2:45 PM - 3:00 PM
[SSS10-21] ICDP DSeis Project: Spatial variation of seismic wave velocity and alteration of ultramafic dike recovered from a seismogenic zone 3 km below the ground surface
Keywords:ICDP, DSeis, Seismogenic Zone, Elastic wave velocity, Vp
In 2014, the fault rupture zone from the M5.5 earthquake (Orkney earthquake) that occurred directly beneath the Moab Khotsong gold mine in South Africa extended from 3.5 to 7 km in depth (almost the entire depth range of the West Rand Group), with the upper margin located several hundred meters below the deepest mine tunnels. Subsequently, in 2017-2018, ICDP; DSeis successfully recovered core samples and conducted downhole logging for a total of 1.6 km in the upper margin and surrounding area of the Orkney earthquake fault rupture zone by drilling Hole A, B, and C from a 2.9 km underground tunnel (Ogasawara et al., 2019; Nkosi et al., 2022).
The West Rand Group, located in the area of the main shock and aftershocks of the Orkney earthquake, is the lower part of the Witwatersrand Supergroup (2.8-2.9 Ga), which is the stratigraphic unit of the Witwatersrand Basin where the Moab Khotsong gold mine is located and consists mostly of metamorphic rocks from sandy marine sediments. The density and P-wave velocity of these rocks are comparable to those of the upper crust of the Japanese Islands (density 2.7 g/cm3 and P-wave velocity about 6 km/s), and some igneous intrusive rocks are found with densities and elastic wave velocities as high as those of the lower crust are found. These intrusive rocks have experienced at least three generations of igneous activity, especially after the formation of the rift zone about 2.7 billion years ago, suggesting that they experienced metamorphic alteration under temperature and pressure close to the bottom of the upper crust of the Japanese islands, and with subsequent uplift and erosion, these intrusive rocks are now distributed 3 km below the surface.
In this study, in addition to Vp measurements (Fujita et al., 2022 JpGU), more detailed comparisons were made between the downhole logging and the alteration of the dikes.
Measurements were taken on core samples recovered from Hole B and C, where intersections of the fault gouge and the core loss zone were observed, for a total of 5 m of samples containing two nearly parallel penetrating dikes. One is a mafic dike with no aftershocks, composed of metamorphic sedimentary rocks, and the other is a lamprophyre dike with many aftershocks, composed of a flood basaltic andesitic lava layer. Within the lamprophyre dikes that have hosted many aftershocks, a decrease in P-wave velocity was observed toward the core loss zone, which is common to Hole B and C. In a section about 2 m from each, the density remained almost constant at about 3.0 g/cm3, while Vp varied from 6.0 km/s to less than 5.0 km/s. Compared to the change in density, the decrease in Vp by more than 1 km/s was the most prominent feature. On the other hand, within the mafic dykes that penetrated the metamorphic sedimentary rocks, the density and Vp were 3.0 g/cm3 and 6.5 km/s, respectively, and no significant changes in Vp were observed, but they were clearly different from the surrounding host rocks. In the core piece, where there is significant decrease in velocity, X-ray fluorescence element maps (tens of micrometers resolution; Tornado) have also successfully captured continuous spatial variation of alteration within the lamprophyre dykes. Ogasawara et al. (2024) also obtained suggestions corresponding to dikes penetrated by partially molten rocks of upper mantle material. In this study, we report details of these results.
Acknowledgments: This work is the result of the activities of the DSeis team, consisting of researchers in seismology, geology, geomicrobiology, rock mechanics, and mining engineering from Japan, South Africa, the United States, Switzerland, and other countries, and was supported by ICDP, JSPS, the Earthquake and Volcano Observation Research Program for Disaster Mitigation (Phase II), NSF, USA, South Africa NRF, Germany DFG, Ritsumeikan University, International Research Institute for Ocean Cores, Kochi University, Tohoku University, etc.
The West Rand Group, located in the area of the main shock and aftershocks of the Orkney earthquake, is the lower part of the Witwatersrand Supergroup (2.8-2.9 Ga), which is the stratigraphic unit of the Witwatersrand Basin where the Moab Khotsong gold mine is located and consists mostly of metamorphic rocks from sandy marine sediments. The density and P-wave velocity of these rocks are comparable to those of the upper crust of the Japanese Islands (density 2.7 g/cm3 and P-wave velocity about 6 km/s), and some igneous intrusive rocks are found with densities and elastic wave velocities as high as those of the lower crust are found. These intrusive rocks have experienced at least three generations of igneous activity, especially after the formation of the rift zone about 2.7 billion years ago, suggesting that they experienced metamorphic alteration under temperature and pressure close to the bottom of the upper crust of the Japanese islands, and with subsequent uplift and erosion, these intrusive rocks are now distributed 3 km below the surface.
In this study, in addition to Vp measurements (Fujita et al., 2022 JpGU), more detailed comparisons were made between the downhole logging and the alteration of the dikes.
Measurements were taken on core samples recovered from Hole B and C, where intersections of the fault gouge and the core loss zone were observed, for a total of 5 m of samples containing two nearly parallel penetrating dikes. One is a mafic dike with no aftershocks, composed of metamorphic sedimentary rocks, and the other is a lamprophyre dike with many aftershocks, composed of a flood basaltic andesitic lava layer. Within the lamprophyre dikes that have hosted many aftershocks, a decrease in P-wave velocity was observed toward the core loss zone, which is common to Hole B and C. In a section about 2 m from each, the density remained almost constant at about 3.0 g/cm3, while Vp varied from 6.0 km/s to less than 5.0 km/s. Compared to the change in density, the decrease in Vp by more than 1 km/s was the most prominent feature. On the other hand, within the mafic dykes that penetrated the metamorphic sedimentary rocks, the density and Vp were 3.0 g/cm3 and 6.5 km/s, respectively, and no significant changes in Vp were observed, but they were clearly different from the surrounding host rocks. In the core piece, where there is significant decrease in velocity, X-ray fluorescence element maps (tens of micrometers resolution; Tornado) have also successfully captured continuous spatial variation of alteration within the lamprophyre dykes. Ogasawara et al. (2024) also obtained suggestions corresponding to dikes penetrated by partially molten rocks of upper mantle material. In this study, we report details of these results.
Acknowledgments: This work is the result of the activities of the DSeis team, consisting of researchers in seismology, geology, geomicrobiology, rock mechanics, and mining engineering from Japan, South Africa, the United States, Switzerland, and other countries, and was supported by ICDP, JSPS, the Earthquake and Volcano Observation Research Program for Disaster Mitigation (Phase II), NSF, USA, South Africa NRF, Germany DFG, Ritsumeikan University, International Research Institute for Ocean Cores, Kochi University, Tohoku University, etc.