Authors: Yabe, Y; Abe, S; Ito, T; Ishida, A; Sugimura, K; Kanematsu, M; Higashi, M; Tadokoro, R; Ogasawara, H; Funato, A; Kato, H; Watson, B; Mngadi, S; Durrheim, R; Hofmann, G; Scheepers, L
Yabe, Y, Abe, S, Ito, T, Ishida, A, Sugimura, K, Kanematsu, M, Higashi, M, Tadokoro, R, Ogasawara, H, Funato, A, Kato, H, Watson, B, Mngadi, S, Durrheim, R, Hofmann, G & Scheepers, L 2019, 'In-situ stress around source faults of seismic events in and beneath South African deep gold mines', in W Joughin (ed.), Deep Mining 2019: Proceedings of the Ninth International Conference on Deep and High Stress Mining
, The Southern African Institute of Mining and Metallurgy, Johannesburg, pp. 399-411, https://doi.org/10.36487/ACG_rep/1952_30_Yabe
To demonstrate the possibility of stress measurement at depths of more than 3 km, in-situ stress states were determined around the source faults of three seismic events: a Mw2.2 seismic event at about 3.3 km below surface in Mponeng gold mine, a Mw3.5 seismic event at about 3.5 km below surface in the Savuka gold mine, and the Orkney earthquake (Mw5.5), with its hypocentre about 5 km below surface, beneath the Moab Khotsong gold mine.
The Mw2.2 seismic event occurred in a 30 m thick gabbroic dyke that intruded into a host rock of quartzite. A 90 m long borehole was drilled to penetrate its source fault. Borehole breakout and core discing were observed in the host rock and the hanging wall of the source fault, i.e., in the dyke. Diametrical core deformation analysis (DCDA) and deformation rate analysis (DRA) were applied to core samples retrieved from the borehole.
The DCDA determines the differential stress in the plane normal to a borehole by measuring the cross-section shape of a core sample, while the DRA reproduces the normal stress in the orientation in which a cyclic loading is applied, i.e., to obtain hysteresis of the stress-strain curves. By integrating these measurements and criteria of the borehole breakout and the core discing, the principal stress states in the host rock, the footwall in the dyke and the hanging wall in the dyke were reproduced. Significant differences were found between the stress states in the footwall and the hanging wall.
The Mw3.5 seismic event occurred in a 36 m thick dyke called BV78. A tunnel that was damaged by the seismic event passed through the source region. The compact conical-ended borehole overcoring (CCBO) technique was applied at two sites along the tunnel; one site was in an area de-stressed by mining and the other about 10 m from the dyke in an area of increased stress owing to the mining abutment above. DCDA was also applied to the core samples.
Three boreholes (Hole A, Hole B and Hole C) that reach an aftershock area in the upper margin of the source fault of the Orkney earthquake (Mw5.5), were drilled by the ICDP-DSeis project (Ogasawara et al, 2019). Since the holes were designed to be drilled in the direction of the maximum compression, borehole breakout or core discing rarely occurred. The DCDA was applied to core samples recovered from Hole A and Hole B. The DRA was also applied to core samples of Hole A. The differential and the normal stresses along Hole A showed a spatial variation that correlates with a variation in lithology and the aftershock distribution.
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