Nucleation process of an M2 earthquake in a deep gold mine in South Africa inferred from on‐fault foreshock activity

Abstract

Using a network of sensitive high-frequency acoustic emission sensors, we observed foreshock activity of an Mw 2.2 earthquake (main shock) in a deep gold mine in South Africa. Foreshock activity, which selectively occurred on a part of the rupture plane of the forthcoming main shock, lasted for at least 6 months until the main shock. Rock samples recovered from the main shock source region showed evidence of ancient hydrothermal alteration on the main shock rupture plane, suggesting that the foreshock activity occurred on a preexisting weakness. The foreshocks during 3 months before the main shock were concentrated in three clusters (F1-F3), which we interpret as representing localized preslip at multiple sites. While the location of mining area, the source of stress perturbations, changed with time, the locations of foreshock clusters did not change, suggesting that the preslip patches were controlled by strength heterogeneity rather than stress distribution. Activity over the entire foreshock area was generally constant, but the largest cluster (F2) showed accelerated activity starting at least 7 days before the main shock, while mining stress did not increase in this period. The main shock initiated at a point close to F1, away from F2. All the six foreshocks during the final 41 h occurred in F1 and F2 and in-between. These suggest that in the last stage of the preparation process of the main shock, preslip patches interacted with each other through the stress concentration ahead of the expanding preslip patch (F2), which should be the only driving force of the preparation process under the constant external load.

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

“…Yabe et al [78] showed that the aftershock AE events clearly delineated a plane in the PG dike with a strike of N22W and a dip of 68 • toward N68E (Figure 17). Because waveforms of the main shock recorded by the AE network were saturated in AE recordings, the main shock was analyzed using waveform data of the in-mine geophone network.…”

“…Therefore, the aftershock plane is considered to correspond to the rupture plane of the main shock plane, which demonstrates clearly that the AE aftershock activity outlines the main shock's rupture plane. Kozlowska [78] show that the aftershocks occur in areas of positive Coulomb stress change as determined using rate and state based stress modeling.…”

“…The area of rupture initiation was subject to foreshock activity that is interpreted as the breakdown of asperities [78]. Interestingly, there are indications that AE events announce the main shock in advance.…”

Scientific Exploration of Induced SeisMicity and Stress (SEISMS)

“…These experiments show that a small amount of dilatant aseismic slip can occur before seismic slip, and that earthquakes can be generated even in velocity strengthening material, which laboratory experiments suggested was unlikely . Furthermore, several current microseismic experiments in underground mines are providing insights into the complexity in small earthquake nucleation (e.g., Yabe et al, 2015). Despite the exciting results of these studies, the earthquakes generated were limited to a small number of small magnitude events.…”

2019 status report: Drilling into seismogenic zones of M2.0–M5.5 earthquakes in South African gold mines (DSeis project)

“…Holes were drilled from mining level 95 L, 2.9 km from surface with a rig used for underground routine drilling of geological exploration. Star: A site where normal-faulting stress regime was measured on mining level 98 L, Ogasawara et al (2012) The research sought to determine how ruptures (Mw>-4) evolve in space and time ahead of mining faces at the Cooke 4 mine and a highly stressed dyke at the Mponeng mine (Nakatani et al (2008), Naoi et al (2011Naoi et al ( , 2014 and Yabe et al (2015), in Figure 1).…”