EGU2020-12094, updated on 12 Jun 2020
https://doi.org/10.5194/egusphere-egu2020-12094
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

The seismogenic zones of an M2.0-5.5 earthquakes successfully recovered in deep South African gold mines: the outcomes and the follow-up plan

Hiroshi Ogasawara1, Bennie Liebenberg2, Yasuo Yabe3, Yuki Yokoyama4, Tetsuro Hirono4, Devan M. Nisson5, Tullis C. Onstott5, Thomas L. Kieft6, Esta van Heerden7, Thomas Wiersberg8, Taku Noda1, Musa S.D. Manzi9, Siyanda B. Mngadi9, Raymond J. Durrheim9, Yuhji Yamamaoto10, Takatoshi Ito3, Akio Funato11, Martin Ziegler12, James J. Mori13, Carsten Dinske14, and the The ICDP DSeis team*
Hiroshi Ogasawara et al.
  • 1Ritsumeikan Univ., Kusatsu, Japan (ogasawar(at)se.ritsumei.ac.jp)
  • 2Moab Khotsong mine, South Africa
  • 3Tohoku Univ., Japan
  • 4Osaka Univ., Japan
  • 5Princeton Univ., USA
  • 6New Mexico Inst. Mining Tech., USA
  • 7iWater, South Africa
  • 8GFZ, Potsdam, Germany
  • 9Univ. Witwatersrand, South Africa
  • 10Kochi Core Center, Japan
  • 11Fukada Geol. Inst.
  • 12ETH, Zurich, Switzerland
  • 13DPRI, Kyoto Univ., Japan
  • 14Freie Universität Berlin
  • *A full list of authors appears at the end of the abstract

This paper reports on the outcomes of the ICDP drilling into seismogenic zones of M2.0-5.5 earthquakes in South African (SA) gold mines (DSeis; 2017-2018), the follow-up work in 2019, and planned post-drilling activity from 2020 onwards.

In deep SA gold mines, seismogenic zones evolve ahead of thin tabular excavations. Normal faulting prevails because mining enhances the vertical maximum principal stress. At 1km depth at the Cooke 4 mine, we elucidated the evolution of the seismogenic zone with a dense acoustic emission network. In 2017, we successfully recovered both the metasedimentary host rock (mainly quartzite ~2.8 Ga) and samples of the seismogenic zone with well-preserved fracture systems using a triple-tube (BQ 1.5m-long). Subsequent laboratory work investigated critical characteristics of rock-rock friction.

In 2014, an M5.5 earthquake, the largest in deep South African gold mining districts, took place. Dense seismic networks, both on the Earth’s surface and at 2-3 km depth, showed that this event was atypical because it was a sinistral event on an unknown geological structure below the mining horizon in West Rand Group strata (~2.9 Ga). Inversion and back-projection of the ground motion showed complicated but unilateral rupture propagation. The densest population of aftershocks shows a sharp upper cut-off and streaks, both dipping to the south.  Its centroid lies outside the significant main rupture zone. In 2017, we commenced drilling at a site at 2.9km depth in a tension quadrant of the sinistral faulting, several hundreds of meters above the upper fringe of the M5.5 aftershock plane. During 2017-2018, we drilled holes, of a total length of 1.6 km. With a 1.5m NQ triple-tube for the critical section, we could recover the fault materials and the host rock with the seismic fracture system well preserved. Borehole logging and core curation in SA and laboratory work at international organizations, including Kochi Core Center Japan (KCC), followed during 2017-2019. With the geology data mapped on the mining horizons and the legacy seismic reflection data as additional information, the following picture is emerging: (a) transition of the stress regime from normal-faulting to sinistral-faulting; (b) stress localization; (c) heterogeneity in the aftershock distribution as well as the segregation between the main rupture and aftershocks, potentially correlated with significant heterogeneity in mechanical properties; (d) a role of an altered lamprophyre dike; (e) hypersaline brine with salinity even higher than measurements at other deep gold mines, potentially as old as brine found at Kidd Creek mine, Canada; and (f) abiogenic gas and organic carbon.

These data sets allow us to address questions in earthquake and deep-life sciences raised in the ICDP Science Plan (2014-2019). In 2019, the ICDP Executive Committee described DSeis as a ‘successful’ project. To integrate and discuss the outcomes in greater depth and plan additional follow-up work, we are planning a post-drilling workshop in November 2020 or January 2021 at KCC before we return the imported critical section of the core to South Africa.

The ICDP DSeis team:

H. Ogasawara, A. Ishida, HY. Ogasawara, K. Sugimura, T. Noda, M. Kanematsu, R. Tadokoro, M. Higashi, S. Yoshida, K. Suzuki, Y. Mima, B. Liebenberg, E. van Heerden, C. van der Nest, B. Freese, R.J. Durrheim, B. Watson, M. Manzi, L. Linzer, S. Mngadi, S. Enslin, S. Tau, D. Ligaraba, T.J. Mabaso, T. Lepote, N. Masondo, M. Ziegler, M. Richenbacher, N. Berset, Y. Yabe, T. Ito, Y. Mukuhira, S. Abe, T.C. Onstott, M. Lau, D.M. Nisson, Z. Garvin, R. Harris, T.L. Kieft, E. Cason, J. Vermeulen, E. van Heerden, J.C. Castillo, L. Scheepers, G. Hofmann, Y. Jooste, M. Rodel, T. Hirono, Y. Yokoyama, A. Funato, D. Roberts, N.Z. Nkhosi, J. Machake, T. Wiesberg, M. Zimmer, C. Kujawa, C. Janssen, A. Schleicher, R. Conze, N. Wechsler, H. Kato, A.K. Ward, S. Morema, V. Visser, K. Bosman, W. Meintjes, M. Grobbelaar, C. Claill, A. Cichowicz, S. Bucibo, G. van Aswegen, T.L. Kieft, J.J. Mori, A.Tsutsumi, M. Naoi, S. Kaneki, T. Yasutomi, W.L. Ellsworth, M. Boettcher, S.A. Shapiro, C. Dinske, S. Sangaraju, S.N. Somala , H. Gupta, P. Dight, Y. Yamamoto, T. Ishikawa, T. Matsuzaki, K. Arai, Tadai, T. Komatsu, N. Shibuya, S. Yanagimoto, Y. Katayama.

How to cite: Ogasawara, H., Liebenberg, B., Yabe, Y., Yokoyama, Y., Hirono, T., Nisson, D. M., Onstott, T. C., Kieft, T. L., van Heerden, E., Wiersberg, T., Noda, T., Manzi, M. S. D., Mngadi, S. B., Durrheim, R. J., Yamamaoto, Y., Ito, T., Funato, A., Ziegler, M., Mori, J. J., and Dinske, C. and the The ICDP DSeis team: The seismogenic zones of an M2.0-5.5 earthquakes successfully recovered in deep South African gold mines: the outcomes and the follow-up plan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12094, https://doi.org/10.5194/egusphere-egu2020-12094, 2020

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