Earthquake Rupture at Focal Depth, Part II: Mechanics of the 2004 M2.2 Earthquake Along the Pretorius Fault, TauTona Mine, South Africa

Abstract

We analyze here the rupture mechanics of the 2004, M2.2 earthquake based on our observations and measurements at focal depth (Part I). This event ruptured the Archean Pretorius fault that has been inactive for at least 2 Ga, and was reactivated due to mining operations down to a depth of 3.6 km depth. Thus, it was expected that the Pretorius fault zone will fail similarly to an intact rock body independently of its ancient healed structure. Our analysis reveals a few puzzling features of the M2.2 rupture-zone:(1) the earthquake ruptured four, non-parallel, cataclasite bearing segments of the ancient Pretorius fault-zone; (2) slip occurred almost exclusively along the cataclasite-host rock contacts of the slipping segments; (3) the local in-situ stress field is not favorable to slip along any of these four segments; and (4) the Archean cataclasite is pervasively sintered and cemented to become brittle and strong. To resolve these observations, we conducted rock mechanics experiments on the fault-rocks and host-rocks and found a strong mechanical contrast between the quartzitic cataclasite zones, with elastic-brittle rheology, and the host quartzites, with damage, elastic-plastic rheology. The finite-element modeling of a heterogeneous fault-zone with the measured mechanical contrast indicates that the slip is likely to reactivate the ancient cataclasitebearing segments, as observed, due to the strong mechanical contrast between the cataclasite and the host quartzitic rock.

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].…”

Field characterization of elastic properties across a fault zone reactivated by fluid injection

“…In this way, damage evolution under various loading conditions was studied to investigate (1) changes in seismic velocity within a fault zone monitor before an earthquake [ Budiansky and O'Connell , ; Hadley , ], (2) acoustic emission associated with irreversible damage in rock subjected to cyclic loading and unloading (Kaiser effect) [ Hamiel et al ., ], (3) the development of the process zones, which is defined as a finite size region of high stress concentration at fault tip in which the intact rock is disintegrated and where the fault grows [ Katz and Reches , ], (4) the portion of elastic strain released during a seismic cycle [ Hamiel et al ., ], (5) the rotation of regional stress within the damage zone [ Faulkner et al ., ], and (6) the suitability of damage rheology models describing an earthquake cycle [ Alm et al ., ; Lyakhovsky et al ., ]. Laboratory experiments have also been done on rock samples taken along fault zones to investigate the role of mechanical heterogeneity on strain localization [ Heesakkers et al ., ] and the hydromechanical coupling within a fault zone [ Bauer et al ., ]. At the meter scale on outcrops with detailed structural description of fractures coupled to Schmidt hammer measurements to estimate the hydromechanical coupling within a fault zone [ Bauer et al ., ; Jeanne et al ., , ; Steer et al ., ] or in boreholes with geophysical logs to study seismic wave propagation within a fault zone [ Isaacs et al ., ]. At the kilometer scale (1) through the modeling of geodetic data where the rigidity of a compliant zone was determined to evaluate changes in the stress field induced by nearby earthquakes [ Fialko , ; Jolivet et al ., ] and to infer slip rate and locking depth [ Hamiel and Fialko , ] or (2) using seismic travel times, trapped waves, and interferometric synthetic aperture radar observations to estimate the fault zone width [ Cochran et al ., ]. In this last case the authors documented a 65% of reduction in shear moduli within the fault zone.…”

“…In this way, damage evolution under various loading conditions was studied to investigate (1) changes in seismic velocity within a fault zone monitor before an earthquake [Budiansky and O'Connell, 1976;Hadley, unloading (Kaiser effect) [Hamiel et al, 2004], (3) the development of the process zones, which is defined as a finite size region of high stress concentration at fault tip in which the intact rock is disintegrated and where the fault grows [Katz and Reches, 2004], (4) the portion of elastic strain released during a seismic cycle [Hamiel et al, 2006], (5) the rotation of regional stress within the damage zone [Faulkner et al, 2006], and (6) the suitability of damage rheology models describing an earthquake cycle [Alm et al, 1985;Lyakhovsky et al, 1997]. Laboratory experiments have also been done on rock samples taken along fault zones to investigate the role of mechanical heterogeneity on strain localization [Heesakkers et al, 2011] and the hydromechanical coupling within a fault zone [Bauer et al, 2015]. 2.…”

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.…”