Emergence and disappearance of very small repeating earthquakes on a geological fault in a gold mine in South Africa

Abstract We investigated the activities of very small (−5.10 ≤  M W  ≤ −2.35) repeating earthquakes that occurred during 14 months on a geological fault at a 1 km depth in the Cooke 4 gold mine in South Africa. Out of 4214 acoustic emissions occurring on the fault, 1785 (42.36% of the total) were identified as repeaters on the basis of the waveform similarity and the proximity of hypocenters. Quite a few large groups, having up to 57 members, were recognized. Activities of some groups certainly continued throughout the 14 months, but we also found some repeater groups that newly emerged or disappeared during this period. Both types often coexisted within a distance as small as ~0.5 m, within which background fault creep should be coherent. Some disappearing groups showed a decrease in the event magnitude with time. These temporal changes of repeater activities may imply the formation and dissipation of topographical asperity contacts on the fault. Our results suggest that ultrasensitive observation of acoustic emissions can reveal the physical evolution of asperities.