Earthquake Rupture at Focal Depth, Part I: Structure and Rupture of the Pretorius Fault, TauTona Mine, South Africa

Abstract

We analyze the structure of the Archaean Pretorius fault in TauTona mine, South Africa, as well as the rupture-zone that recently reactivated it. The analysis is part of the Natural Earthquake Laboratory in South African Mines (NELSAM) project that utilizes the access to 3.6 km depth provided by the mining operations. The Pretorius fault is a *10 km long, oblique-strikeslip fault with displacement of up to 200 m that crosscuts fine to very coarse grain quartzitic rocks in TauTona mine. We identify here three structural zones within the fault-zone: (1) an outer damage zone, *100 m wide, of brittle deformation manifested by multiple, widely spaced fractures and faults with slip up to 3 m; (2) an inner damage zone, 25-30 m wide, with high density of anastomosing conjugate sets of fault segments and fractures, many of which carry cataclasite zones; and (3) a dominant segment, with a cataclasite zone up to 50 cm thick that accommodated most of the Archaean slip of the Pretorius fault, and is regarded as the 'principal slip zone' (PSZ). This fault-zone structure indicates that during its Archaean activity, the Pretorius fault entered the mature fault stage in which many slip events were localized along a single, PSZ. The mining operations continuously induce earthquakes, including the 2004, M2.2 event that rejuvenated the Pretorius fault in the NELSAM project area. Our analysis of the M2.2 rupture-zone shows that (1) slip occurred exclusively along four, pre-existing large, quasi-planer segments of the ancient fault-zone; (2) the slipping segments contain brittle cataclasite zones up to 0.5 m thick; (3) these segments are not parallel to each other; (4) gouge zones, 1-5 mm thick, composed of white 'rock-flour' formed almost exclusively along the cataclasite-host rock contacts of the slipping segments; (5) locally, new, fresh fractures branched from the slipping segments and propagated in mixed shear-tensile mode; (6) the maximum observed shear displacement is 25 mm in oblique-normal slip. The mechanical analysis of this rupture-zone is presented in Part II (HEESAKKERS et al.,

Assessment of P and S wave energy radiated from very small shear‐tensile seismic events in a deep South African mine

“…Such mechanisms are expected from the results of Lucier et al . [] on normal faulting stress conditions in the nearby TauTona mine, and that faults dipping 50°–85° to the east‐northeast or west‐southwest are critically stressed based on the Coulomb failure criterion [ Heesakkers et al ., , ]. Faults with such parameters are also supported by direct in situ observations at the exploitation level [ McGarr , , ; Cichowicz et al ., ; van Aswegen , 2008].…”

Geogas transport in fractured hard rock – Correlations with mining seismicity at 3.54km depth, TauTona gold mine, South Africa

“…TauTona`s gold deposits are currently mined at depths of 2-3.5 km, and shafts are under construction to extend mining to below 4 km. The shafts cross through the about 10 m wide and about 10 km long Pretorius Fault Zone (PFZ), the largest fault in the western deep area of the mine (Heesakkers et al, 2011a;Heesakkers et al, 2011b). Figure 1 illustrates the location of an approximately 25 m 2 cubby that was excavated particularly for the purpose of the DAFSAM-NELSAM earthquake laboratory (Reches, 2006) directly within the PFZ at 3.54 km depth.…”

Steady activity of microfractures on geological faults loaded by mining stress

“…Deep mines, where many earthquakes are induced by the stress concentration produced by excavation, provide a good opportunity to deploy an observation network that can detect very small earthquakes. However, even in observations in mines, there are no published examples of a preexisting fault that was clearly delineated by steady activity of microseismic events, though some studies reported that a larger earthquake (M N~2) reactivated part of a geological fault (e.g., Heesakkers et al, 2011). For example, Snelling et al (2013) investigated hypocenter distributions of -2.3 ≤ M W ≤ 0.8 events and reported that there was no correlation between the hypocenters and geological faults near the microseismic network.…”