5:15 PM - 6:30 PM
[SEM14-P01] A research report on the fundamental investigations of an electrical resistivity structure beneath Chugoku and Shikoku regions, southwestern Japan(2020)
Keywords:electrical resistivity, fundamental investigations, Chugoku and Shikoku regions
The purpose of this study is to investigate the spatial and structural heterogeneity of the crust and upper mantle by conducting basic resistivity surveys in Chugoku and Shikoku regions in order to contribute to the mitigation of disasters due to earthquakes and volcanic eruptions.
So far, our research group has shown that there is a close relationship between resistivity structure and seismic activity in the San'in and Shikoku regions. For example, in the eastern part of the Sanin region, there are prominent earthquake occurrence areas and belt-shaped seismic activities along the coast of the Sea of Japan, including the Shikano-Yoshioka fault, which is the Tottori earthquake (1943, M = 7.2). MT surveys were conducted on survey lines that cross the active area, and the existence of conductive regions were clarified in the deep part of the crust under the seismogenic layer, which is resistive region, along the seismic active zone extending in the almost east-west direction. If an inland earthquake is caused by local stress concentration caused by an inhomogeneous structure directly under the seismic active zone (Iio, 2009), it is important to carefully examine the homogeneous structure and consider seismic activity and stress concentration / relaxation.
On the other hand, in the Shikoku region, the survey results mainly in the outer zone indicate that there is a remarkable conductive region in the upper crust, and that it is clearly related to aseismic region in the central and western regions. For a unified understanding of seismic phenomena, it is important to elucidate not only the activity pattern of slow earthquakes but also the environment and principle of occurrence (Ohara (2017)), so the regional characteristics of the overall resistivity structure must be deternmined. Fundamental investigation of resistivity structure for this purpose is required.
MT observation point surveys were conducted in the central and northern regions of the Chugoku region, taking into consideration the electromagnetic noise environment, topography, roads and land use conditions. In addition, the existing two survey line data in this area (for example, acquired in the area around Mt. Sanbe and the seismic gap in the eastern part of Shimane Prefecture) were examined. In particular, in the area around Mt. Sanbe, we reevaluated the data obtained from past observations and introduced Phase Tensor (PT) analysis (Caldwell et al. (2004)), which was not included in the previous analysis, to examine the 3-dimensionality of the structure and to compare the findings in the seismic gap area of eastern Shimane to investigate the spatial and electrical characteristics of these areas.
As a result, in the area around Mt. Sanbe, three-dimensional nature was implied for the some period bands, and it is pointed out that further investigation is necessary for the structure elucidation. At present, paying attention to this periodic zone, the strike of the resistivyty structure was estimated to be N50E from the direction of the PT and induction vector analysis, and Groom and Bailey (1989) decomposition was performed to try to extract the two-dimensional structure. From the results of structural analysis obtained using the code of Ogawa and Uchida (1996), the crust was generally estimated as a high resistivity region.
The characteristics of this resistivity structure are in harmony with the research results of Shiozaki (1993). The estimated northern conduvtive / resistive boundary may be related to the 2018 Western Shimane Earthquake (M6.1) that occurred around the intersection of the two seismic zones pointed out by Asano et al. (1986). In addition, the seismic activity from around Mt. Sanbe to the southeast direction corresponds to the resistive region.
For the 3D resistivity structure analysis of the Shikoku region, we created the input data according to the format of the 3D analysis program code (Siripinvaraporn and Egbert (2009)) by utilizing and integrating the existing wideband MT observation data. In addition, ETOPO1 Ice Surface data published by NOAA was used for the land and sea distribution information of the created initial model.
So far, our research group has shown that there is a close relationship between resistivity structure and seismic activity in the San'in and Shikoku regions. For example, in the eastern part of the Sanin region, there are prominent earthquake occurrence areas and belt-shaped seismic activities along the coast of the Sea of Japan, including the Shikano-Yoshioka fault, which is the Tottori earthquake (1943, M = 7.2). MT surveys were conducted on survey lines that cross the active area, and the existence of conductive regions were clarified in the deep part of the crust under the seismogenic layer, which is resistive region, along the seismic active zone extending in the almost east-west direction. If an inland earthquake is caused by local stress concentration caused by an inhomogeneous structure directly under the seismic active zone (Iio, 2009), it is important to carefully examine the homogeneous structure and consider seismic activity and stress concentration / relaxation.
On the other hand, in the Shikoku region, the survey results mainly in the outer zone indicate that there is a remarkable conductive region in the upper crust, and that it is clearly related to aseismic region in the central and western regions. For a unified understanding of seismic phenomena, it is important to elucidate not only the activity pattern of slow earthquakes but also the environment and principle of occurrence (Ohara (2017)), so the regional characteristics of the overall resistivity structure must be deternmined. Fundamental investigation of resistivity structure for this purpose is required.
MT observation point surveys were conducted in the central and northern regions of the Chugoku region, taking into consideration the electromagnetic noise environment, topography, roads and land use conditions. In addition, the existing two survey line data in this area (for example, acquired in the area around Mt. Sanbe and the seismic gap in the eastern part of Shimane Prefecture) were examined. In particular, in the area around Mt. Sanbe, we reevaluated the data obtained from past observations and introduced Phase Tensor (PT) analysis (Caldwell et al. (2004)), which was not included in the previous analysis, to examine the 3-dimensionality of the structure and to compare the findings in the seismic gap area of eastern Shimane to investigate the spatial and electrical characteristics of these areas.
As a result, in the area around Mt. Sanbe, three-dimensional nature was implied for the some period bands, and it is pointed out that further investigation is necessary for the structure elucidation. At present, paying attention to this periodic zone, the strike of the resistivyty structure was estimated to be N50E from the direction of the PT and induction vector analysis, and Groom and Bailey (1989) decomposition was performed to try to extract the two-dimensional structure. From the results of structural analysis obtained using the code of Ogawa and Uchida (1996), the crust was generally estimated as a high resistivity region.
The characteristics of this resistivity structure are in harmony with the research results of Shiozaki (1993). The estimated northern conduvtive / resistive boundary may be related to the 2018 Western Shimane Earthquake (M6.1) that occurred around the intersection of the two seismic zones pointed out by Asano et al. (1986). In addition, the seismic activity from around Mt. Sanbe to the southeast direction corresponds to the resistive region.
For the 3D resistivity structure analysis of the Shikoku region, we created the input data according to the format of the 3D analysis program code (Siripinvaraporn and Egbert (2009)) by utilizing and integrating the existing wideband MT observation data. In addition, ETOPO1 Ice Surface data published by NOAA was used for the land and sea distribution information of the created initial model.