It is impossible to access the rupture zone of an earthquake at its focal depth, and thus the perception of earthquake rupture zones is inferred from indirect observations of surface rupture and exhumed fault-zones. A different approach is used here: direct observations in deep gold mines of South Africa where the PI recently mapped the rupture zones of three earthquakes. These mapped zones are associated with moderate earthquakes of magnitudes 3.7 to 5.1 that cut across the thick quartzite sequences of the western Witwatersrand basin. He found that a typical rupture zone of a single earthquake consists of 5-20 crosscutting gouge zones that were active during the earthquake. The gouge zones contain crushed, quartzitic powder, rock fragments or clay minerals; some of these zones are 15 cm thick. The PI proposes to analyze in detail the composition, microstructure and geochemical stability of the gouge found in these rupture zones. He plans to determine gouge mineralogy, high-pressure phases, gouge surface area and gouge solubility. He will examine the fragmentation processes at the microscopic scale and the transition from intact rock to gouge powder. The gouge material will be subjected to long term (1-3 months) cementation experiments at hydrothermal condition (< 500) to explore its stability and strengthening under mid-crust conditions. This study will resolve some central questions of earthquake processes and seismic hazard.
1. Is the slip during the earthquake localized along a single frictional surface or is it distributed along many fractures? 2. Do high-pressure and high-temperature mineralogical phases form during an earthquake (they could be unstable and not be found in exhumed faults)? 3. Could pseudotachylites form during shallow earthquakes? 4. How much gouge is formed during a single earthquake? 5. What portion of the mechanical energy of the earthquake is invested in gouge formation? 6. Is the formed gouge good starting material for fault-zone cementation during the interseismic stage? 7. Which properties of ERZ are indicators of the seismic activity and could be used for distinguishing between an exhumed seismic fault and an exhumed creeping? 8. How fast does gouge compact and cement (lithify), and how does that affect fault strength?
