TY - JOUR
T1 - Characteristics of Fault Rocks Within the Aftershock Cloud of the 2014 Orkney Earthquake (M5.5) Beneath the Moab Khotsong Gold Mine, South Africa
AU - Miyamoto, T.
AU - Hirono, T.
AU - Yokoyama, Y.
AU - Kaneki, S.
AU - Yamamoto, Y.
AU - Ishikawa, T.
AU - Tsuchiyama, A.
AU - Katayama, I.
AU - Yabe, Y.
AU - Ziegler, M.
AU - Durrheim, R. J.
AU - Ogasawara, H.
N1 - Funding Information:
Although core samples have been recovered from various active faults by numerous drilling projects such as the San Andreas Fault Observatory at Depth project (Zoback et al., 2011 ) and the Taiwan Chelungpu Fault Drilling project (Ma et al., 2006 ), to our knowledge, none of those wells reached the seismogenic depths at which regular earthquakes nucleate. Historically, deep mines close to earthquake hypocenters have provided rare opportunities to drill into the focal areas of earthquakes and recover seismogenic fault materials (Durrheim, 2015 ). Several such wells were recently drilled under the “Drilling into Seismogenic zones of M2.0–5.5 earthquakes in deep South African Gold Mines (DSeis)” project, which commenced in June 2017 and concluded in July 2018. DSeis was supported by the International Continental Scientific Drilling Program (ICDP). The main goals of DSeis were to characterize the in situ stress fields and rocks in and around seismogenic faults and, in particular, to understand the process and mechanism of the 2014 Orkney earthquake (Ogasawara et al., 2017 ; Voosen, 2017 ).
Funding Information:
We thank AngloGold Ashanti and Harmony Gold for kindly facilitating the DSeis project, providing relevant data from the Moab Khotsong mine, and for their permission for publication. We also thank Mandela Mining Precinct, the Council for Scientific and Industrial Research, for allowing us to use their core shed for core curation. We thank all other scientists and operations staff on the DSeis Project and ICDP. This work was conducted in part under the cooperative research program of the Center for Advanced Marine Core Research, Kochi University (19A032, 19B029, 20A066, and 20B058) and was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI (21224012, JP19K04039, JP21H01194, and JP22K03778), JSPS-National Research Foundation bilateral research project, JSPS Core-to-Core program, the Science and Technology Research Partnership for Sustainable Development, the Earthquake and Volcano Hazards Observation and Research Program, and the South African Research Chair Initiative.
Publisher Copyright:
© 2022. The Authors.
PY - 2022/7/28
Y1 - 2022/7/28
N2 - 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.
AB - 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.
KW - earthquake
KW - fault drilling
KW - lamprophyre
UR - http://www.scopus.com/inward/record.url?scp=85135173401&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85135173401&partnerID=8YFLogxK
U2 - 10.1029/2022GL098745
DO - 10.1029/2022GL098745
M3 - Article
AN - SCOPUS:85135173401
SN - 0094-8276
VL - 49
JO - Geophysical Research Letters
JF - Geophysical Research Letters
IS - 14
M1 - e2022GL098745
ER -