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[SSS12-P04] Distribution and characteristics of deformation structures from borehole survey of the Neodani Fault
Keywords:deformation structures, Neodani Fault
The Neodani fault is an active fault mainly located in Neo area, Motosu City, Gifu Prefecture, central Japan. It caused the Nobi earthquake in 1891, and a maximum left lateral displacement of 8 m and a maximum longitudinal displacement of 6 m occurred at that time. Although many geographic studies have been conducted on the Neodani Fault, the internal structure of the fault zone of the Neodani Fault remains unclear. In 2019, the Nuclear Regulation Authority drilled two boreholes penetrating the Neodani fault at Neomidori, Motosu City (Nuclear Regulation Authority, 2019). It is expected that detailed observation of the boring core will reveal the internal structure of the fault. In addition to the latest slip plane, many deformation structures are developed in the fault zone. Therefore, it is necessary to understand the distribution and characteristics of deformation structures in order to reveal the internal structure of the fault zone. The purpose of this study is to reveal the internal structure of the fault zone by understanding the distribution and characteristics of the deformation structure in the Neodani fault borehole core and its relationship with the latest slip surface.
Two boreholes, NDFP-1 and NDFD-1-S1, were drilled to penetrate the latest slip plane. NDFP-1 is an inclined hole, and its length is 140.0 m. NDFD-1-S1 is an almost vertical hole, and it is 524.8 m. Both holes were drilled in the direction perpendicular to the strike of the Neodani fault. The Jurassic accretionary complex, Mino Belt, is distributed near the drilling site, and it consists of a mélange of mudstone matrix and blocks of sandstone, chert and basalt.
Based on the core description and the observation of the borehole cores, type, drilling depth and angle of the deformation structures are recorded to understand the distribution of the deformation structure in the fault zone. When a displacement of a marker object is observed in a boring core, a shear plane is recognized there. Some shear planes show isolated distribution and the others are cluster with parallel distribution. The former is defined as "single shear plane" and the latter as "multiple shear planes". The angle θ of a shear plane is defined as the angle between it and the plane perpendicular to the core axis.
The Diagram of the distribution of shear plane angles developed in NDFD-1-S1 shows that the angle of the shear plane is distributed from 30° to 60° in a whole depth, and the most angle concentration is around 60°. Since the appearance ratio of the shear plane in the core depends on the angle α between the shear plane and the core axis, we need to correct the appearance ratio of the shear plane. The correction is made by multiplying the appearance ratio of shear planes by 1/sinα = 1/cosθ. The angles of the deformation structures are measured in the NDFD-1-S1, because it is an almost vertical hole. The angle of the latest slip plane in the NDFD-1-S1 is 74° (Yatabe et al., 2021). From the angle relationship between the latest slip plane and the other shear planes in NDFD-1-S1, the shear planes tend to be inclined at a low angle to the latest slip surface.
Considering the composite planar fablic, the R1 shear plane is inclined at a low angle to the Y-plane, which is the main fault plane. Because of the northeast side uplift at Neomidori, the R1 shear plane is inclined to the northeast with a moderate to steep dipping. This is consistent with the fact that the shear planes in NDFD-1-S1 are inclined at a small angle to the latest slip plane and have an angular concentration around 60°, suggesting that most of the shear planes in NDFD-1-S1 are R1 shear planes of the Neodani fault.
Reference: Nuclear Regulation Authority (2019) FY 2018 report on active fault drilling to construct a method of fault activity assessment.
Yatabe et al. (2021) The latest slip zone with low density and mineral precipitation from
borehole survey of the Neodani Fault. Abst. 128th Annual Meeting Geol. Soc. Japan.
Two boreholes, NDFP-1 and NDFD-1-S1, were drilled to penetrate the latest slip plane. NDFP-1 is an inclined hole, and its length is 140.0 m. NDFD-1-S1 is an almost vertical hole, and it is 524.8 m. Both holes were drilled in the direction perpendicular to the strike of the Neodani fault. The Jurassic accretionary complex, Mino Belt, is distributed near the drilling site, and it consists of a mélange of mudstone matrix and blocks of sandstone, chert and basalt.
Based on the core description and the observation of the borehole cores, type, drilling depth and angle of the deformation structures are recorded to understand the distribution of the deformation structure in the fault zone. When a displacement of a marker object is observed in a boring core, a shear plane is recognized there. Some shear planes show isolated distribution and the others are cluster with parallel distribution. The former is defined as "single shear plane" and the latter as "multiple shear planes". The angle θ of a shear plane is defined as the angle between it and the plane perpendicular to the core axis.
The Diagram of the distribution of shear plane angles developed in NDFD-1-S1 shows that the angle of the shear plane is distributed from 30° to 60° in a whole depth, and the most angle concentration is around 60°. Since the appearance ratio of the shear plane in the core depends on the angle α between the shear plane and the core axis, we need to correct the appearance ratio of the shear plane. The correction is made by multiplying the appearance ratio of shear planes by 1/sinα = 1/cosθ. The angles of the deformation structures are measured in the NDFD-1-S1, because it is an almost vertical hole. The angle of the latest slip plane in the NDFD-1-S1 is 74° (Yatabe et al., 2021). From the angle relationship between the latest slip plane and the other shear planes in NDFD-1-S1, the shear planes tend to be inclined at a low angle to the latest slip surface.
Considering the composite planar fablic, the R1 shear plane is inclined at a low angle to the Y-plane, which is the main fault plane. Because of the northeast side uplift at Neomidori, the R1 shear plane is inclined to the northeast with a moderate to steep dipping. This is consistent with the fact that the shear planes in NDFD-1-S1 are inclined at a small angle to the latest slip plane and have an angular concentration around 60°, suggesting that most of the shear planes in NDFD-1-S1 are R1 shear planes of the Neodani fault.
Reference: Nuclear Regulation Authority (2019) FY 2018 report on active fault drilling to construct a method of fault activity assessment.
Yatabe et al. (2021) The latest slip zone with low density and mineral precipitation from
borehole survey of the Neodani Fault. Abst. 128th Annual Meeting Geol. Soc. Japan.