science and Technology, Kochi Instutute for Core Sample Research), Hideki Mukoyoshi(Department of Geoscience Interdisciplinary Graduate School of Science and Engineering, Shimane University), Chair:Eiichi Fukuyama(National Research Institute for Earth Science and Disaster Prevention), Takehito Suzuki(Department of Physics and Mathematics, Aoyama Gakuin University)

2:15 PM - 2:30 PM

[SSS27-03] Emergence and disappearance of repeating earthquakes on a geological fault in a deep gold mine in South Africa

*Junya Yamaguchi1, Makoto Naoi1, Masao Nakatani2, Moriya Hirokazu3, Thabang Kgarume4, Toshihiro Igarashi2, Osamu Murakami5, Thabang Masakale6, Yasuo Yabe3, Kenshiro Otsuki3, Hironori Kawakata7, Tsuyoshi Ishida1, Anthony Ward8, Ray Durrheim4,9, Hiroshi Ogasawara7 (1.Kyoto University, 2.The University of Tokyo, 3.Tohoku University, 4.CSIR, 5.Tono Research Institute of Earthquake Science, 6.OHMS, 7.Ritsumeikan University, 8.SeismoGen, 9.University of the Witwatersrand, Johannesburg)

Keywords:Acoustic Emission, Induced Earthquake, Repeating Earthquake

We deployed an Acoustic Emission (AE) monitoring network consisting of 30 very sensitive AE sensors and 7 accelerometers at 1-km depth in the Cooke 4 gold mine in South Africa, where many earthquakes up to ~M 3 are induced by stress buildup by mining. Naoi et al. [2015] analyzed data obtained by the AE network during 2 months, and they found very small repeating earthquakes of –5.1 ≤ Mw ≤ –3.6 which occurred on a geological fault. In this study, we extended the analysis period to 14 months, investigating a time variation of the repeating earthquakes during longer periods.
Firstly, we relocated 5869 events that occurred along the geological fault during the 14 months (from 7 April 2011 to 30 may 2012) by using the double-difference method [Waldhauser and Ellsworth, 2000] with the cross-correlation travel-time reading technique. Of the relocated AEs, we chose 3735 events within 3 m from an approximation plane of the two-dimensional distribution of the AEs, which delineates the fault. We then cross-correlated waveforms of all event pairs whose interevent distances D were less than 2 m. We chose event pairs whose seismograms had cross-correlation coefficient greater than 0.90 at 20 percent or more working stations at the time and their rupture areas evaluated from a circular crack model overlapped significantly. We finally assembled them into “repeater groups” whose event pairs shared one event or more.Out of the 3735 events (35.6%), 1328 events belonging to 308 groups were identified as repeaters. The number of recurrence reached 45 times for the largest repeater group.
Activities of some groups continues for the whole 14 months (Type A), but we also found groups that newly emerged (Type B) or disappeared (Type C) in the analysis period. We also found areas of ~10-m scale where only Type-B or Type-C groups existed, which likely corresponds to a newly emerged or terminated macroscopic slow slip respectively. Meanwhile, there were areas where Type A–C existed within a tiny area of a-few-meter scale. Only in such area, we found some Type C groups whose events size decreased with time. We consider the emergence and disappearance of the repeaters in such areas represent formation and dissipation of unstable patches of the fault, resulting from newly encountered protruding portions or frictional wear of the contacts by the progress of fault creeping.