13:45 〜 15:15
[SEM14-P17] A research report on the fundamental investigations of an electrical resistivity structure beneath Chugoku and Shikoku regions(2022)
キーワード： 地殻比抵抗構造、中国・四国地方、基盤的調査
The purpose of this study is to investigate the spatial and structural heterogeneity of the crust and upper mantle by conducting MT investigations 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 large 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). 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. 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.
Based on this research background, from the late October to the early December 2022, we conducted the wideband MT observations at 9 points in the central part of the Chugoku region (Shimane / Izumo – Hiroshima / Miyoshi survey line) using the Phoenix MTU5A systems. The northern part of the survey area corresponds to the western edge of the seismic gap (Ishikawa, 1990), which is an area of low seismic activity within the inland seismic zone in the San'in region. In addition, the southern region is located in the seismic zone, extending from Nakanohara through Mikawa to Mt.Sanbe in the northwest - southeast direction, pointed out by Asano et al(1986). The 2018 Western Shimane Earthquake (M6.1) occurred around the crossing point between another seismic zone extending in the southwest direction from Mt.Sanbe parallel to the coastline and the seismic zone mentioned above. It is expected that new knowledge will be obtained regarding the seismic gap and the continuity of the features of the active zone structure around these regions.
The characteristics obtained from the sounding curves, such as apparent resistivity and phase value can be divided into two groups: the northern Izumo plain area and the southern mountainous region. It was found that the shapes of the curves are similar to each other at each region. Especially in the northern part, a gradual increase in the apparent resistivityρyx over the long period band and almost constant phase-difference curve forφxy over a wide period band were observed. On the other hand, in the southern part, ρxy and ρyx increase over the period band around 1 second, and then both components decrease until the period around 100 seconds. The phase curve is also consistent with this variation. These features suggest the existence of a high resistivity crust and a low resistivity deep part in the southern region.
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, 2-D model shows that the crust was generally characterized by a high resistivity region (several kΩ or more). This structure was inferred as a high resistivity structure compared with the structure of the eastern Shimane earthquake gap survey result by the 2003 Eastern Shimane Resistivity Research Group (2005) and the deep part of the high resistivity region shows discontinuous around a few points. This feature of discontinuing high resistivity is also seen in the resistivity structure of the eastern Shimane. In addition, the remarkable deep low resistivity part of 10Ωm or less, which has been pointed out under the inland seismogenic zone exceeding M6 in the San'in region, is not seen except for the northern area structure.
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 determined. Fundamental investigation of resistivity structure for this purpose is required.
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 large 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). 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. 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.
Based on this research background, from the late October to the early December 2022, we conducted the wideband MT observations at 9 points in the central part of the Chugoku region (Shimane / Izumo – Hiroshima / Miyoshi survey line) using the Phoenix MTU5A systems. The northern part of the survey area corresponds to the western edge of the seismic gap (Ishikawa, 1990), which is an area of low seismic activity within the inland seismic zone in the San'in region. In addition, the southern region is located in the seismic zone, extending from Nakanohara through Mikawa to Mt.Sanbe in the northwest - southeast direction, pointed out by Asano et al(1986). The 2018 Western Shimane Earthquake (M6.1) occurred around the crossing point between another seismic zone extending in the southwest direction from Mt.Sanbe parallel to the coastline and the seismic zone mentioned above. It is expected that new knowledge will be obtained regarding the seismic gap and the continuity of the features of the active zone structure around these regions.
The characteristics obtained from the sounding curves, such as apparent resistivity and phase value can be divided into two groups: the northern Izumo plain area and the southern mountainous region. It was found that the shapes of the curves are similar to each other at each region. Especially in the northern part, a gradual increase in the apparent resistivityρyx over the long period band and almost constant phase-difference curve forφxy over a wide period band were observed. On the other hand, in the southern part, ρxy and ρyx increase over the period band around 1 second, and then both components decrease until the period around 100 seconds. The phase curve is also consistent with this variation. These features suggest the existence of a high resistivity crust and a low resistivity deep part in the southern region.
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, 2-D model shows that the crust was generally characterized by a high resistivity region (several kΩ or more). This structure was inferred as a high resistivity structure compared with the structure of the eastern Shimane earthquake gap survey result by the 2003 Eastern Shimane Resistivity Research Group (2005) and the deep part of the high resistivity region shows discontinuous around a few points. This feature of discontinuing high resistivity is also seen in the resistivity structure of the eastern Shimane. In addition, the remarkable deep low resistivity part of 10Ωm or less, which has been pointed out under the inland seismogenic zone exceeding M6 in the San'in region, is not seen except for the northern area structure.
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 determined. Fundamental investigation of resistivity structure for this purpose is required.
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.