Two mining induced tremors that occurred in a South African deep-level gold mine are considered, with the aim to simulate the shear slip area and source mechanism using static boundary element numerical modelling. Large seismic tremors, induced by the extraction of the tabular ore body typically 1 m high over horizontal spans of kilometres, are a major seismic hazard in South African gold mines. For the cases considered here, virgin stress levels are y54 MPa, and seismic related damage is mainly due to geological faults failing in shear under gravitational loading. The tremors occurred y2 km below surface, 700 m apart in space, and in consecutive months. Seismic recordings and underground observations suggested that the tremors, of local magnitudes 3?0 and 4?0 respectively, were associated with normal slip on similar geological faults. A boundary element numerical model was implemented to some degree of accuracy, since the fault geometries could be inferred from extensive mining spans and fault intersections, and an in situ stress state could be estimated from other available information. Furthermore, the rock mass stress changes and associated surface stress changes on faults could be modelled from monthly measured advance of the mining excavations. Simulation of the seismic sources entailed finding appropriate Mohr-Coulomb strength parameters to allow yielding of the faults at the actual mining stage, and quantitatively in agreement with the seismic moment as inferred from seismic waveform recordings. A critical input into the model was the use of non-zero cohesion in addition to friction angle as specified in the Mohr-Coulomb shear failure criterion, allowing the simulation of coseismic slip. Hence, the breaking of fault cohesion and subsequent stress transfer to the surrounding fault area is simulated, albeit statically, conforming to the perceived triggering of mining related seismic tremors by induced stress changes. The back analysis furthermore suggested that coseismic stope closure can comprise a significant part of the total recorded seismic moment, depending on the proximity of the hypocentre to mining excavations. From the back analysis a modelling methodology and input parameters are proposed, aiming at identifying vulnerable mining excavations subject to seismic hazard posed by geological structures, hence providing relevant knowledge towards safer mining.

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 conicalended 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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