4:15 PM - 4:30 PM
[SCG46-09] Three-dimensional deep electrical resistivity structure beneath the Kii Peninsula, Southwestern Japan
Keywords:electrical resistivity structure, Kii Peninsula, Deep Low-Frequency tremor, high-temperature spring, Network-MT method
The Kii Peninsula is located on the fore-arc side of southwest Japan and has high seismic activity areas (fast earthquakes, swarm earthquakes, deep low-frequency tremors, and slow slip), high-temperature springs with various 3He/4He isotopic ratios, and huge acidic rock bodies that extend from the surface to the top of the slab. These are considered to be related to the subducting Philippine Sea slab and deep fluids. Despite various investigations, including chemical composition analysis of hot spring water and seismic surveys, a unified interpretation of the relationship between the above tectonic features and the slab and deep fluids has not yet been obtained.
Electrical resistivity is a specific physical property of matter that opposes the flow of electric current. It is sensitive to the presence of fluids and temperature. The estimated electrical resistivity values of the crust and upper mantle generally range from the resistivity of dry rock to that of pore fluids. Since the change in resistivity of dry rock due to temperature change alone cannot explain the small resistivity values that are close to that of pore water, these values suggest that the presence of subsurface pore fluids may significantly contribute to the change in resistivity. The resistivity value reflects the quantity, composition, and connectivity of pore fluids. Therefore, it is important to estimate the resistivity structure to understand the distribution of fluids in the subsurface.
Several resistivity structure surveys have been conducted to solve this problem. In the latest study, Kinoshita (2018) estimated 3-D resistivity structure using ordinary MT data. However, his results are inconsistent with the 2-D resistivity structure using Network-MT data [Yamaguchi et al. (2009)], which is superior to the ordinary MT method in estimating the structure of the regional depths. They have a particularly different view of the resistivity values in the deep low-frequency seismogenic zone. Therefore, we estimated a 3-D wide-area deep resistivity structure model using the Network-MT data for the first time in the Kii Peninsula.
The resultant resistive structure shows high resistivity around the deep low-frequency tremor source zone on the slab. The resistive area is considered to be a part of the Kumano acidic rock body and corresponds well with a high seismic velocity zone and a high gravity anomaly zone. A low resistivity zone surrounds the high resistivity zone, and the contrast between the high-resistive acidic rock body and the surrounding low-resistivity region is clearly noticeable. There is a prominent low resistivity region extending from the top of the slab to the crust in the vertical section along a WSW-ENE section. In the center of the low-resistivity region, there is a seismic source zone that rises at a wide angle in the west-southwest direction. The low-resistivity zone may be a path of fluid ascent.
Our 3-D model shows the following resistivity structure beneath the Kii Peninsula: a high resistivity region, which corresponds well with the high seismic velocity region and the high gravity anomaly region and is thought to be the Kumano acid rock body, exists beneath the peninsula, and a low resistivity region exists surrounding the high resistivity region. In the same cross-section of the previous studies, deep low-frequency earthquakes occur at the boundary between the high-resistivity region and the slab. In addition, a cross-section in the west-southwest-northeast-east direction, which shows a clear resistivity contrast, reveals a prominent low resistivity region extending from the top of the slab to the crustal surface. This result is consistent with the presence of high-temperature springs with high 3He/4He isotope ratios on the Kii Peninsula, which surrounds the margin of the Kumano acid rock body. In its low resistivity area around the top of the slab, there is a dense hypocenter area of the Fast earthquake that rises at a wide angle in the west-southwest direction in the center of the low resistivity area, indicating that it may be a pathway of fluid ascent. There is another dense source area of fast earthquakes that rises vertically just above the dense source area of fast earthquakes, and this area is located at the boundary between the area of high resistivity and the area of low resistivity. This correspondence between the resistivity structure and the source distribution may explain the fluid contribution to the subsurface of the Kii Peninsula.
Electrical resistivity is a specific physical property of matter that opposes the flow of electric current. It is sensitive to the presence of fluids and temperature. The estimated electrical resistivity values of the crust and upper mantle generally range from the resistivity of dry rock to that of pore fluids. Since the change in resistivity of dry rock due to temperature change alone cannot explain the small resistivity values that are close to that of pore water, these values suggest that the presence of subsurface pore fluids may significantly contribute to the change in resistivity. The resistivity value reflects the quantity, composition, and connectivity of pore fluids. Therefore, it is important to estimate the resistivity structure to understand the distribution of fluids in the subsurface.
Several resistivity structure surveys have been conducted to solve this problem. In the latest study, Kinoshita (2018) estimated 3-D resistivity structure using ordinary MT data. However, his results are inconsistent with the 2-D resistivity structure using Network-MT data [Yamaguchi et al. (2009)], which is superior to the ordinary MT method in estimating the structure of the regional depths. They have a particularly different view of the resistivity values in the deep low-frequency seismogenic zone. Therefore, we estimated a 3-D wide-area deep resistivity structure model using the Network-MT data for the first time in the Kii Peninsula.
The resultant resistive structure shows high resistivity around the deep low-frequency tremor source zone on the slab. The resistive area is considered to be a part of the Kumano acidic rock body and corresponds well with a high seismic velocity zone and a high gravity anomaly zone. A low resistivity zone surrounds the high resistivity zone, and the contrast between the high-resistive acidic rock body and the surrounding low-resistivity region is clearly noticeable. There is a prominent low resistivity region extending from the top of the slab to the crust in the vertical section along a WSW-ENE section. In the center of the low-resistivity region, there is a seismic source zone that rises at a wide angle in the west-southwest direction. The low-resistivity zone may be a path of fluid ascent.
Our 3-D model shows the following resistivity structure beneath the Kii Peninsula: a high resistivity region, which corresponds well with the high seismic velocity region and the high gravity anomaly region and is thought to be the Kumano acid rock body, exists beneath the peninsula, and a low resistivity region exists surrounding the high resistivity region. In the same cross-section of the previous studies, deep low-frequency earthquakes occur at the boundary between the high-resistivity region and the slab. In addition, a cross-section in the west-southwest-northeast-east direction, which shows a clear resistivity contrast, reveals a prominent low resistivity region extending from the top of the slab to the crustal surface. This result is consistent with the presence of high-temperature springs with high 3He/4He isotope ratios on the Kii Peninsula, which surrounds the margin of the Kumano acid rock body. In its low resistivity area around the top of the slab, there is a dense hypocenter area of the Fast earthquake that rises at a wide angle in the west-southwest direction in the center of the low resistivity area, indicating that it may be a pathway of fluid ascent. There is another dense source area of fast earthquakes that rises vertically just above the dense source area of fast earthquakes, and this area is located at the boundary between the area of high resistivity and the area of low resistivity. This correspondence between the resistivity structure and the source distribution may explain the fluid contribution to the subsurface of the Kii Peninsula.