5:15 PM - 6:30 PM
[SSS10-P09] Characteristics of the latest slip zone of the Neodani Fault drilling by X-ray CT
Keywords:latest slip zone, active fault drilling, X-ray CT, Neodani Fault
The Neodani Fault is a left-lateral fault with a length of 35km that is distributed from Nogohakusan on the border of Gifu and Fukui Prefectures to the northwestern part of Gifu city, Gifu Prefecture. Neomidori area is located in the fault jog dividing into two faults and a vertical displacement of 6 m occurred during the Nobi earthquake that occurred in 1891. The fault drilling was performed in 2019 at Neomidori to clarify the characteristics of the latest slip zone of the Neodani Fault by the Nuclear Regulation Authority. The purpose of this study is to clarify the characteristics of fault rock distribution and the latest slip zone by X-ray CT.
The boreholes consist of NDFP-1 of the pilot hole, NDFD-1 of the main hole and NDFD-1-S1 of the side track hole (Nuclear Regulation Authority, 2019). NDFP-1 is an inclined borehole, and the length of the borehole is 140 m. The depth of the bottom of the hole is 110.75 m. NDFD-1 and NDFD-1-S1 is almost vertical. The length of the borehole is 524.8 m, and the depth of the bottom of the hole is 516.9 m.
NDFP-1 penetrates the latest slip zone at 110.75 m along the borehole from the top of the hole. The latest slip zone consists of the shale fault gouge. Fault rocks in the hanging wall are shale fault breccia, fractured chert and weakly fractured basalt, and those in the footwall are shale fault breccia and fractured shale. NDFD-1-S1 penetrates the latest slip zone at 387.7 m along the borehole from the top of the hole. The latest slip zone consists of the shale fault gouge. Fault rocks in the hanging wall are shale fault breccia, fractured shale, foliated and non-foliated basalt cataclasite, and those in the footwall are shale fault breccia and fractured shale. These indicates that the fault rocks in the hanging wall are more fractured than those in the footwall. Fault rocks in the hanging wall are located in the fault jog with compression. This would result in the above described fault rocks distribution.
We performed X-ray CT imaging of the borehole core including the latest slip zone. 2D cross sections and 3D images are reconstructed by Horos that is a software of CT image viewer. The latest slip zone of NDFP-1 110.75 m exhibits foliation by the difference of CT number, but the clear characteristics of the latest slip zone cannot be recognized on the CT image. The latest slip zone of NDFD-1-S1 387.7m is the clear boundary of the fault gouge. The CT number is very low inside of the fault gouge, which is assumed to be the latest slip zone, and that neighboring to the fault gouge is higher than general rocks including the borehole. This result suggests that the density is very low inside of the latest slip zone due to higher porosity and very high of the neighboring rocks due to the concentration of heavy minerals.
Reference: Nuclear Regulation Authority (2019) FY 2018 report on active fault drilling to construct a method of fault activity assessment.
The boreholes consist of NDFP-1 of the pilot hole, NDFD-1 of the main hole and NDFD-1-S1 of the side track hole (Nuclear Regulation Authority, 2019). NDFP-1 is an inclined borehole, and the length of the borehole is 140 m. The depth of the bottom of the hole is 110.75 m. NDFD-1 and NDFD-1-S1 is almost vertical. The length of the borehole is 524.8 m, and the depth of the bottom of the hole is 516.9 m.
NDFP-1 penetrates the latest slip zone at 110.75 m along the borehole from the top of the hole. The latest slip zone consists of the shale fault gouge. Fault rocks in the hanging wall are shale fault breccia, fractured chert and weakly fractured basalt, and those in the footwall are shale fault breccia and fractured shale. NDFD-1-S1 penetrates the latest slip zone at 387.7 m along the borehole from the top of the hole. The latest slip zone consists of the shale fault gouge. Fault rocks in the hanging wall are shale fault breccia, fractured shale, foliated and non-foliated basalt cataclasite, and those in the footwall are shale fault breccia and fractured shale. These indicates that the fault rocks in the hanging wall are more fractured than those in the footwall. Fault rocks in the hanging wall are located in the fault jog with compression. This would result in the above described fault rocks distribution.
We performed X-ray CT imaging of the borehole core including the latest slip zone. 2D cross sections and 3D images are reconstructed by Horos that is a software of CT image viewer. The latest slip zone of NDFP-1 110.75 m exhibits foliation by the difference of CT number, but the clear characteristics of the latest slip zone cannot be recognized on the CT image. The latest slip zone of NDFD-1-S1 387.7m is the clear boundary of the fault gouge. The CT number is very low inside of the fault gouge, which is assumed to be the latest slip zone, and that neighboring to the fault gouge is higher than general rocks including the borehole. This result suggests that the density is very low inside of the latest slip zone due to higher porosity and very high of the neighboring rocks due to the concentration of heavy minerals.
Reference: Nuclear Regulation Authority (2019) FY 2018 report on active fault drilling to construct a method of fault activity assessment.