5:15 PM - 6:45 PM
[MGI31-P02] Sealing Process of the Neodani Fault Zone at Neo-Midori, Motosu City
Keywords:Neodani-Fault, fault zone, Sealing process
The Neodani Fault is a left-lateral active fault located in the northwestern part of Gifu Prefecture, Central Japan and ruptured during the 1891 Nobi Earthquake, which is one of the largest inland earthquakes with a magnitude of 8.0. Since approximately 130 years have passed since the latest earthquake, we expect that the sealing process after the fault activity will be clarified by fault rock study in the Neodani Fault.
The drillings penetrating to the Neodani Fault were conducted in Neo-Midori, Motosu City by the Nuclear Regulation Authority in FY2018 as part of a study on a method to evaluate fault activity using fault rocks. The fault is divided into two in this area, and the area between these two is the compressional fault jog. The vertical displacement at the boundary of the fault jog is 6 m in the 1891 earthquake, and the top of the borehole is located in the uplift side, where is in the northeastern part of the fault. The borehole NDFD-1-S1 penetrates perpendicular to the surface trace of the Neodani fault, and penetrated the latest slip surface at a depth of 382 m. The Mélange with a matrix of mudstone of Jurassic accretionary prism is distributed, and the core is composed of mudstone, basalt, and chert.
Okada et al (2023) suggested that the density of fault gouge decreases with the activity of the Neodani Fault and recovers by calcite filling . However, the occurrence of calcite filling is not clarified. The purpose of this study is to confirm the filling with calcite and other minerals in the fault gouge at and near the latest slip zone and to clarify the sealing process of the fault zone associated with the activity of the Neodani Fault.
We performed scanning electron microscope (SEM) observation of rock thin sections by backscattered electron (BSE) image and element analysis of SEM-EDX. In the latest slip zone of the fault gouge, no calcite vein was observed, suggesting that no calcite precipitation occurs after the 1891 earthquake.
In the fault gouge adjacent to the latest slip surface, two types of Ca veins are included; one is a linear mineral vein of a constant width and the other is of variable. The veins are mainly composed of numerous calcite grains with a size of a few mm, fragmented quartz and other minerals, clay matrix of the fault gouge. Weakly euhedral calcite grains are included in the vein with variable width. This suggests that calcite rapidly crystallized in the fractures formed during faulting and the surrounding minerals and matrix were involved.
In the fault gouge, 40 cm away from the latest slip zone, linear veins with Ca cut perpendicularly through the foliation of the fault gouge. These veins are composed of multiple carbonate veins, and each vein is developed parallel to the other. This suggests that carbonate minerals crystallized in fractures after faulting, and that a fracture opened along an existing vein during later faulting activity, and the fracture is filled again with carbonate minerals.
Boullier et al. (2004) suggests that the rise of groundwater along the fault and the decrease in the partial pressure of CO2 resulted in the crystallization of euhedral carbonate, ankerite and siderite. From this and the hypothesis of fault-valve behavior (Sibson, 1990), the sealing process of the Neodani Fault Zone is considered to be as follows.
Before fault activity, a low permeability layer is formed across the fault plane, and pore water pressure becomes higher than hydrostatic pressure below the layer. During faulting, the low permeability layer is ruptured by faulting and fractures are formed in the fault rock, mainly in the direction oblique to the slip plane. Immediately after faulting, groundwater rises rapidly along the fractures, involving mineral fragments and erodes the boundaries of fractures. During the rise of groundwater, euhedral carbonate rapidly crystallize and seal the fractures. After faulting, dissolution of calcite proceeds very slowly due to CO2-rich fluid along the fault, and euhedral calcite becomes weakly euhedral. Fracture by faulting are slowly sealed, and veins are formed after longer time without being destroyed by later faulting.
As described above, the sealing process of the Neodani Fault Zone is composed of rapid crystallization of carbonate minerals such as calcite due to groundwater rise immediately after faulting and slow crystallization of minerals after faulting, and these processes would have been repeated in the past.
The drillings penetrating to the Neodani Fault were conducted in Neo-Midori, Motosu City by the Nuclear Regulation Authority in FY2018 as part of a study on a method to evaluate fault activity using fault rocks. The fault is divided into two in this area, and the area between these two is the compressional fault jog. The vertical displacement at the boundary of the fault jog is 6 m in the 1891 earthquake, and the top of the borehole is located in the uplift side, where is in the northeastern part of the fault. The borehole NDFD-1-S1 penetrates perpendicular to the surface trace of the Neodani fault, and penetrated the latest slip surface at a depth of 382 m. The Mélange with a matrix of mudstone of Jurassic accretionary prism is distributed, and the core is composed of mudstone, basalt, and chert.
Okada et al (2023) suggested that the density of fault gouge decreases with the activity of the Neodani Fault and recovers by calcite filling . However, the occurrence of calcite filling is not clarified. The purpose of this study is to confirm the filling with calcite and other minerals in the fault gouge at and near the latest slip zone and to clarify the sealing process of the fault zone associated with the activity of the Neodani Fault.
We performed scanning electron microscope (SEM) observation of rock thin sections by backscattered electron (BSE) image and element analysis of SEM-EDX. In the latest slip zone of the fault gouge, no calcite vein was observed, suggesting that no calcite precipitation occurs after the 1891 earthquake.
In the fault gouge adjacent to the latest slip surface, two types of Ca veins are included; one is a linear mineral vein of a constant width and the other is of variable. The veins are mainly composed of numerous calcite grains with a size of a few mm, fragmented quartz and other minerals, clay matrix of the fault gouge. Weakly euhedral calcite grains are included in the vein with variable width. This suggests that calcite rapidly crystallized in the fractures formed during faulting and the surrounding minerals and matrix were involved.
In the fault gouge, 40 cm away from the latest slip zone, linear veins with Ca cut perpendicularly through the foliation of the fault gouge. These veins are composed of multiple carbonate veins, and each vein is developed parallel to the other. This suggests that carbonate minerals crystallized in fractures after faulting, and that a fracture opened along an existing vein during later faulting activity, and the fracture is filled again with carbonate minerals.
Boullier et al. (2004) suggests that the rise of groundwater along the fault and the decrease in the partial pressure of CO2 resulted in the crystallization of euhedral carbonate, ankerite and siderite. From this and the hypothesis of fault-valve behavior (Sibson, 1990), the sealing process of the Neodani Fault Zone is considered to be as follows.
Before fault activity, a low permeability layer is formed across the fault plane, and pore water pressure becomes higher than hydrostatic pressure below the layer. During faulting, the low permeability layer is ruptured by faulting and fractures are formed in the fault rock, mainly in the direction oblique to the slip plane. Immediately after faulting, groundwater rises rapidly along the fractures, involving mineral fragments and erodes the boundaries of fractures. During the rise of groundwater, euhedral carbonate rapidly crystallize and seal the fractures. After faulting, dissolution of calcite proceeds very slowly due to CO2-rich fluid along the fault, and euhedral calcite becomes weakly euhedral. Fracture by faulting are slowly sealed, and veins are formed after longer time without being destroyed by later faulting.
As described above, the sealing process of the Neodani Fault Zone is composed of rapid crystallization of carbonate minerals such as calcite due to groundwater rise immediately after faulting and slow crystallization of minerals after faulting, and these processes would have been repeated in the past.