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Pure and Applied Geophysics

, Volume 168, Issue 12, pp 2395–2425 | Cite as

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

  • V. Heesakkers
  • S. Murphy
  • Z. RechesEmail author
Article

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-strike-slip 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., Earthquake Rupture at Focal Depth, Part II: Mechanics of the 2004 M2.2 Earthquake Along the Pretorius Fault, TauTona mine, South Africa 2011, this volume).

Keywords

Brittle faulting fault reactivation earthquake mechanics earthquake rupture zone deep mine fault rocks 

Notes

Acknowledgments

We are in debt to many people and organizations. Foremost to Gerrie van Aswegen of ISS International who guided and advised us throughout this entire study. This work was not possible without the invaluable help and support by Hannes Moller, Pieter van Zyl, Rob Burnet, and many other workers in TauTona mine and ISSI. We greatly appreciate the help in underground work by Tom Dewers, Amir Allam, Kate Moore, and Matthew Zechmeister of University of Oklahoma, Reginald Domoney, Selwyn Adams, and Curnell Campher, of the University of Western Cape, South Africa, Amie Lacier of Stanford University, and Malcolm Johnston, US Geological Survey. This work could not be completed without the important advice, suggestions and encouragement by the NELSAM team of Tom Jordan, Malcolm Johnston, Mark Zoback, TC Onstott, and well as Hiroshi Ogasawara, Ritsumeikan University, Japan, and Uli Harms, GFZ, Germany. Many thanks to AngloGoldAshanti for the permission to work in the TauTona mine and the generous logistic support. The thoughtful comments of two anonymous reviewers significantly improved the manuscript. This work was supported by the National Science Foundation under Grant No. 0409605 (NELSAM project), and the drilling grant by ICDP (DAFSAM project). Other sponsors of this work include US Geological survey, AngloGoldAshanti, ISS International, and National Research Foundation (NRF).

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Copyright information

© Springer Basel AG 2011

Authors and Affiliations

  1. 1.School of Geology and GeophysicsUniversity of OklahomaNormanUSA
  2. 2.Chevron ETCHoustonUSA
  3. 3.AngloGold AshantiCarletonvilleSouth Africa

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