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[SSS12-05] Characteristics of Latest Slip Plane and Strength Recovery Process of Fault Gouges of the Shallow Fault Zone of the Neodani Fault
Keywords:latest slip plane, Neodani Fault Zone, strength recovery process
X-ray CT observation, powder X-ray diffraction analysis (XRD), X-ray fluorescence analysis (XRF), and micro XRF analysis (XGT) of the latest slip zone of the shallow fault zone of the Neodani Fault ruptured in 1891 Nobi Earthquake were performed using boring cores drilled by the Nuclear Regulation Authority. The purpose of this study is to clarify the characteristics of the latest slip plane and the strength recovery process of the fault based on the comparison between the latest and the other fault gouges.
The drilling site is Neomidori, Motosu City, Gifu Prefecture, central Japan in which a vertical displacement of 6 m occurred during the Nobi Earthquake. It is located in the compressional fault jog. In this study, the studied boring cores are NDFP-1 which is a pilot hole and NDFD-1-S1 which is a main hole drilled in 2019. NDFP-1 is an inclined well. The length of NDFP-1 is 140.0 m and the bottom of the hole is at depth of 106.8 m. NDFD-1-S1 is an almost vertical well. The length of the hole is 524.8 m and a bottom of the hole is at depth of 516.9 m (Nuclear Regulation Authority, 2019).
X-ray CT observation shows that the latest slip zones in NDFP-1 and NDFD-1-S1 have extremely lower CT values, whereas the fault gouges except the latest slip zone, fault breccias and fractured rocks have higher values. This means that the latest slip zone has lower density.
XRD results exhibit that the fault gouges including the latest slip zone contain calcite and smectite. XRF results represent that Ca is higher in the latest slip zone and its vicinity of NDFP-1. XGT results display that Ca is higher in the vicinity of the latest slip zone, whereas it is relatively lower in the latest slip zone.
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. Therefore, the lower density of the latest slip zone would be caused by volume expansion of the fault gouge during the Nobi earthquake despite it is a compressional fault jog. The CT values in the other gouges are higher and the density is higher, suggesting that their densities are fully recovered. This density recovery would be caused by mineral filling, compression by the effect of the fault jogs and shortening of the earth crust.
The filling of calcite generally occurs in the slip zone, and the fault gouges next to the latest slip zone show the higher concentration of Ca. This implies that it is fully filled by calcite. It is unclear whether the calcite has filled voids in the latest slip zone after the Nobi earthquake. Both of the latest slip zone and the other fault gouges contain smectite, indicating the formation condition is almost same. Therefore, the calcite filling will occur in the latest slip zone and its density will be recovered in future.
The average recurrence time of the Neodani Fault is about 2100 to 3600 years (Headquarters for Earthquake Research Promotion, 2005). The strength recovery of the latest slip zone is not enough 130 years after the Nobi Earthquake. The other fault gouges, which is not the latest slip zone show the
has completed the process of strength recovery of the fault by density recovering over time. But, since the target boring core of this time is a shallow sample and the confined pressure is small, it is considered that high fault strength is not required. Therefore, since a large strain energy for the next earthquake of the Neodani Fault cannot be stored at this depth, it is possible that the fault strength will not be completely restored by the next fault movement.
Nuclear Regulation Authority (2019) FY 2018 report on active fault drilling to construct a method of fault activity assessment.
Headquarters for Earthquake Research Promotion (2005) Long-term assessment of the Nobi Fault Zone. 49pp.
Scaringi et al. (2018) Geophysical Research Letters, 45, 766–777.
The drilling site is Neomidori, Motosu City, Gifu Prefecture, central Japan in which a vertical displacement of 6 m occurred during the Nobi Earthquake. It is located in the compressional fault jog. In this study, the studied boring cores are NDFP-1 which is a pilot hole and NDFD-1-S1 which is a main hole drilled in 2019. NDFP-1 is an inclined well. The length of NDFP-1 is 140.0 m and the bottom of the hole is at depth of 106.8 m. NDFD-1-S1 is an almost vertical well. The length of the hole is 524.8 m and a bottom of the hole is at depth of 516.9 m (Nuclear Regulation Authority, 2019).
X-ray CT observation shows that the latest slip zones in NDFP-1 and NDFD-1-S1 have extremely lower CT values, whereas the fault gouges except the latest slip zone, fault breccias and fractured rocks have higher values. This means that the latest slip zone has lower density.
XRD results exhibit that the fault gouges including the latest slip zone contain calcite and smectite. XRF results represent that Ca is higher in the latest slip zone and its vicinity of NDFP-1. XGT results display that Ca is higher in the vicinity of the latest slip zone, whereas it is relatively lower in the latest slip zone.
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. Therefore, the lower density of the latest slip zone would be caused by volume expansion of the fault gouge during the Nobi earthquake despite it is a compressional fault jog. The CT values in the other gouges are higher and the density is higher, suggesting that their densities are fully recovered. This density recovery would be caused by mineral filling, compression by the effect of the fault jogs and shortening of the earth crust.
The filling of calcite generally occurs in the slip zone, and the fault gouges next to the latest slip zone show the higher concentration of Ca. This implies that it is fully filled by calcite. It is unclear whether the calcite has filled voids in the latest slip zone after the Nobi earthquake. Both of the latest slip zone and the other fault gouges contain smectite, indicating the formation condition is almost same. Therefore, the calcite filling will occur in the latest slip zone and its density will be recovered in future.
The average recurrence time of the Neodani Fault is about 2100 to 3600 years (Headquarters for Earthquake Research Promotion, 2005). The strength recovery of the latest slip zone is not enough 130 years after the Nobi Earthquake. The other fault gouges, which is not the latest slip zone show the
has completed the process of strength recovery of the fault by density recovering over time. But, since the target boring core of this time is a shallow sample and the confined pressure is small, it is considered that high fault strength is not required. Therefore, since a large strain energy for the next earthquake of the Neodani Fault cannot be stored at this depth, it is possible that the fault strength will not be completely restored by the next fault movement.
Nuclear Regulation Authority (2019) FY 2018 report on active fault drilling to construct a method of fault activity assessment.
Headquarters for Earthquake Research Promotion (2005) Long-term assessment of the Nobi Fault Zone. 49pp.
Scaringi et al. (2018) Geophysical Research Letters, 45, 766–777.