Quasi-static slip patch growth to 20 m on a geological fault inferred from acoustic emissions in a South African gold mine

Makoto Naoi, Masao Nakatani, Thabang Kgarume, Sifiso Khambule, Thabang Masakale, Luiz Ribeiro, Joachim Philipp, Shigeki Horiuchi, Kenshiro Otsuki, Koji Miyakawa, Atsushi Watanabe, Hirokazu Moriya, Osamu Murakami, Yasuo Yabe, Hironori Kawakata, Nana Yoshimitsu, Anthony Ward, Raymond Durrheim, Hiroshi Ogasawara

Research output: Contribution to journalArticlepeer-review

15 Citations (Scopus)


Three months of acoustic emission (AE) monitoring in a South African gold mine down to Mw -5 revealed a newly emergent planar cluster of 7557 events -3.9 ≤ Mw ≤ -1.8 (typical rupture radius of 6-70 cm) that expanded with time to reach a size of 20 m on a preexisting geological fault near an active mining front 1 km beneath the ground. It had a sharply defined, planar configuration, with hypocenters aggregated within a thickness of only several decimeters. We infer that the zone defines an aseismic slip patch on the fault, wherein the individual AEs represent failures of very small asperities being loaded by the aseismic slip. Additional support for the interpretation was obtained by analyzing composite focal mechanisms and repeating events. The patch expansion over 2 months was likely quasistatic because all individual AEs ruptured much smaller areas than the cluster size at the corresponding time. The b values dropped gradually from 2.6 to 1.4, consistent with a significant increase in shear stress expected of the mining style. Another cluster with similar characteristics emerged later on a neighboring part of the same fault and grew to a 10 m extent in the last weeks of the study period. The quasi-static expansion of inferred localized slow-slip patches to sizes of 10-20 m suggests that the critical crack length on natural faults can be at least as large, much exceeding the decimeter range derived from laboratory stick-slip experiments on saw-cut rocks.

Original languageEnglish
Pages (from-to)1692-1707
Number of pages16
JournalJournal of Geophysical Research: Solid Earth
Issue number3
Publication statusPublished - 2015 Mar 1


  • acoustic emission
  • aseismic slip
  • b value
  • induced earthquake
  • microseismicity

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science


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