Unexpectedly frequent occurrence of very small repeating earthquakes (-5.1 ≤ Mw ≤ -3.6) in a South African gold mine: Implications for monitoring intraplate faults

Makoto Naoi, Masao Nakatani, Toshihiro Igarashi, Kenshiro Otsuki, Yasuo Yabe, Thabang Kgarume, Osamu Murakami, Thabang Masakale, Luiz Ribeiro, Anthony Ward, Hirokazu Moriya, Hironori Kawakata, Shigeru Nakao, Raymond Durrheim, Hiroshi Ogasawara

Research output: Contribution to journalArticlepeer-review

14 Citations (Scopus)


We observed very small repeating earthquakes with -5.1 ≤ Mw ≤ -3.6 on a geological fault at 1 km depth in a gold mine in South Africa. Of the 851 acoustic emissions that occurred on the fault during the 2 month analysis period, 45% were identified as repeaters on the basis of waveform similarity and relative locations. They occurred steadily at the same location with similar magnitudes, analogous to repeaters at plate boundaries, suggesting that they are repeat ruptures of the same asperity loaded by the surrounding aseismic slip (background creep). Application of the Nadeau and Johnson (1998) empirical formula (NJ formula), which relates the amount of background creep and repeater activity and is well established for plate boundary faults, to the present case yielded an impossibly large estimate of the background creep. This means that the presently studied repeaters were produced more efficiently, for a given amount of background creep, than expected from the NJ formula. When combined with an independently estimated average stress drop of 16 MPa, which is not particularly high, it suggests that the small asperities of the presently studied repeaters had a high seismic coupling (almost unity), in contrast to one physical interpretation of the plate boundary repeaters. The productivity of such repeaters, per unit background creep, is expected to increase strongly as smaller repeaters are considered (∝ Mo -1/3 as opposed to Mo -1/6 of the NJ formula), which may be usable to estimate very slow creep that may occur on intraplate faults.

Original languageEnglish
Pages (from-to)8478-8493
Number of pages16
JournalJournal of Geophysical Research: Solid Earth
Issue number12
Publication statusPublished - 2015 Dec 1


  • acoustic emission
  • deep mine in South Africa
  • mining-induced earthquake
  • repeating earthquakes
  • scaling relationship

ASJC Scopus subject areas

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


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