11:00 AM - 1:00 PM
[SEM16-P03] A research report on the fundamental investigations of an electrical resistivity structure beneath Chugoku and Shikoku regions, southwestern Japan(2021)
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 San'in 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 activity 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 activity 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 are remarkable conductive regions 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.
In this background, wideband MT observation was conducted in the eastern Tottori area (A-W area) and the Tottori-Hyogo prefectural border area (A-E area) from early November to early December 2021. The A-W area is located at the eastern end of the Shikano-Yoshioka fault and the A-E area is located on the survey line that crosses the Amedaki-Kamado fault. The three geomagnetic and two electric or two electric field components were measured at a total of 8 points using the Phoenix MTU5A.
The characteristics obtained from the sounding curves such as apparent resistivity and phase are similar and both curves imply that the existence of a high resistivity region in the upper crust and a low resistivity region in the deep part. In addition, YX phase values exceeded 90 degrees for the period of more than several 100 seconds were observed in all sites.
2-D inversion using the program code of Ogawa and Uchida (1996) was performed on the impedance data derived from PT and GB decomposition analysis. As a general feature, both 2-D model show that the upper crust was generally characterized by a high resistivity region (several kΩ or more). However, in A-W area, discontinuity was appeared, in which the resistivity was lower by about an order of magnitude compared to the surroundings, at the point exactly at the eastern end of the Shikano-Yoshioka fault. On the other hand, in A-E area, in the area from the south point of the Amedaki-Kamado fault to the south, the resistivity value of the crust was found to be about an order of magnitude lower than that of the north side and the southernmost part.
Looking at the relationship between the resistivity structure and seismic activity, seismic activity was also observed at the boundary between high and low resistivity or on the high resistivity region. However, the deep low resistivity region pointed out under the seismic band in the San'in region has not been found. This may be related to the magnitude of the inland earthquake in this region.
In Shikoku region, 3-D model analysis is carried out by utilizing and integrating existing data. It is important to clarify the relationship between the resistivity structure and the mode and environment of slow earthquakes in the eastern and western part of Shikoku.
[Acknowledgment] In this research, we received support from the Ministry of Education, Culture, Sports, Science and Technology for the Earthquake and Volcano Observation Research Program to contribute to the mitigation of disasters. The observation equipment and reference magnetic field data were provided by Nittetsu Mining Consultant Co., Ltd.
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 San'in 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 activity 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 activity 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 are remarkable conductive regions 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.
In this background, wideband MT observation was conducted in the eastern Tottori area (A-W area) and the Tottori-Hyogo prefectural border area (A-E area) from early November to early December 2021. The A-W area is located at the eastern end of the Shikano-Yoshioka fault and the A-E area is located on the survey line that crosses the Amedaki-Kamado fault. The three geomagnetic and two electric or two electric field components were measured at a total of 8 points using the Phoenix MTU5A.
The characteristics obtained from the sounding curves such as apparent resistivity and phase are similar and both curves imply that the existence of a high resistivity region in the upper crust and a low resistivity region in the deep part. In addition, YX phase values exceeded 90 degrees for the period of more than several 100 seconds were observed in all sites.
2-D inversion using the program code of Ogawa and Uchida (1996) was performed on the impedance data derived from PT and GB decomposition analysis. As a general feature, both 2-D model show that the upper crust was generally characterized by a high resistivity region (several kΩ or more). However, in A-W area, discontinuity was appeared, in which the resistivity was lower by about an order of magnitude compared to the surroundings, at the point exactly at the eastern end of the Shikano-Yoshioka fault. On the other hand, in A-E area, in the area from the south point of the Amedaki-Kamado fault to the south, the resistivity value of the crust was found to be about an order of magnitude lower than that of the north side and the southernmost part.
Looking at the relationship between the resistivity structure and seismic activity, seismic activity was also observed at the boundary between high and low resistivity or on the high resistivity region. However, the deep low resistivity region pointed out under the seismic band in the San'in region has not been found. This may be related to the magnitude of the inland earthquake in this region.
In Shikoku region, 3-D model analysis is carried out by utilizing and integrating existing data. It is important to clarify the relationship between the resistivity structure and the mode and environment of slow earthquakes in the eastern and western part of Shikoku.
[Acknowledgment] In this research, we received support from the Ministry of Education, Culture, Sports, Science and Technology for the Earthquake and Volcano Observation Research Program to contribute to the mitigation of disasters. The observation equipment and reference magnetic field data were provided by Nittetsu Mining Consultant Co., Ltd.