Characteristics of Fault Rocks Within the Aftershock Cloud of the 2014 Orkney Earthquake (M5.5) Beneath the Moab Khotsong Gold Mine, South Africa

Abstract

Cores recovered during the International Continental Scientific Drilling Program project “Drilling into Seismogenic zones of M2.0 to M5.5 earthquakes in deep South African Gold Mines” include fault breccia from within the aftershock cloud of the 2014 Orkney earthquake (M5.5). The breccia and surrounding intrusive rocks, probably lamprophyres rich in talc, biotite, calcite, and amphibole, had high magnetic susceptibilities owing to the presence of magnetite. All of these characteristics can be attributed to fluid‐related alteration. Both the breccia and the lamprophyres had low friction coefficients and showed evidence of velocity strengthening, which is inconsistent with the occurrence of earthquakes. Variable amounts of talc, biotite, calcite, and amphibole within the lamprophyres might have produced complex frictional properties and spatial heterogeneity of fault stability. The altered lamprophyres may be the host rocks of the 2014 Orkney earthquake, but frictional complexity may have governed the magnitudes of the main‐ and aftershocks and their distributions.

Frictional properties of rocks recovered form aftershock cloud of the 2014 Orkney earthquake (M5.5), South Africa, by ICDP-DSeis

“…The steel fragments were removed using a magnet before grinding. As noted by Miyamoto et al (2022), lamprophyres contain minerals with high magnetic susceptibility, probably magnetite. Some of these minerals might also be removed using this procedure.…”

“…The initial friction and hardening rate are 𝜇 𝑖 = 0.627-0.691 and 𝜇 ℎ = 3.814-8.042 × 10 −4 mm −1 , respectively (Figure 6). Miyamoto et al (2022) measured the frictional behaviors of UFZ and Lamprophyre just below the LFZ under ambient conditions using a direct shear apparatus. The friction coefficient at yielding was 0.40-0.45 and increased to 0.65 at 𝑑 ≈ 16 mm (green square in Figure 6a).…”

“…Those of the dike are ρ = 2880-3090 km/m 3 , φ = 0.96-1.02%, Vp = 5399-6743 m/s, and Vs = 3083-4126 m/s. Miyamoto et al (2022) analyzed the mineralogical and chemical components of the dike hosting the source fault and Crown Lava on both sides of the dike between 530 and 660 m of Hole B. As the dike is characterized by low contents of quartz and feldspar and high contents of amphibole, biotite, talc, and calcite, it is defined as an altered lamprophyre (hereafter, the dike is referred to as Lamprophyre).…”

“…In this study, we examined the frictional properties of the Crown Lava and Lamprophyre recovered from Hole B of the ICDP-DSeis project under wider conditions than those of Miyamoto et al (2022) in terms of the water content and normal stress.…”

Broadband Strong Ground Motion Modeling Using Planar Dynamic Rupture With Fractal Parameters

“…A promising alternative to improve high‐frequency radiation is introducing fractal stress and frictional properties, as also suggested by in‐situ fault rock investigations (Miyamoto et al., 2022). Mikumo and Miyatake (1978) numerically demonstrated how non‐uniform friction results in irregular rupture propagation.…”