日本地球惑星科学連合2018年大会

講演情報

[EJ] Eveningポスター発表

セッション記号 M (領域外・複数領域) » M-IS ジョイント

[M-IS08] 地球掘削科学

2018年5月22日(火) 17:15 〜 18:30 ポスター会場 (幕張メッセ国際展示場 7ホール)

コンビーナ:山田 泰広(海洋研究開発機構 海洋掘削科学研究開発センター)、黒田 潤一郎(東京大学大気海洋研究所 海洋底科学部門)、氏家 恒太郎(筑波大学生命環境系、共同)、菅沼 悠介(国立極地研究所)

[MIS08-P15] Stress measurement using cores of drilling into seismognic zone of M2.0-M5.5 earthquakes in South African gold mines (ICDP DSeis project)

石田 亮壮1Liebeberg Bennie2Rickenbacher Michael3Siyanda Mngadi4Kato Harumi5阿部 周平6矢部 康男6、*杉村 幸祐1野田 拓1船戸 明雄7伊藤 高敏8中谷 正生9Ward Anthony10Durrheim Raymond11小笠原 宏12安富 逹哉13 (1.立命館大学大学院 理工学研究科、2.Moab Khotsong mine, Anglogold Ashanti, South Africa、3.スイス連邦工科大学、4.University of the Witwatersrand, South Africa、5.3D Geoscience, Tokyo, Japan、6.東北大学大学院、7.深田地質研究所、8.東北大学流体研究所、9.東京大学地震研究所、10.Seismogen CC, Carletonville, South Africa、11.School of Geoscience, University of Witwatersrand、12.立命館大学理工学部、13.京都大学)

It has been difficult to drill into seismogenic zones and measure stress. However, an ICDP project "Drilling into seismogenic zones of M2.0-5.5 earthquakes in South African gold mines (2016 - on going; Ogasawara et al. 2018 JpGU)" offers us cores with total length > 1.3 km with minimal drilling-induced damage. It was drilled from 2.9km depth at the Moab Khotsong gold mine toward a M5.5 seismogenic zone. Although damage induced by stress or drilling are sometimes very severe, a JST-JICA SATREPS project "Observational studies in South Africa mines to mitigate seismic risks (2010-2015) (hereinafter SATREPS project)" and mine's geology drilling offer us cores drilled out from seismogenic zones of M2-3 earthquakes at the Cooke 4, Mponeng, and Savuka gold mines.



From the above-mentioned drilled-cores, we selected 276 samples with various lithology (e.g., quartzite, siltstone, basaltic lava and intrusive in Archean era; >2.8 Ga), various drilling diameters (AX, BX and NQ), and stored periods (from < 1 month to > 6 years). In order to measure stress we used Diametrical Core Deformation Analysis (Funato and Ito, 2017). This method measures variation in core diameter with roll angle. Maximum differential stress on a plane normal to core axes can be calculated by assuming elastic deformation during drilling.



We measured diametrical variation with roll angle on several sections normal to core axes for each cores. We looked into measurement errors, which included residuals in fitting sinusoidal curves, and differences in sinusoidal curve phases and amplitude (strain). It was found that fitting residuals was not dependent on the lithology, while not the case for the others. We could statistically quantitatively set thresholds to rule out outliers for each lithology.



At the Moab Khotsong mine, an 817m inclined NQ hole was collared at 2.9km depth and drilled obliquely toward the left-lateral M5.5 fault. Differential stress initially decreases with distance from mine working, while turned to increase when drilling reached the depth of upper edge of the M5.5 aftershock zone.



At the Savuka mine, differential stress along an AX hole intersecting a M3.5 seismic fault at high angle were compared with an overcoring 3D stress measurements and other stress analysis (DCDA in other hole, DRA, borehole breakout and core discing analysis, numerical stress modeling; Abe 2017).



We could set a threshold to select reliable DCDA results at a highly stressed shaft pillar at the Cooke 4 mine, where Naoi et al. (2013, 2015abc) discussed in detail spatio-temporal evolution of microfracturing activity. The differential stress thus selected were consistent between points close to each other and systematically different in space in association with Zebra fault.