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[SSS10-P19] Relationship between deformation structure and seismic slip in the shallow part of the Neodani Fault zone
Keywords:fault, thermal pressurization, Barite
A Shallow fault zone has not been considered an important part because they cannot accumulate large strain energy. However, during the 2011 off the Pacific coast of Tohoku Earthquake, fault slip was estimated to have increased in shallower areas, with a 65 m slip at the trench axis (Sun et al., 2017). Lin et al. (2017) also found that a comparison of stress conditions before and after the Tohoku-Pacific Ocean Earthquake showed that the shallow part of the plate boundary fault near the Japan Trench released accumulated stress and energy, which was one of the factors causing the large fault slip. This suggests that in earthquakes occurring at plate boundaries, the stress release in shallow fault zone is responsible for the increased slip. On the other hand, it is unclear whether this phenomenon occurs at the shallow fault zone in intraplate earthquakes. This should be clarified.
The Neodani Fault is rupture during the 1891 Nobi earthquake, one of the world's largest inland earthquakes. The maximum displacement in the Nobi Earthquake was 8 m in left lateral displacement and 6 m in vertical displacement (Muramatsu et al., 2002). Ohtani et al. (2024) observed core samples taken from a borehole drilling at Neomidori, and found a high possibility that the fault was weakened by thermal pressurization at a depth of 380 m. Therefore, the purpose of this study is to observe deformation structures of the Neodani Fault at depths shallower than 100 m, and to clarify the relationship between depth and phenomena occurring in and near the slip zone caused by seismic faulting.
In this study, we observed surface outcrops and borehole core samples containing the latest slip surface of the Neodani Fault. The surface outcrops and borehole locations are located in the Neonagamine, Motosu City, Gifu Prefecture, Japan. Holes R3NDFP-1 and R3NDFD-1 were drilled by the Nuclear Regulation Authority at an angle perpendicular to the strike of the Neodani Fault. Hole R3NDFP-1 was drilled toward the direction of 60° from the horizontal plane with a total length of 30 m and penetrates the latest slip zone at a drilling depth of 16 m. Hole R3NDFD-1 was drilled toward the direction of 82° from the horizontal plane with a total length of 80 m and penetrates the latest slip zone at a drilling depth of 65 m. Rock thin sections were prepared from each sample at surface outcrop, 16 m depth, and 65 m depth, and BSE images were observed by SEM.
Observations show that barite (barium sulfate) is included in all rock thin sections. Barite was only found around the latest slip zone in the surface outcrop. Barite distributed in fractures and radial barite was found at depths of 16 m and 65 m. Barite vein is recognized at depth of 65 m.
Barium sulfate is known to be almost insoluble in water, but its solubility increases with increasing temperature from 0 °C to 100 °C and decreases at temperatures higher than 100 °C, and its solubility increases with increasing pressure (Shi et al., 2023). This suggests that barite was formed through the following processes.
First, faulting causes thermal pressurization, which increases the temperature and pressure around the slip zone and dissolved barium sulfate into the pore water. Next, cracks caused by faulting are filled by the pore water with dissolved barium sulfate. Finally, the temperature of the pore water decreases and barium sulfate precipitates in the cracks.
Thus, barite may be evidence of increased temperature and pressure due to thermal pressurization during faulting, which may have promoted seismic slip in the shallow part of the inland fault.
References:
Ohtani et al. (2024) Proceedings of the 131st Annual Meeting of Geological Society of Japan, G5-O-2.
Muramatsu et al. (2002) The 1891 Nobi Earthquake and Neo-dani Fault Zone, Kokon Shoin, Tokyo (in Japansese)
Lin et al. (2017) Journal of Geography, 126, pp.223-246.
Sun et al. (2017) Nature Communication, 8, 14044.
Shi et al. (2023) ACS Omega, 8, pp.20440-20449.
The Neodani Fault is rupture during the 1891 Nobi earthquake, one of the world's largest inland earthquakes. The maximum displacement in the Nobi Earthquake was 8 m in left lateral displacement and 6 m in vertical displacement (Muramatsu et al., 2002). Ohtani et al. (2024) observed core samples taken from a borehole drilling at Neomidori, and found a high possibility that the fault was weakened by thermal pressurization at a depth of 380 m. Therefore, the purpose of this study is to observe deformation structures of the Neodani Fault at depths shallower than 100 m, and to clarify the relationship between depth and phenomena occurring in and near the slip zone caused by seismic faulting.
In this study, we observed surface outcrops and borehole core samples containing the latest slip surface of the Neodani Fault. The surface outcrops and borehole locations are located in the Neonagamine, Motosu City, Gifu Prefecture, Japan. Holes R3NDFP-1 and R3NDFD-1 were drilled by the Nuclear Regulation Authority at an angle perpendicular to the strike of the Neodani Fault. Hole R3NDFP-1 was drilled toward the direction of 60° from the horizontal plane with a total length of 30 m and penetrates the latest slip zone at a drilling depth of 16 m. Hole R3NDFD-1 was drilled toward the direction of 82° from the horizontal plane with a total length of 80 m and penetrates the latest slip zone at a drilling depth of 65 m. Rock thin sections were prepared from each sample at surface outcrop, 16 m depth, and 65 m depth, and BSE images were observed by SEM.
Observations show that barite (barium sulfate) is included in all rock thin sections. Barite was only found around the latest slip zone in the surface outcrop. Barite distributed in fractures and radial barite was found at depths of 16 m and 65 m. Barite vein is recognized at depth of 65 m.
Barium sulfate is known to be almost insoluble in water, but its solubility increases with increasing temperature from 0 °C to 100 °C and decreases at temperatures higher than 100 °C, and its solubility increases with increasing pressure (Shi et al., 2023). This suggests that barite was formed through the following processes.
First, faulting causes thermal pressurization, which increases the temperature and pressure around the slip zone and dissolved barium sulfate into the pore water. Next, cracks caused by faulting are filled by the pore water with dissolved barium sulfate. Finally, the temperature of the pore water decreases and barium sulfate precipitates in the cracks.
Thus, barite may be evidence of increased temperature and pressure due to thermal pressurization during faulting, which may have promoted seismic slip in the shallow part of the inland fault.
References:
Ohtani et al. (2024) Proceedings of the 131st Annual Meeting of Geological Society of Japan, G5-O-2.
Muramatsu et al. (2002) The 1891 Nobi Earthquake and Neo-dani Fault Zone, Kokon Shoin, Tokyo (in Japansese)
Lin et al. (2017) Journal of Geography, 126, pp.223-246.
Sun et al. (2017) Nature Communication, 8, 14044.
Shi et al. (2023) ACS Omega, 8, pp.20440-20449.