Investigating Earthquake Rupture Processes in a Deep South African Gold Mine (Invited)
Abstract
A primary objective of the NELSAM (Natural Earthquake Laboratory in South African Mines) Project was to record the rupture processes in the near field of a mining-induced earthquake of M>3. When this project was initiated, it seemed likely that an earthquake this large would rupture the ancient Pretorius fault where it intersected the gold-bearing reef at a depth of 3600 m, near the bottom of TauTona Mine. To prepare, a small-scale network of borehole seismic and strain instrumentation was installed. Also, much effort was expended in geologic investigations in the immediate environs of the portion of the Pretorius fault where earthquake rupture was expected. Contrary to expectations, the M>3, earthquake never occurred, at least as of this writing. Nonetheless, this project has provided many insights regarding earthquake ruptures because of an opportunity that came up during the initial field investigations of the Pretorius fault. That is, a M2.2 earthquake ruptured the Pretorius fault in December 2004, before the NELSAM borehole instrumentation was installed. This earthquake was, however, well recorded by the mine-wide seismic network and so these seismograms, together with the extensive mapping of the rupture within the Pretorius fault zone, have enhanced considerably our understanding of earthquake source processes and the conditions causing an earthquake. The initial field studies revealed that the Pretorius fault is about 10 km long and dips nearly vertically, on average, with oblique slip up to 200 m. This slip occurred more than 2 billion years ago during the Archean. In the meantime, the Pretorius fault has been inactive until affected by mining operations. This extensive period of inactivity has allowed the fault zone material to recover much of its initial strength; thus, rupture across this fault is probably more like fresh rock fracture than frictional slip. After the M2.2 earthquake in December 2004, a comprehensive study of its effects on the Pretorius fault revealed oblique-normal slip across four non-parallel segments of the fault zone, with a maximum observed slip of 25 mm. The ambient state of stress measured in the environs of this study area is extensional with a vertically oriented, maximum principal stress due to the overburden that is about twice the minimum horizontal principal stress. These ambient stresses were altered substantially by the nearby mining so as to induce this M2.2 earthquake. Analysis of the ground motion data from the mine-wide seismic network indicated a seismic moment tensor with a shear component of 2.3e12 N-m as well as an implosive component, associated with coseismic collapse of the nearby mine stopes. Laboratory rock mechanics results were used to interpret the seismic moment and the maximum slip to infer a failure stress in the seismogenic zone of about 120 MPa. This high failure stress is consistent with the strength of the Pretorius fault zone material measured in the laboratory as well as with the radiated energy, 5.4e8 J, which is quite high for an earthquake of M2.2. In short, the rupture zone of this well-recorded earthquake was studied in detail to reveal many important insights concerning the source mechanics of earthquakes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.H11M..02M
- Keywords:
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- 7209 SEISMOLOGY / Earthquake dynamics;
- 7215 SEISMOLOGY / Earthquake source observations;
- 8163 TECTONOPHYSICS / Rheology and friction of fault zones;
- 8168 TECTONOPHYSICS / Stresses: general