In 2014, a M5.5 earthquake ruptured the range of depths between 3.5 km and 7 km near Orkney, South Africa. The main and aftershocks were very well monitored in the nearfield by dense, surface, strong motion meters and a dense underground seismic network in the deep gold mines. The mechanism of this M5.5 earthquake was left-lateral strike-slip faulting, differing from typical mining-induced earthquakes with normalfaulting mechanisms on the mining horizons shallower than 3.5 km depth. To understand why such an unusual event took place, the aftershock zone was probed by full-core NQ drilling during 2017-2018, with a total length of about 1.6 km, followed by in-hole geophysical logging, core logging, core testing, and monitoring in the drilled holes. These holes also presented a rare opportunity to investigate deep life. In addition, seismogenic zones of M2-M3 earthquakes were probed on mine horizons that were also very well monitored by acoustic emission networks. This paper reviews the early results of the project.

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<p>The 2014 Orkney earthquake (M5.5) occurred below the Moab Khotsong gold mine in South Africa. The shallowest aftershocks were located only several hundred meters below the deepest level of the mine. Two boreholes (Holes A (817 m) and B (700 m)) were drilled toward the upper margin of the aftershock zone from a specially excavated chamber at 2.9 km depth by the ICDP-DSeis project. Hole A deflected from the aftershock zone, while Hole B intersected it. Hole C was branched from Hole B to recover more samples from the aftershock zone. Except for the intersection in Hole B, the drill core recovery was ~100%. In-hole geophysical logging, including the surveys of the borehole wall geometry were carried out along the entire length of Hole A, while they could be done only as far as the intersection with the aftershock zone in Hole B due to hole closure. Hole C was not logged.</p><p>The focal mechanism solutions of mining induced earthquakes shallower than 3 km are usually of the normal faulting type, while those of the Orkney earthquake and its aftershocks deeper than 3.5 km have a strike-slip signature. In this study, we applied the Deformation Rate Analysis (DRA) and the Diametrical Core Deformation Analysis (DCDA) techniques to rock cores recovered from Holes A, B and C to explore the depth variation in the stress state that would cause the depth variation in the faulting regime. In the DRA, a cyclic loading is applied to a sub-sample cut from a drill core to determine the normal stress in the loading direction from hystereses of the stress-strain curve. We determined the normal stresses in 9 directions at each depth to recover the principal stress state redundantly. However, because it takes much time for sub-sample preparations and loading, we applied this technique only at 3 depths in Hole A. With the DCDA, the differential stress in the plane normal to a borehole is evaluated from the ellipsoidal cross-sectional shape of the rock cores. Though only the differential stress can be measured by the DCDA, it takes only several minutes for measurement at each depth. We evaluated the differential stress as densely as every several meters along Holes A, B and C.</p><p>Rock cores of Hole A were oriented by comparing joints and veins identified on the borehole wall optical-televiewer images and in the cores. Thus, the stress orientations in the plane normal to Hole A can be determined as the orientation of the maximum and the minimum core diameter. The stress orientation is obtained also from the breakout of borehole wall identified by the acoustic televiewer. Further, by combining the differential stress magnitude evaluated by the DCDA and the width of the breakout, magnitudes of the maximum and the minimum compression are estimated. We introduce the depth variations in the stress state along Holes A, B and C, as well as those of in-hole logging data to discuss spatial heterogeneity of stresses in the source region of the Orkney earthquake.</p>

<p>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.</p><p>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.</p><p>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.</p><p>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.</p>

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