3:30 PM - 5:00 PM
[HDS09-P04] Characteristics of Fault Fracture Zone along the central- northern part of the Atera Fault Zone
Keywords:Atera fault, fracture zone, sense of displacement, fault gouge
The structural characteristics in and around the active fault are important for the model of inland earthquake. Furthermore, information on the geological structure is needed to consider deeper underground and longer-term movements.
In this study, we focused on the Atera fault zone, especially on the central-northern part characterized by parallel active fault traces, and obtained information on fracture characteristics around active fault outcrops. Fault outcrops were observed on active faults. A fault gouge about 50 cm wide is distributed on the riverbed of Norimasa River bordering the Nouhi rhyolite and riverbed rocks. Based on the surrounding tectonic landform and the strike of the fault surface, it is an outcrop of the Yugamine Fault. Fault gouge of a few mm to a few cm in width is distributed along the continuous fault surface in the Nouhi rhyolite in the riverbed of the Hida River. Based on the surrounding tectonic landform and the strike of the fault surface, it is an outcrop of the Nishiueda Fault. Fractures have developed in the bedrock surrounding these fault outcrops.
The number of fractures was surveyed in the surrounding area of the Yugamine Fault. Although there are some points that have not been confirmed, high-density fracture zones of more than 10 fractures/m are widely distributed along the Norimasa River for more than 1 km. 0n the other hand, the fracture zone in the southern part of the Atera Fault Zone, where the fault trace has clear continuity, is about 50-200 m wide (Yamada,1975). There is a clear difference in the width of the fracture zone between the central-northern part and southern part. In the 500 m south of the Yugamine Fault, volcanic dykes with a formation age of 100,000 to 120,000 years ago are distributed in the fracture zone (Tsukuda et al, 1991). This dyke is not fractured and there are no fresh fractures due to fault movement. This suggests that the widely fractured zone was formed by fault movement at an older age.
To understand the process of fault movement, we measured the strike of fractures and observed small faults and fault gouges. Small faults that strike at low angles are distributed clearly around a few meters from the fault surface of the active fault. On this small fault, especially on the Nishiueda Fault, there are two types of faults: one is left slip and more to the right, and the other is right slip and more to the left. In addition, a structure in which a fault with right slip is cut by a fault with left slip is observed. Therefore, it is possible that this fault was once right slip and then changed to the present left slip. This change in displacement sense may be related to the formation of fracture zones.
We plan to examine these fault outcrops in detail to understand changes in displacement sense and to compare fracture characteristics between sections where active faults run parallel or dip and those where there is good continuity.
In this study, we focused on the Atera fault zone, especially on the central-northern part characterized by parallel active fault traces, and obtained information on fracture characteristics around active fault outcrops. Fault outcrops were observed on active faults. A fault gouge about 50 cm wide is distributed on the riverbed of Norimasa River bordering the Nouhi rhyolite and riverbed rocks. Based on the surrounding tectonic landform and the strike of the fault surface, it is an outcrop of the Yugamine Fault. Fault gouge of a few mm to a few cm in width is distributed along the continuous fault surface in the Nouhi rhyolite in the riverbed of the Hida River. Based on the surrounding tectonic landform and the strike of the fault surface, it is an outcrop of the Nishiueda Fault. Fractures have developed in the bedrock surrounding these fault outcrops.
The number of fractures was surveyed in the surrounding area of the Yugamine Fault. Although there are some points that have not been confirmed, high-density fracture zones of more than 10 fractures/m are widely distributed along the Norimasa River for more than 1 km. 0n the other hand, the fracture zone in the southern part of the Atera Fault Zone, where the fault trace has clear continuity, is about 50-200 m wide (Yamada,1975). There is a clear difference in the width of the fracture zone between the central-northern part and southern part. In the 500 m south of the Yugamine Fault, volcanic dykes with a formation age of 100,000 to 120,000 years ago are distributed in the fracture zone (Tsukuda et al, 1991). This dyke is not fractured and there are no fresh fractures due to fault movement. This suggests that the widely fractured zone was formed by fault movement at an older age.
To understand the process of fault movement, we measured the strike of fractures and observed small faults and fault gouges. Small faults that strike at low angles are distributed clearly around a few meters from the fault surface of the active fault. On this small fault, especially on the Nishiueda Fault, there are two types of faults: one is left slip and more to the right, and the other is right slip and more to the left. In addition, a structure in which a fault with right slip is cut by a fault with left slip is observed. Therefore, it is possible that this fault was once right slip and then changed to the present left slip. This change in displacement sense may be related to the formation of fracture zones.
We plan to examine these fault outcrops in detail to understand changes in displacement sense and to compare fracture characteristics between sections where active faults run parallel or dip and those where there is good continuity.