Earth Planets Space, Vol. 61 (No. 10), pp. e49-e52, 2009E-LETTER
Yasuo Yabe1, Joachim Philipp2, Masao Nakatani3, Gilbert Morema4, Makoto Naoi3, Hironori Kawakata5, Toshihiro Igarashi3, Georg Dresen6, Hiroshi Ogasawara5, and JAGUARS
1Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan
2GMuG Gesellschaft für Materialprüfung und Geophysik, Dieselstraße 9, 61231 Bad Nauheim, Germany
3Earthquake Research Institute, the University of Tokyo, 1-1-1 Yayoi, Bunkyo, Tokyo 113-0032, Japan
4SeismoGen cc, PO Box 1177, Carletonville, 2500, South Africa
5Faculty of Science and Engineering, Ritsumeikan University, 1-1-1 Noji Higashi, Kusatsu 525-8577, Japan
6Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany
(Received September 30, 2009; Revised October 31, 2009; Accepted November 4, 2009; Online published November 24, 2009)
This is the first report from the JAGUARS (JApanese-German
Underground Acoustic Emission Research in South Africa) project, the overall aim of which is to observe ultra-small fracturing in a more or less natural environment. We installed a local (∼40-m span) network of eight acoustic emission (AE) sensors, which have the capability to observe up to 200 kHz at a depth of 3.3 km in a South African gold mine. Our specific objective was to monitor a 30-m thick dyke that remains as a dip pillar against active mining ∼90 m above our network. An Mw 1.9 earthquake whose hypocenter was ∼30 m above the network occurred in the dyke. Although the mine-owned geophone (4.5 Hz) network detected only five earthquakes in the surrounding 200×200×150-m3 volume within the first 150 h following the main shock, our AE network detected more than 20,000 earthquakes in the same period. More than 13,000 of these formed a distinct planar cluster (∼100×80 m2) on which the main shock hypocenter lay, suggesting that this cluster delineates the main shock rupture plane. Most of the aftershocks were presumably very small, probably as low as M ∼ -4. The aftershock cluster dipped ∼60°. This is consistent with normal faulting under a nearly vertical compression field, as indicated by nearly horizontal breakouts found in a borehole crossing the rupture plane.
Key words: Semi-controlled earthquake generation experiment, acoustic emission, mining-induced earthquake, deep South African gold mines, aftershocks, main shock rupture plane.