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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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The authors document strain changes up to ~10 -4 associated with two M>2 events 2.4 km deep at distances less than ~100 m from a strainmeter. This corresponds to a ~7 MPa stress change recorded only within the hypocentral area. This change was recorded with a sensitive, wide-dynamic-range, Ishii strainmeter. A 15-m hole was drilled subparallel to the fault strike, in order to continuously monitor slip-driving shear and normal strains with a sampling frequency of 25 Hz. Two M>2 events took place around the fault. For both events, relaxation in the maximum principal stress at a rate of 10 -6 /week was observed for several days prior to the main shock. In one of the two events, foreshocks were concentrated in the last several tens of seconds, accompanied by strain steps and logarithmic post-seismic deformation. However, no acceleration in deformation was observed, even with a resolution of 1/10,000 of a coseismic step for each event.