5:15 PM - 7:15 PM
[SGC38-P06] Characteristics of the Latest Slip Zone of the Neodani Fault based on physical properties
Keywords:Neodani Fault, magnetic susceptibility, CT value
When only basement rock is distributed, it is not possible to evaluate faulting history, so a method of evaluating activity using basement rocks is required. Physical properties are expected to change due to mineral filling in older fault gouge. If we can find changes in physical properties associated with fault activity, it may be possible to evaluate activity by wireline logging along a borehole.
In previous studies, magnetic susceptibility measurements and powder X-ray diffraction analysis (XRD) were performed to consider the characteristics of the latest slip zone based on the relationship between magnetic susceptibility and contained minerals (Aoki and Ohtani, 2023). However, the relationship of the other physical parameters is still unknown. Therefore, in this study, I aim to clarify the characteristics of the latest slip zone from the gamma ray spectrum, magnetic susceptibility, and X-ray CT.
This study focuses on the Neodani Fault, which caused the 1891 Nobi earthquake. In this study, we use the drilling core R3NDFD-1-S1, which was drilled by the Nuclear Regulation Authority in 2021 at Neonagamine in Motosu City. I also use rock samples collected from surface outcrops at Shikiharadani, Nagamine, Neo, Motosu City, and Kinbaradani, Motosu City. R3NDFD-1-S1 is inclined 82° from the horizontal plane and was drilled 80m, and the latest slip zone can be seen at a depth of 64.80 to 66.16 m. In addition, the boring core mainly consists of mudstone. The fault plane of polished slab from Shikiharadani is N25W83NE, and the fault plane of polished slab from Kinbaradani is N22W67NE. The polished slabs of both samples were cut perpendicular to the fault plane and parallel to the striations. In Shikiharadani, fault rocks are formed from breccia, gouge, cataclasite, and gouge from quartz veins originating from mudstone. The polished slab from Shikiharadani consists of approximately half black clayey gouge and the other half greenish-gray clayey gouge. In Kinbaradani, fault rocks are formed from gouges from mudstone and basalt. The polished slab from Kinbaradani consists of approximately half light brown clayey gouge and the other half dark brown clayey gouge.
Magnetic susceptibility was measured at 5 mm intervals using a portable magnetic susceptibility meter SM-30. Polished slab from Shikiharadani shows a maximum value of 1.60 in the greenish-gray gouge, while that from Kinbaradani is a maximum value of 3.57 in the light brown gouge. Both polished slabs show a circular distribution of higher value in magnetic susceptibility.
For the X-ray CT, the image data of previous X-ray CT scans were reanalyzed using SedCT running on MATLAB, and compared with the measurement results of physical properties. Comparison with other physical properties showed a positive correlation with gamma ray density and a negative correlation with porosity. No direct correlation was found with magnetic susceptibility or natural gamma rays.
The magnetic susceptibility of drilling core samples is 15 to 20 in basalt, 0.1 to 0.2 in mudstone, and 1.0 to 2.0 near the latest slip zone in mudstone (Aoki et al. 2024). Polished slab from Shikiharadani showed a value of 1.60 in mudstone fault gouge, suggesting that magnetic susceptibility becomes higher due to the frictional heating near the surface. Polished slab from Kinbaradani showed a considerably higher value, implying that this reflects basalt distribution in the fault gouge
Gamma ray density is measured by detecting the amount of attenuation when gamma rays pass through a sample. From this phenomenon, gamma ray density is correlated to the CT value, which represents the density distribution of an object. Yatabe (2021) analyzed the CT values of drilling core samples excavated at Neo Midori and found that the latest slip zone showed extremely low CT values. Therefore, based on the correlation with gamma ray density, it is expected that the latest slip zone can be detected by measuring the gamma ray density.
Aoki (2023) Abst JpGU 2023, SSS13-P15.
Ohtani et al. (2023) Abst The 130th annual meeting of The Geological Society of Japan T1-O-13.
Yatabe (2022) Abst JpGU 2022, SSS13-05.
In previous studies, magnetic susceptibility measurements and powder X-ray diffraction analysis (XRD) were performed to consider the characteristics of the latest slip zone based on the relationship between magnetic susceptibility and contained minerals (Aoki and Ohtani, 2023). However, the relationship of the other physical parameters is still unknown. Therefore, in this study, I aim to clarify the characteristics of the latest slip zone from the gamma ray spectrum, magnetic susceptibility, and X-ray CT.
This study focuses on the Neodani Fault, which caused the 1891 Nobi earthquake. In this study, we use the drilling core R3NDFD-1-S1, which was drilled by the Nuclear Regulation Authority in 2021 at Neonagamine in Motosu City. I also use rock samples collected from surface outcrops at Shikiharadani, Nagamine, Neo, Motosu City, and Kinbaradani, Motosu City. R3NDFD-1-S1 is inclined 82° from the horizontal plane and was drilled 80m, and the latest slip zone can be seen at a depth of 64.80 to 66.16 m. In addition, the boring core mainly consists of mudstone. The fault plane of polished slab from Shikiharadani is N25W83NE, and the fault plane of polished slab from Kinbaradani is N22W67NE. The polished slabs of both samples were cut perpendicular to the fault plane and parallel to the striations. In Shikiharadani, fault rocks are formed from breccia, gouge, cataclasite, and gouge from quartz veins originating from mudstone. The polished slab from Shikiharadani consists of approximately half black clayey gouge and the other half greenish-gray clayey gouge. In Kinbaradani, fault rocks are formed from gouges from mudstone and basalt. The polished slab from Kinbaradani consists of approximately half light brown clayey gouge and the other half dark brown clayey gouge.
Magnetic susceptibility was measured at 5 mm intervals using a portable magnetic susceptibility meter SM-30. Polished slab from Shikiharadani shows a maximum value of 1.60 in the greenish-gray gouge, while that from Kinbaradani is a maximum value of 3.57 in the light brown gouge. Both polished slabs show a circular distribution of higher value in magnetic susceptibility.
For the X-ray CT, the image data of previous X-ray CT scans were reanalyzed using SedCT running on MATLAB, and compared with the measurement results of physical properties. Comparison with other physical properties showed a positive correlation with gamma ray density and a negative correlation with porosity. No direct correlation was found with magnetic susceptibility or natural gamma rays.
The magnetic susceptibility of drilling core samples is 15 to 20 in basalt, 0.1 to 0.2 in mudstone, and 1.0 to 2.0 near the latest slip zone in mudstone (Aoki et al. 2024). Polished slab from Shikiharadani showed a value of 1.60 in mudstone fault gouge, suggesting that magnetic susceptibility becomes higher due to the frictional heating near the surface. Polished slab from Kinbaradani showed a considerably higher value, implying that this reflects basalt distribution in the fault gouge
Gamma ray density is measured by detecting the amount of attenuation when gamma rays pass through a sample. From this phenomenon, gamma ray density is correlated to the CT value, which represents the density distribution of an object. Yatabe (2021) analyzed the CT values of drilling core samples excavated at Neo Midori and found that the latest slip zone showed extremely low CT values. Therefore, based on the correlation with gamma ray density, it is expected that the latest slip zone can be detected by measuring the gamma ray density.
Aoki (2023) Abst JpGU 2023, SSS13-P15.
Ohtani et al. (2023) Abst The 130th annual meeting of The Geological Society of Japan T1-O-13.
Yatabe (2022) Abst JpGU 2022, SSS13-05.