Mechanics of the 2004 M2.2 Earthquake Along the Pretorius Fault, Tautona Mine, South Africa

Abstract:

The 2004, M2.2 earthquake in TauTona mine ruptured the Archean Pretorius fault that has been inactive for at least 2 Ga, and was reactivated by the mining at ~3.6 km depth. The analysis is based on structural mapping, rock mechanics experiments, numerical modeling, and in-situ stress measurements at focal depth. The Archean cataclasite of the Pretorius fault was pervasively sintered and cemented to become brittle and strong, and thus, it was expected that the fault zone will fail similarly to an intact rock body, e.g., by array of tensile fractures. However, the study revealed 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; and (3) the local in-situ, static stress is not favorable to slip along any of these four segments, and slip could occur only if the friction coefficient was very low, below 0.12. We conducted rock mechanics experiments on the fault-rocks and host-rocks at relevant confining pressure (up to 120 MPa), 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 cataclasite bearing segments at the contact between the cataclasite and the host quartzitic rock (as observed) due to the strong mechanical contrast. We propose that the earthquake slip was facilitated by very intense dynamic weakening as observed in high-velocity friction experiments.