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[SSS13-P15] Characteristics of magnetic susceptibility distribution along the latest slip surface in the drilling core of the Neodani fault, Neonagamine, Central Japan: from the viewpoint
Keywords:fault, magnetic susceptibility
Trench survey is generally used to evaluate the activity history of active faults. However, if there is no Quaternary strata , it is impossible to know the activity history using this method. Therefore, a method to evaluate the activity history from the fault fracture zone at the site where there is no Quaternary strata is required.
In the Neodani fault zone, previous studies have attempted to understand the characteristics of the latest slip zone by the comparison of the characteristics between the latest and older slip zone from a surface peel of the surface exposure. (Aoki and Ohtani, 2022).Fault drilling near the above-mentioned surface exposure was conducted in 2021 by the Nuclear Regulation Authority. The core sample from this borehole enablesto evaluate the characteristics of the subsurface fault zone. Therefore, the alteration of materials and the formation of magnetic minerals, in the fault gouge due to frictional heating of the fault is discussed from the relationship between magnetic susceptibility and contained minerals assemblage in this study
The Neodani fault, ruptured during the 1891 Nobi earthquake, is an active fault with a length of about 35 km from Mt. Nogo Hakusan to the northwestern part of Gifu City, via Neonagamine, Motosu City, Gifu Prefecture. Two borehole cores, R3NDFD-1 and R3NDFP-1, which were drilled in FY 2021 at Neonagamine. R3NDFD-1 is the length of 80 m in the direction of N61E with an inclination of 82 degrees from the horizontal, and the latest slip zone is from 64.73 to 65.96 m in depth. R3NDFP-1 is the length of 30 m in the direction of N60E with an inclination of 60 degrees from the horizontal, and the latest slip zone is from 15.93 to 16.07 m in depth.
Fault rocks, including the latest slip surface, was observed on the half-cut borehole core to confirm the fault rock distribtion. Magnetic susceptibility was measured using a portable magnetic susceptibility meter to understand the distribution of magnetic susceptibility. Minerals assemblages are obtained by XRD analysis using samples collected from the surface outcrop. In XRD, non-oriented and oriented method are implemented.
The half-core (63.70-66.48 m depth) near the latest slip surface of R3NDFD-1 shows that fault rocks can be classified according to color . Dark gray and black fault gouges are discontinuously distributed along the latest slip zone.
The magnetic susceptibility is about 0.5 in the latest slip zone, and 1.0 to 2.0 in the dark gray and black fault gouges , which are higher than their surroundings. Weakly fractured basalt, sandstone/mudstone, and chert far from the latest slip surface shows 15 to 20, and 0.01 to 0.2, respectively.
XRD analysis clarifies magnetite in dark gray/black fault gouges and basalt, and smectite in some fault gouges.
Yang et al. (2019) shows that smectite forms fine-grained magnetite when heated above 250 °C. Magnetite in the dark gray and black fault gouges would have been formed by alteration of smectite due to frictional heating of the fault. Since the latest slip zone of the borehole core is located at a depth of about 60 m, the frictional heating is considered to be not so large. This suggests that the dark gray and black fault gouges were formed at a deeper depth. The dark gray and black fault gouges do not seem to involve basalt with higher magnetic susceptibility, because the mineral assemblage is greatly different. The latest slip zone also show a slightly higher value than the surroundings, and it suggests that dark gray and black fault gouges near the latest slip zone were fractured and involved into the latest slip zone.
References
Aoki and Ohtani (2022) Abst. JpGU2022 , SSS12-P05.
Yang et al. (2020) Rev. Geophys., 58, e2019RG000690.
In the Neodani fault zone, previous studies have attempted to understand the characteristics of the latest slip zone by the comparison of the characteristics between the latest and older slip zone from a surface peel of the surface exposure. (Aoki and Ohtani, 2022).Fault drilling near the above-mentioned surface exposure was conducted in 2021 by the Nuclear Regulation Authority. The core sample from this borehole enablesto evaluate the characteristics of the subsurface fault zone. Therefore, the alteration of materials and the formation of magnetic minerals, in the fault gouge due to frictional heating of the fault is discussed from the relationship between magnetic susceptibility and contained minerals assemblage in this study
The Neodani fault, ruptured during the 1891 Nobi earthquake, is an active fault with a length of about 35 km from Mt. Nogo Hakusan to the northwestern part of Gifu City, via Neonagamine, Motosu City, Gifu Prefecture. Two borehole cores, R3NDFD-1 and R3NDFP-1, which were drilled in FY 2021 at Neonagamine. R3NDFD-1 is the length of 80 m in the direction of N61E with an inclination of 82 degrees from the horizontal, and the latest slip zone is from 64.73 to 65.96 m in depth. R3NDFP-1 is the length of 30 m in the direction of N60E with an inclination of 60 degrees from the horizontal, and the latest slip zone is from 15.93 to 16.07 m in depth.
Fault rocks, including the latest slip surface, was observed on the half-cut borehole core to confirm the fault rock distribtion. Magnetic susceptibility was measured using a portable magnetic susceptibility meter to understand the distribution of magnetic susceptibility. Minerals assemblages are obtained by XRD analysis using samples collected from the surface outcrop. In XRD, non-oriented and oriented method are implemented.
The half-core (63.70-66.48 m depth) near the latest slip surface of R3NDFD-1 shows that fault rocks can be classified according to color . Dark gray and black fault gouges are discontinuously distributed along the latest slip zone.
The magnetic susceptibility is about 0.5 in the latest slip zone, and 1.0 to 2.0 in the dark gray and black fault gouges , which are higher than their surroundings. Weakly fractured basalt, sandstone/mudstone, and chert far from the latest slip surface shows 15 to 20, and 0.01 to 0.2, respectively.
XRD analysis clarifies magnetite in dark gray/black fault gouges and basalt, and smectite in some fault gouges.
Yang et al. (2019) shows that smectite forms fine-grained magnetite when heated above 250 °C. Magnetite in the dark gray and black fault gouges would have been formed by alteration of smectite due to frictional heating of the fault. Since the latest slip zone of the borehole core is located at a depth of about 60 m, the frictional heating is considered to be not so large. This suggests that the dark gray and black fault gouges were formed at a deeper depth. The dark gray and black fault gouges do not seem to involve basalt with higher magnetic susceptibility, because the mineral assemblage is greatly different. The latest slip zone also show a slightly higher value than the surroundings, and it suggests that dark gray and black fault gouges near the latest slip zone were fractured and involved into the latest slip zone.
References
Aoki and Ohtani (2022) Abst. JpGU2022 , SSS12-P05.
Yang et al. (2020) Rev. Geophys., 58, e2019RG000690.