2019 status report: Drilling into seismogenic zones of M2.0–M5.5 earthquakes in South African gold mines (DSeis project)

Abstract

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.

Characteristics of Fault Rocks Within the Aftershock Cloud of the 2014 Orkney Earthquake (M5.5) Beneath the Moab Khotsong Gold Mine, South Africa

“…The strike‐slip focal mechanism differs from the mechanisms generally associated with mining‐induced seismicity but is consistent with the regional stress field associated with the East African rift system (Manzunzu et al., 2017). The hypocenter was considerably deeper than those of typical of mining‐induced earthquakes (2–3 km below the surface; Ogasawara et al., 2017). Therefore, the mainshock and aftershocks of the 2014 Orkney earthquake might be in response to the regional tectonic stress field.…”

“…The Moab Khotsong gold mine exploits the Vaal Reef, a gold‐bearing quartz‐pebble conglomerate in the Central Rand Group of the Witwatersrand Supergroup (Figure 1a; Catuneanu & Biddulph, 2001; Dankert & Hein, 2010; Frimmel & Minter, 2002). The deepest formations exposed in the Moab Khotsong mine include the uppermost formations of the underlying West Rand Group (Ogasawara et al., 2017), dated at 2.9 Ga (Tucker et al., 2016). Hole B core samples include strata from the Roodepoort, Crown, and Babrosco Formations of the Jeppestown Subgroup of the West Rand Group (Figure 1d).…”

“…DSeis was supported by the International Continental Scientific Drilling Program (ICDP). The main goals of DSeis were to characterize the in situ stress fields and rocks in and around seismogenic faults and, in particular, to understand the process and mechanism of the 2014 Orkney earthquake (Ogasawara et al., 2017; Voosen, 2017).…”

“…The spatial relationship of the damaged core zone with the aftershock cloud suggests that it corresponds to the source fault of the 2014 Orkney earthquake (Figure 1c; Ogasawara et al, 2019). The DSeis onsite team undertook post-drilling core logging, downhole logging, and core curation, as documented by Ogasawara et al (2019) and Nkosi et al (2022). Except for the core-loss zone, continuous core samples were successfully recovered from Hole B. Subsamples for the analyses of this study were taken from the Hole B cores at downhole distances between 530 and 660 m below the drill collar (hereafter, all core positions are downhole distances).…”

“…However, Hole B (drilled length, 700 m) intersected the cloud (Figure 1c), after which it transected a 3.2-m zone of non-recovery of core (hereafter, core-loss zone), loss of drilling fluid, and recovery of clearly damaged core fragments. The spatial relationship of the damaged core zone with the aftershock cloud suggests that it corresponds to the source fault of the 2014 Orkney earthquake (Figure 1c; Ogasawara et al, 2019). The DSeis onsite team undertook post-drilling core logging, downhole logging, and core curation, as documented by Ogasawara et al (2019) and Nkosi et al (2022).…”