[S02-1] World overview of anthropogenic seismicity I

[S02-1-03] Source parameters of the 2014 M5.5 Orkney earthquake sequence, South Africa, estimated by using near-field underground seismic arrays in gold mines

^○Kazutoshi Imanishi¹, Hiroshi Ogasawara², Yasuo Yabe³, Shigeki Horiuchi⁴, Makoto Okubo⁵, Osamu Murakami⁶ (1.Geological Survey of Japan, AIST, Tsukuba, Japan, 2.Ritsumeikan University, Kusatsu, Japan, 3.Tohoku University, Sendai, Japan, 4.Home Seismometer Corporation, Shirakawa, Japan, 5.Kochi University, Kochi, Japan, 6.Tono Research Institute of Earthquake Science, Association for the Development of Earthquake Prediction, Mizunami, Japan)

The M5.5 Orkney earthquake occurred on August 5, 2014, near Orkney town, South Africa. The mainshock and aftershocks were recorded by underground arrays in gold mines, which are composed of 46 three-component geophones installed at 2-3 km depths. The sampling rate is 6 kHz. The observed waveforms display high signal-to-noise ratios and contain high frequency components up to at least 1 kHz, providing us an unprecedented opportunity for precise determination of aftershock distribution and source parameters. We determined hypocenters of 2000+ earthquakes from 4 August to 31 October 2014 by automatic earthquake location software from Home Seismometer Corp. (Horiuchi et al., 2011). Aftershocks distribute at a depth range of 4–7 km and form a 8 km-long near-vertical plane in the NNW-SSE direction. This distribution agrees with the mainshock focal mechanism (left lateral strike-slip faulting). We also determined 500+ aftershock focal mechanisms by using P-wave polarity data as well as body wave amplitudes, where almost all aftershocks could be explained by a double-couple solution. Stress tensor inversion reveals that strike-slip faulting with a NW-SE compression prevails throughout the region, which differs from the stress state on the mining levels (e.g., Ogasawara et al., 2012, 2013) but explains the occurrence of the mainshock. We also found that normal-faulting stress fields prevail around the southern part of the aftershock distribution, where low stress drop aftershocks are also prominent (Kazuki Sato BSc thesis, Ritsumei University). We infer that the mainshock terminated at the area where the stress field is different from the mainshock area and the stress level is low.

Acknowledgements. The seismic network developed by Inst. Mine Seismol. Ltd (IMS) used in this study is operated and maintained by Anglogold Ashanti Ltd (AGA) and Open House Management Solutions Ltd. The data processing was performed by IMS. The data is owned by AGA.