Observation of numerous aftershocks of an Mw 1.9 earthquake with an AE network installed in a deep gold mine in South Africa

Yasuo Yabe,Hiroshi Ogasawara,Hironori Kawakata,Toshihiro Igarashi,Masao Nakatani,Makoto Naoi,Gilbert Morema,Georg Dresen,Joachim Philipp

doi:10.1186/bf03352963

Abstract

Abstract 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 M w 1.9 earthquake whose hypocenter was ∼30 m above the network occurred in the dyke. Although the mineowned 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.

Highlights

Deep mining facilities provide an excellent opportunity to observe earthquakes at the closest proximity possible (e.g., Nicolaysen, 1992)
Since 1994, Japanese and South African researchers have carried out a series of experiments within the framework of the SeeSA project (Semi-controlled Earthquake generation Experiments in South African mines) (Iio and Fukao, 1992; Ogasawara et al, 2002a) in which various kinds of sensors have been installed in the close proximity of the source region of forthcoming earthquakes
By deploying sensitive high-frequency acoustic emission (AE) sensors, we were able to observe a large number of aftershocks, most of which are presumably very small, following an Mw 1.9 earthquake that occurred in our AE network

Summary

Introduction

Deep mining facilities provide an excellent opportunity to observe earthquakes at the closest proximity possible (e.g., Nicolaysen, 1992). By deploying sensitive high-frequency (up to 200 kHz) AE sensors, we were able to observe a large number of aftershocks, most of which are presumably very small, following an Mw 1.9 earthquake that occurred in our AE network. The reef is 90 m above the location of our sensors; as such, our network is situated in solid rock that has not been damaged by stoping, but it is still close enough to mining activities to expect the occurrence of relatively large earthquakes due to the non-local redistribution of mining-induced stress. Several events with a local magnitude >2 occurred along the PG dyke prior to our commencing with observations in June 2007 These events were presumably affected by earlier mining activity at shallower levels to the NNW of our site. We would like to investigate in detail whether or not cluster N belongs to the main shock rupture plane

Discussion

Conclusion