[S-SS27] 地震発生の物理・断層のレオロジー

[SSS27-17] 南アフリカ金鉱山の地震観測網による2014年オーク二ー地震の余震分布と発震機構解

*今西 和俊¹、小笠原 宏²、矢部 康男³、堀内 茂木⁴、大久保 慎人⁵、村上 理⁶ (1.産業技術総合研究所、2.立命館大学理工学部、3.東北大学大学院理学研究科付属地震・噴火予知研究観測センター、4.株式会社ホームサイスモメータ、5.高知大学教育研究部自然科学系理学部門、6.地震予知総合研究振興会　東濃地震科学研究所)

キーワード：2014年オークニー地震、金鉱山、南アフリカ、余震分布、発震機構解

The M_w5.4 Orkney earthquake occurred on August 5, 2014, near Orkney town, South Africa. The mainshock and aftershocks were recorded by underground networks 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 have high signal-to-noise ratios and contain higher frequency components up to at least 1 kHz, which provide the opportunity for precise determination of aftershock distribution and source parameters. We determined hypocenters of 2000+ aftershocks by automatic earthquake location software from Home Seismometer Corp. (Horiuchi et al., 2011). Aftershocks distributed at depths from about 4 to 7 km forming a 8 km-long in the NNW-SSE direction. The distribution agrees with one of nodal planes of the mainshock focal mechanism, suggesting that the mainshock represents a left lateral strike-slip fault. Aftershock focal mechanisms were determined from P-wave polarity data as well as body wave amplitudes. As a preliminary analysis, we analyzed aftershocks with at least 15 P-wave polarities and obtained 137 well-determined solutions. Most of aftershocks show a pure strike-slip mechanism that is similar to the mainshock. We also found some aftershocks whose P- and T- axis deviates from the general trend and contain normal or reverse faulting components. These events seem to distribute at the middle and the north of the aftershock distribution, suggesting the existence of local stress heterogeneity. Further analysis of aftershocks is needed to elucidate whether the heterogeneity was caused by stress changes due to the mainshock and/or associated with locally formed pre-mainshock stress regime.
Acknowledgements. The seismic network used in this study is operated by Anglogold Ashanti and Open House Management Solutions. The data processing was performed by Institute of Mine Seismology. The data ownership belongs to Anglogold Ashanti.