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

Abstract

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.

Review on In Situ Acoustic Emission Monitoring in the Context of Structural Health Monitoring in Mines

“…Naoi et al [84][85][86] relocated clustered AE events by using the double difference technique [94]. After a pre-selection of events, the cross-correlation technique for all event pairs whose inter-event distance was smaller than 4 m was applied.…”

“…Interestingly the frequent observation of repeating AE events on faults monitored during the SATRAPS project suggests that AE events on faults loaded by stress-redistribution due to mining undergo a similar process as tectonic faults subject to tectonic stresses [86]. Note that both Ortlepp shear fractures as well as the re-activation of faults cause on a regular basis violent rock burst events (M1 to M4), which are a significant threat to people working underground [96][97][98].…”

“…Using the continuous data recordings from the Äspö experiment, López-Comino et al [86] demonstrated that using automated full waveform detection algorithm during post-processing could significantly increase the amount of triggered AE events. No seismic fracture event was recorded by the microseismic monitoring network or the Wilcoxon accelerometers, although the latter is capable of measuring the frequency range of the observed AE events.…”

A Repeating Earthquake Catalog From 2003 to 2020 for the Raukumara Peninsula, Northern Hikurangi Subduction Margin, New Zealand

“…Repeating earthquakes are identified primarily on the basis of their highly similar waveforms, observed at multiple stations, which imply similar hypocenters and focal mechanisms (e.g., Chen, Bürgmann, & Nadeau, 2013; Li et al., 2018; Nadeau, Antolik, et al., 1994; Nadeau & Johnson, 1998; Naoi et al., 2015; Senobari & Funning, 2019; Uchida, Matsuzawa, Hasegawa, et al., 2003; Zhang et al., 2008). Due to their similarities, it is hypothesized that repeating earthquakes represent the repeated rupture of the same strong asperity or fault patch (e.g., Nadeau & McEvilly, 1999; Uchida, Matsuzawa, Hasegawa, et al., 2003).…”

Optimization of the Match‐Filtering Method for Robust Repeating Earthquake Detection: The Multisegment Cross‐Correlation Approach

“…The match‐filtering (MF) method uses waveform cross‐correlation to determine the similarity between a pair of events. It is a powerful tool in modern seismology to identify repeating earthquakes (e.g., Huang & Meng, 2018; Igarashi et al, 2003; Matsuzawa et al, 2004; Meng et al, 2015; Nadeau et al, 1995; Naoi et al, 2015; Schaff & Richards, 2004, 2011; Schmittbuhl et al, 2016; Uchida et al, 2003; Yamashita et al, 2012) and to detect events that can be easily missed by conventional phase arrival‐based methods (e.g., Chamberlain & Townend, 2018; Gibbons & Ringdal, 2006; Peng & Zhao, 2009; Ross et al, 2019; Shelly et al, 2007; Schultz et al, 2014, 2017; Skoumal et al, 2014, 2015, 2019; Warren‐Smith et al, 2017, 2018; Zhang & Wen, 2015). The extensive applications of this technique have led to major observational breakthroughs (e.g., Shelly et al, 2007).…”