Japan Geoscience Union Meeting 2016

Presentation information


Symbol M (Multidisciplinary and Interdisciplinary) » M-IS Intersection

[M-IS16] Drilling Earth Science

Thu. May 26, 2016 10:45 AM - 12:15 PM 103 (1F)

Convener:*Yasuhiro Yamada(Japan Agency for Marine-Earth Science and Technology (JAMSTEC), R&D Center for Ocean Drilling Science (ODS)), Minoru Ikehara(Center for Advanced Marine Core Research, Kochi University), Yusuke Suganuma(National institute of Polar Research), Kazuno Arai(Japan Agency for Marine-Earth Science and Technology), Keita Umetsu(Japan Agency for Marine Earth Science and Technology), Chair:Kazuya Shiraishi(Japan Agency for Marine-Earth Science and Technology), Saneatsu Saito(Japan Agency for Marine-Earth Science and Technology)

11:15 AM - 11:30 AM

[MIS16-09] Scientific drilling into seismogenic zones of M2.0 – M5.5 earthquakes in deep South African gold mines (DSeis)

*Hiroshi Ogasawara1,2, Yasuo Yabe1,3, Takatoshi Ito3, Gerrie van Aswegen1,4, Artur Cichowicz1,5, Raymond J Durrheim1,6,7, James J Mori8, Tullis C Onstott9, Thomas L Kieft10, Margaret Boettcher11, Wiemer Stefan12, Ziegler Martin12, Christoph Janssen13, Shapiro Serge14, Harsh Gupta15, Phil Dight16, ICDP DSeis team (1.JST-JICA SATREPS, 2.Ritsumeikan Univ., 3.Tohoku Univ., 4.Inst. Mine Seismol., South Africa, 5.Council Geosci., South Africa, 6.Council Sci. Ind. Res., South Africa, 7.Univ. Witwatersrand, South Africa, 8.Kyoto Univ., 9.Princeton Univ., USA, 10.New Mexico Inst. Min. Tech., USA, 11.Univ. New Hampshire, USA, 12.ETH Zurich, Switzerland, 13.GFZ Potsdam, Germany, 14.Freie Univ. Berlin, Germany, 15.Nat. Geophys. Res. Inst., India, 16.Univ. W. Australia, Australia)

Keywords:South African gold mines, Seismogenic zones, Scientific drilling

Several times a year, small (M2) mining-induced earthquakes occur only a few tens of meters from active workings in South African gold mines at depths of up to 3.4 km. The source regions of these events are accessible with short boreholes from the deep mines, and provide a very cost-effective method to directly study the earthquake sources. Recently, the largest event (M5.5) recorded in a mining region, took place near Orkney, South Africa on 5 August 2014, with the upper edge of the activated fault being several hundred meters below the nearest mine workings (3.0 km depth). This event has rare detailed seismological data available both from surface and underground seismometers and strainmeters, allowing for a detailed seismological analysis. Drilling into the source area of this earthquake while aftershocks are still occurring will enable important near-field seismological observations as well as a rare opportunity to study possible presence of H2 that is important for microbiological activity.
We intend to drill several tens of holes into and around seismogenic zones to study the rupture details and scaling of both small (M2.0) and larger (M5.5) earthquakes. An advantage of the relatively low cost of drilling is that multiple holes can be drilled. Past fault zone drilling projects have been limited to 1 or 2 boreholes, severely limiting the ability to resolve spatial variability. The value of the project will be maximized if we combine results from a number of boreholes drilled into the source area of the M5.5 seismogenic zone, and also compare with boreholes in source regions of small earthquakes in other mining horizons. Additionally, the combination of logging, fault sampling, and earthquake monitoring, will be enhanced in some cases by the direct visual observations of exhumed faults, leading to a unique complete picture of the earthquake source.
In seismogenic zones in a critical state of stress, it is difficult to delineate reliably local spatial variations in both the directions and magnitudes of principal stresses (3D full stress tensor). We have overcome this problem and can numerically model stress better, enabling orientations of boreholes that minimize stress-induced damage during drilling and overcoring. We can also reliably measure the stress tensor even when stresses are as large as those expected in seismogenic zones. Better recovery of cores with less stress-induced damage is also feasible. These studies will allow us to address key scientific questions in earthquake science and deep biosphere activities.