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

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.

Automated summary

The cores recovered from deep South African Gold Mines during the International Continental Scientific Drilling Program project reveal fault breccia and altered lamprophyres. The breccia and lamprophyres have high magnetic susceptibilities and low friction coefficients, which are inconsistent with earthquakes. The altered lamprophyres may be the host rocks of the 2014 Orkney earthquake, but the frictional complexity may have influenced the magnitudes and distributions of the main- and aftershocks.