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[SSS11-04] Estimation of Resistivity Structure of Sanageyama-Kita Fault Zone based on Electrical Prospecting
Keywords:electrical prospecting, Sanageyama-Kita Fault, fault fracture zone
The Enasan Sanageyama-Kita Fault Zone extends from Nakatsugawa City in Gifu Pref. to Seto City in Aichi Pref. The Byobuyama Fault Zone continues to the north, and the Takahama Fault Zone and Kagiya Fault Zone continue to the south, and their total length is over 100 km. However, their subsurface structure and continuity have not been clarified. Because a densely populated city (Nagoya City) and an industrial city (Toyota City) are located nearby the fault zones, proper evaluation of fault activity and appropriate earthquake countermeasures are required. The trench surveys (Aichi Pref. et al., 1996; Azuma et al., 2020, 2022), the borehole surveys (Aichi Pref. et al., 1996, 1998), and the seismic prospecting (Watanabe et al., 2021) were conducted in Higashi-Shirasaka site, Seto City, Aichi Pref., that is located almost at the southern end of the Sanageyama-Kita Fault Zone. The severely fractured granite was recognized in the borehole core. The width of the fracture zone is estimated to be approximately 16 m based on the range of the fractured granite in the borehole core and the part of fault observed in the trench survey. The seismic prospecting (ultra-shallow reflection method) by Watanabe et al. (2021) detected regions with weak reflection. As rock weathering due to fault fracture progresses, the boundary of physical properties between sedimentary layers and bedrock becomes unclear, so the weak reflection areas are interpreted to reflect fault fracture zones. To elucidate a more detailed distribution of the fracture zone, we conducted electrical prospecting and estimated subsurface resistivity structure in the Higashi-Shirasaka site. The study area is appropriate to discuss how the fracture zone affects resistivity distribution because the prospecting area shows simple geology mainly formed with granites. Therefore, the second purpose of this study is to discuss the utility of electrical prospecting in estimation of underground structure of fault zone.
Electrical prospecting was conducted on the survey line that was set almost perpendicular to the fault strike. In the resistivity method, electrical current is sent from current electrodes to the ground and potential difference V is measured between potential electrodes, and apparent resistivity ρa=G(V/I) is calculated. For data analysis, we utilized the two-dimensional inversion method (Uchida, 1993). In this method, the previous model is improved to the new model based on the minimum condition of the objective function consisting of Misfit (weighted residual sum of squares of observed values and model response) and Roughness (sum of squares of second order difference of model parameters). Repeating this process, we obtain the model minimizing objective function. The resistivity structure with balance between the data misfit and the smoothness was selected as the optimum one based on the L-curve method (Hansen, 2001).
Significant low resistivity regions are detected in the optimum resistivity structure. The regions are located in the areas where the fault zone was estimated from the trench and borehole surveys. When rocks have been fractured by fault activity, the resistivity decreases due to inter-connected pore water by increasing porosity and the increase of clay mineral content, etc. Therefore, the low resistivity regions may reflect fault fracture zones. These low resistivity areas also correspond to the weak reflection areas in the seismic prospecting. The fault fracture zone may be more developed than the width of approximately 16 m estimated from the trench and borehole surveys. It is expected that the width of the fracture zone may be correctly estimated by extending the survey line further than the present survey. The above discussion indicates that electrical prospecting is useful when used as one of the clues for interpretation of fault structure and for narrowing down the target areas for other surveys.
Electrical prospecting was conducted on the survey line that was set almost perpendicular to the fault strike. In the resistivity method, electrical current is sent from current electrodes to the ground and potential difference V is measured between potential electrodes, and apparent resistivity ρa=G(V/I) is calculated. For data analysis, we utilized the two-dimensional inversion method (Uchida, 1993). In this method, the previous model is improved to the new model based on the minimum condition of the objective function consisting of Misfit (weighted residual sum of squares of observed values and model response) and Roughness (sum of squares of second order difference of model parameters). Repeating this process, we obtain the model minimizing objective function. The resistivity structure with balance between the data misfit and the smoothness was selected as the optimum one based on the L-curve method (Hansen, 2001).
Significant low resistivity regions are detected in the optimum resistivity structure. The regions are located in the areas where the fault zone was estimated from the trench and borehole surveys. When rocks have been fractured by fault activity, the resistivity decreases due to inter-connected pore water by increasing porosity and the increase of clay mineral content, etc. Therefore, the low resistivity regions may reflect fault fracture zones. These low resistivity areas also correspond to the weak reflection areas in the seismic prospecting. The fault fracture zone may be more developed than the width of approximately 16 m estimated from the trench and borehole surveys. It is expected that the width of the fracture zone may be correctly estimated by extending the survey line further than the present survey. The above discussion indicates that electrical prospecting is useful when used as one of the clues for interpretation of fault structure and for narrowing down the target areas for other surveys.