17:15 〜 18:45
[SEM12-P20] Electrical resistivity structure by using Wide-Band MT data beneath the Gomura fault zone in the Tango Peninsula, Southwestern Japan
キーワード：郷村断層帯、電気比抵抗構造、Wide-Band MT法、活断層、断層低比抵抗領域
The Gomura fault zone is located in the Tango Peninsula, northern Kyoto Prefecture, Southwestern Japan, and consists of three faults; the Go-seiho, Gomura, and Chuzenji Faults, which are parallel and adjacent to each other. The Go-seiho Fault is very short (~2.8 km long) and its lateral displacement is unclear, and it is interpreted as a secondary fault that is not continuous with the source fault (Mimura et al., 2023; Yamaguchi et al., 2021; Okada and Togo, 2000). The Gomura Fault has a length of about 13 km on land and is thought to extend further offshore, and produced a prominent surface earthquake fault with a left lateral displacement component at the 1927 Kita-tango Earthquake (The Headquarters for Earthquake Research Promotion, 2004). The Chuzenji Fault is characterized by a remarkable left lateral displacement of up to 200 m along its ridges and river valleys and a large cumulative displacement (The Headquarters for Earthquake Research Promotion, 2004). However, it was not active at the 1927 Kita-Tango earthquake and is thought to have been active before 12,000 years ago, especially until 20,000 years ago (Sugiyama and Tsukuda, 1993). As described above, in the Gomura Fault Zone, these faults show different features in fault activity (e.g. mean length, slip rates, and the latest events) and all of them are located in the Miyazu granite body, which is a desirable condition for investigating the relationship between fault characteristics and basement structure.
In the Gomura fault zone, the Audio-frequency MT method survey by Mimura et al. (2023) revealed a subsurface electrical resistivity structure to a depth of 1.5 km. As a result, a prominent low resistivity zone was identified directly below the Gomura Fault, while no such zone was found directly below the Chuzenji Fault. In addition, an unclear low resistivity zone was observed only in the shallow part directly below the Go-seiho Fault, which was interpreted to be a secondary fault. However, to clarify the characteristics of the fault zone, i.e., the relationship between these faults, especially the Gomura and Chuzenji faults, it is necessary to clarify the subsurface structure in more depth.
Therefore, we analyzed the Wide-Band MT data obtained by Mimura et al. (2017) around the Gomura fault zone and calculated a new resistivity model in this study. We paid attention to the following 3 points, in the analysis.
1. In the spectral analysis, we paid attention to the appropriate selection of the analysis section and parameters.
2. In the model analysis, we paid attention to the number and size of resistivity elements used in the 3-D inversion calculation.
3. In the model analysis, we improved the reproducibility of the sea area to be set up where the resistivity value is fixed.
As a result, the GMR deep resistivity structure model is characterized by two resistive zones in the shallower area (~3km) and three conductive zones in the deeper area (3km~). These are classified as either predominant structures around the study area, two-dimensional structures parallel to the fault, or localized structures. In this presentation, we will discuss the relationship of these structures to geology, topography, and seismic source distribution.
In the Gomura fault zone, the Audio-frequency MT method survey by Mimura et al. (2023) revealed a subsurface electrical resistivity structure to a depth of 1.5 km. As a result, a prominent low resistivity zone was identified directly below the Gomura Fault, while no such zone was found directly below the Chuzenji Fault. In addition, an unclear low resistivity zone was observed only in the shallow part directly below the Go-seiho Fault, which was interpreted to be a secondary fault. However, to clarify the characteristics of the fault zone, i.e., the relationship between these faults, especially the Gomura and Chuzenji faults, it is necessary to clarify the subsurface structure in more depth.
Therefore, we analyzed the Wide-Band MT data obtained by Mimura et al. (2017) around the Gomura fault zone and calculated a new resistivity model in this study. We paid attention to the following 3 points, in the analysis.
1. In the spectral analysis, we paid attention to the appropriate selection of the analysis section and parameters.
2. In the model analysis, we paid attention to the number and size of resistivity elements used in the 3-D inversion calculation.
3. In the model analysis, we improved the reproducibility of the sea area to be set up where the resistivity value is fixed.
As a result, the GMR deep resistivity structure model is characterized by two resistive zones in the shallower area (~3km) and three conductive zones in the deeper area (3km~). These are classified as either predominant structures around the study area, two-dimensional structures parallel to the fault, or localized structures. In this presentation, we will discuss the relationship of these structures to geology, topography, and seismic source distribution.