11:00 AM - 1:00 PM
[SSS12-P05] Characteristics of the very shallow fault zone of the Neodani Fault in Neonagamine, Motosu City, Gifu Prefecture
Keywords:the latest slip plane, very shallow, Neodani Fault
Trench surveys are commonly used to reveal the history of active faults (Yoshioka, 2004). Since this requires Quaternary strata, a new assessment method for active faults is desired using the fault zone of the basement rock. Although Tsuru and Ohtani (2020) reported that the latest slip plane of near-surface fault gouge of the Neodani Fault Zone includes small gravels from the upper strata, the maximum depth at which fault gouges contain dropped gravels is unclear. In addition, as the fault zone consists of not only the latest slip plane but also the old slip planes, these allow to understand the history of the Neodani Fault. The Nuclear Regulation Authority has been performing outcrop surveys of the Neodani Fault Zone in recent years (Nuclear Regulation Authority, 2021), and we have had an opportunity to observe the stripped outcrop in detail. The purpose of this study is to clarify the characteristics of the latest slip plane and the long-term history of the Neodani Fault at very shallow fault zone using this stripped outcrop.
The Neodani Fault, which caused the Nobi Earthquake in 1891, is an active fault with a length of about 35 km. The study area is Neonagamine in Motosu city, Gifu Prefecture, central Japan. A detailed sketch of the deformed structures that develops in the fault zone is drawn using the stripped outcrop, and the deformational features contained in the old slip planes are observed.
The stripped outcrop is composed of a gravel layer on the upper part, a quartz-rich fault zone on the left side, and a black mudstone fault zone on the right side. The latest slip plane consists of a fault gouge, has a slightly different color from the dark gray gouge, has an almost straight structure cutting other shear planes and deformed structures, and contains gravel that is not from neighboring basement rocks. In addition, the latest slip plane does not coincide with the boundary between the quartz-rich and the black mudstone fault zones, and is located slightly to the right of the boundary. The small gravels are contained in the latest slip plane, and are spread from the top of the latest slip plane in the black mudstone fault zone to 80 cm below the top. The bottom surface of the gravel layer shows a displacement of 30 cm with uplift on the right side, while the composite planer fabric in the quartz-rich fault zone suggests an opposite sense of shear.
Scaringi et al. (2018) performed the ring shear experiments using the landslide clay under the confined pressure of 600 kPa and showed that the volume expansion of the landslide clay becomes larger when the displacement speed is higher. This suggests that the gravel falling is caused by the volume expansion of the fault gouge in deeper part and the accompanied opening in very shallow part during the displacement of the Nobi Earthquake.
A displacement with 30 cm can be confirmed in the gravel layer. During the Nobi earthquake, pure left lateral displacement occurred near the outcrops in the Neonagamine area. This suggests that the displacement of the gravel layer with 30 cm did not occur during the Nobi earthquake, but the previous earthquakes. The gravel layer was deposited in the Quaternary, suggesting that it was displaced by the Quaternary fault activity. The shear sense of the composite planar fabric is different from the displacement of the gravel layer, indicating that the composite planar fabric formed earlier than the displacement of the gravel layer.
References
Nuclear Regulation Authority (2021) FY 2020 report on sampling and laboratory analyses of fault zone material.
Scaringi et al. (2018) Geophys. Res. Let., 45, 767-770.
Tanna Fault Excavation Research Group (1983) Bull. Earthq. Res. Inst. Univ. Tokyo, 58, 816-817.
Tsuru and Ohtani (2021) Abst JpGU 2021.
Yoshioka (2004) Earthquakes and active faults-learning from the past and predicting the future-, Maruzen, p.237
The Neodani Fault, which caused the Nobi Earthquake in 1891, is an active fault with a length of about 35 km. The study area is Neonagamine in Motosu city, Gifu Prefecture, central Japan. A detailed sketch of the deformed structures that develops in the fault zone is drawn using the stripped outcrop, and the deformational features contained in the old slip planes are observed.
The stripped outcrop is composed of a gravel layer on the upper part, a quartz-rich fault zone on the left side, and a black mudstone fault zone on the right side. The latest slip plane consists of a fault gouge, has a slightly different color from the dark gray gouge, has an almost straight structure cutting other shear planes and deformed structures, and contains gravel that is not from neighboring basement rocks. In addition, the latest slip plane does not coincide with the boundary between the quartz-rich and the black mudstone fault zones, and is located slightly to the right of the boundary. The small gravels are contained in the latest slip plane, and are spread from the top of the latest slip plane in the black mudstone fault zone to 80 cm below the top. The bottom surface of the gravel layer shows a displacement of 30 cm with uplift on the right side, while the composite planer fabric in the quartz-rich fault zone suggests an opposite sense of shear.
Scaringi et al. (2018) performed the ring shear experiments using the landslide clay under the confined pressure of 600 kPa and showed that the volume expansion of the landslide clay becomes larger when the displacement speed is higher. This suggests that the gravel falling is caused by the volume expansion of the fault gouge in deeper part and the accompanied opening in very shallow part during the displacement of the Nobi Earthquake.
A displacement with 30 cm can be confirmed in the gravel layer. During the Nobi earthquake, pure left lateral displacement occurred near the outcrops in the Neonagamine area. This suggests that the displacement of the gravel layer with 30 cm did not occur during the Nobi earthquake, but the previous earthquakes. The gravel layer was deposited in the Quaternary, suggesting that it was displaced by the Quaternary fault activity. The shear sense of the composite planar fabric is different from the displacement of the gravel layer, indicating that the composite planar fabric formed earlier than the displacement of the gravel layer.
References
Nuclear Regulation Authority (2021) FY 2020 report on sampling and laboratory analyses of fault zone material.
Scaringi et al. (2018) Geophys. Res. Let., 45, 767-770.
Tanna Fault Excavation Research Group (1983) Bull. Earthq. Res. Inst. Univ. Tokyo, 58, 816-817.
Tsuru and Ohtani (2021) Abst JpGU 2021.
Yoshioka (2004) Earthquakes and active faults-learning from the past and predicting the future-, Maruzen, p.237