Nucleation process of an M2 earthquake in a deep gold mine in South Africa inferred from on-fault foreshock activity
Tuesday, 16 December 2014
We observed foreshock activity of an Mw2.2 earthquake (the mainshock) that occurred in a gabbroic dyke at a depth of about 3.3 km from the surface in a deep gold mine in South Africa. Foreshock activity, selectively occurring on a plane on which the mainshock would occur, lasted for at least six months until the mainshock. Rock samples in the mainshock source region were recovered by drilling afterward. Indication of ancient hydrothermal alteration on the rupture plane of the mainshock suggests that the foreshock activity occurred on a pre-existing weakness, probably a healed joint, to nucleate the mainshock. The foreshocks during the three months leading up to the mainshock concentrated to three clusters (F1-F3), which, we interpreted, represent the nucleation at multiple sites. The temporal variation in the foreshock activity in the three months can be well explained by the temporal variation of the stressing state in the source region of the mainshock due to nearby mining. One of these clusters (cluster F2) showed an accelerated activity from about 10 days before the mainshock, while activity over the entire foreshock area was rather constant. The foreshock sources in the final 41 hours, during which the stress state was constant, migrated from F2 to F1 that neighbored to the mainshock hypocenter, suggesting coalescence of the two nuclei. The occurrence of mainshock was 0.4-2.3 days earlier than the time expected from an extrapolation of the accelerated foreshock activity in F2. The nucleation of mainshock may have been advanced to the criticality for dynamic instability in a stepwise manner upon the coalescence of nuclei.
While the heterogeneity of geological structures obscures the straightforward manifestation of self-driven quasi-static nucleation, the present careful analysis suggests that some essence of such nucleation as known from the fracture theory and laboratory experiments was caught in the pre-M2 AE data on a natural joint at a depth of 3.3 km.