[M-IS22] [JJ] 地球掘削科学

[MIS22-02] ICDP 大深度南アフリカ金鉱山におけるM2.0-5.5地震発生場掘削計画（DSeis）ー掘削の開始

*小笠原 宏^1,2、矢部 康男^3,1、伊藤 高敏³、van Aswegen Gerrie⁴、Grobbelaar Michelle⁵、Durrheim Raymond^6,1、Zeigler Martin⁷、Boettcher Margaret⁸、Onstott Tullis⁹、Craill Christo⁵、石田 亮壮²、小笠原 宏幸²、Manzi Musa⁶、加藤 春實¹⁰、船戸 明雄¹¹、Mngadi Siyanda^6,1、安富 達就¹²、掘内 茂木¹³、Milev Alex^14,1、Moyer Pamela⁸、Ellsworth William¹⁵、阿部 周平³、大久保 慎人^1,16、今西 和俊^1,17、Ward Tony^18,1、Birch Denver⁵、Wechsler Neta¹⁹、Liebenberg Bennie²⁰、Berset Nicolas⁷、Hunt John⁵、Bucibo Sifiso⁵、Morema Sylvester¹⁸、Dight Phil²¹、Kieft Tom²²、James Mori¹²、Gupta Harsh²³、Janssen Christoph²⁴、Shapiro Serge²⁵、椋平 祐輔²⁶、Wiemer Stefan⁷、Philipp Joachim²⁸、Plenkers Katrin²⁸、直井 誠^12,1、森谷 祐一^3,1、小村 健太朗²⁹、Somala Surendra²⁷、坂口 清敏³、Harris Rachel^24,9、Cason Errol³⁰、van Heerden Esta³⁰ (1.JST-JICA 地球規模課題対応国際科学技術協力 (2009-2015)、2.立命館大学、3.東北大学、4.鉱山地震研究所（株）, 南アフリカ (SA)、5.地質調査所, SA、6.ウィットワータースランド大学, SA、7.スイス連邦工科大学,スイス、8.ニューハンプシャー大学, USA、9.プリンストン大学, USA、10.（株）3D地科学研究所、11.（公財）深田地質研究所、12.京都大学、13.（株）ホームサイスモメータ、14.科学産業研究所, SA、15.スタンフォード大，USA、16.高知大学、17.産総研、18.サイスモジェン（有）, SA、19.テルアビブ大学，イスラエル、20.アングロゴールド・アシャンティ，SA、21.西オーストラリア大学、22.ニュー・メキシコ工科大学，USA、23.インド地球物理学研究所、24.GFZ, ドイツ、25.ベルリン自由大学，ドイツ、26.マサチューセッツ工科大学, USA、27.インド工科大学、28.GMuG, ドイツ、29.防災科技研、30.自由州大学, SA)

キーワード：国際陸上科学掘削、地震発生場、大深度南アフリカ金鉱山

The International Continental Scientific Drilling Program (ICDP) approved our proposal to drill into and around seismogenic zones where critically-stressed faults initiated ruptures at depth. The drilling targets include four ruptures equivalent to M2.0, 2.8, 3.5, and 5.5 earthquakes that dynamically and quasi-statically evolved in 2.9 Ga hard rock in the Witwatersrand basin, South Africa. A major advantage of our proposed project is the large quantity of high-quality data recorded by existing dense seismic arrays, both on surface and near-field underground, in three deep gold mines. Additionally, the great depths (1.0 to 3.3 km from surface) at which drilling starts reduces costs significantly and allows a larger number of holes to be drilled with the available budget. Flexibility in the drilling direction will also allow us to minimize damage to the borehole or the drilled cores. With ICDP funds, we will conduct full-core drilling of 16 holes at ranges of 50 to 750 m to recover both solid and fractured material in and around the seismogenic zones. This will be followed by core and borehole logging. Additional in-hole monitoring of rock deformation, ground motion, hydrology and geomicrobiology will be supported by co-mingled funds. We will also determine the 3D stress tensor near the collars of the holes using an overcoring technique that has been optimized for the highly-stressed ground and the working conditions found in deep South African mines. The measurement of the differential stress is based on the assumption that anisotropic variation in the diameter of the recovered core is caused by elastic expansion after drilling.

The M5.5 earthquake that took place near Orkney, South Africa on 5 August 2014 offers a special opportunity to compare models of the spatio-temporal evolution of both the main rupture and the aftershock activity determined by the inversion of ground motion measurements with direct observations.

Drilling will commence in early 2017.