[S02-4] Studies of seismicity at Koyna, India and other holistic approach projects

[S02-4-03] An overview of an ICDP project to drill into seismogenic zones of M2.0 – M5.5 earthquakes in deep South African gold mines (DSeis)

^○Hiroshi Ogasawara^{1, 2}, Yasuo Yabe^{1, 3}, Takatoshi Ito³, Gerrie Van Aswegen^1,4, Artur Cichowicz¹, Michelle Grobbelaar^1,5, Ray Durrheim^1,6, Martin Ziegler⁷, Margaret Boettcher⁸, Tullis C Onstott⁹, DSeis Team¹⁰ (1.JST-JICA SATREPS, Tokyo, Japan, 2.Ritsumeikan Univ., Kusatsu, Japan, 3.Tohoku Univ., Sendai, Japan, 4.Inst. Mine Seismol., Summerset West, South Africa, 5.CGS, Pretoria, South Africa, 6.Univ. Witwatersrand, South Africa, 7.ETH, Zurich, Switzerland, 8.Univ. New Hampshire, USA, 9.Princeton Univ., USA, 10.CrailC IshidaA OgasawaraHY ManziM KatoH FunatoA MngadiS YasutomiT HoriuchiS MilevA MoyerP EllsworthB AbeS OkuboM ImanishiK WardT WechslerN LiebenbergB BersetN HuntJP BuciboS MoremaS DightP KieftT GuptaH JanssenC ShapiroS MukuhiraY SomalaSN)

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 axi-symmetric 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.