17:15 〜 19:15
[SCG52-P04] Three-dimensional resistivity structure beneath the Chugoku and Shikoku districts in Southwestern Japan using Network-Magnetotelluric method
キーワード：三次元電気比抵抗構造、西南日本、Network-MT法、地磁気地電流法
Two oceanic plates are subducting in the Japan Islands. In the Chugoku and Shikoku districts in Southwestern Japan, Philippine Sea plate is subducting from the south-east to the north-west. This subduction causes various types of tectonic activities in the districts. Among them, the Low-Frequency Earthquakes (LFEs) in the forearc side and the intense crustal activities such as existence of linear seismically active zone with high- temperature hot springs in the back-arc side are interesting targets of investigation in geophysics. Subsurface fluid is thought to be one of the key factors causing these activities.
Electrical resistivity (hereafter expressed just as resistivity) is a physical parameter, which is especially sensitive to existence of interstitial fluids. In the Chugoku and Shikoku districts, previous studies have revealed two-dimensional (2-D) resistivity structures. However, bathymetry distribution in these districts is very complex and purely three-dimensional (3-D). Considering highly conductive sea water, 2-D interpretation may lead some fake structure. In order to mitigate this problem, this study focuses on the 3-D interpretation.
This study uses Network-Magnetotelluric (Network-MT) data acquired from 1994 to 1996, a period during which long baseline metallic telephone cables were available. In the Network-MT method, we use metallic telephone line network to measure potential differences between long distant points. Owing to the long electrode spacings, Network-MT data are relatively free from the static shift and the method enables us to cover a target area with less efforts compared with the conventional MT method.
The regional resistivity structure was obtained through a series of steps. First, the data with low artificial noise were carefully selected. Then, we were processed using the robust BIRRP code to estimate the Network-MT response function between each voltage difference and the horizontal components of the magnetic field in the frequency domain. Finally, the FEMTIC inversion code was applied to these response functions to obtain a 3-D resistivity structure.
The final 3-D resistivity model revealed three main features in these districts. First, the subducting Philippine Sea Plate is identified as a high-resistivity body. Second, a low-resistivity body is detected in the back-arc region, providing evidence of fluid upwelling in the mantle wedge and potentially contributing to intense crustal activities. Finally, a low-resistivity body in the fore-arc is associated with activities of LFEs, but more detailed discussion on relationship between location and intensity of the low-resistivity body and LFE activities will be necessary.
Electrical resistivity (hereafter expressed just as resistivity) is a physical parameter, which is especially sensitive to existence of interstitial fluids. In the Chugoku and Shikoku districts, previous studies have revealed two-dimensional (2-D) resistivity structures. However, bathymetry distribution in these districts is very complex and purely three-dimensional (3-D). Considering highly conductive sea water, 2-D interpretation may lead some fake structure. In order to mitigate this problem, this study focuses on the 3-D interpretation.
This study uses Network-Magnetotelluric (Network-MT) data acquired from 1994 to 1996, a period during which long baseline metallic telephone cables were available. In the Network-MT method, we use metallic telephone line network to measure potential differences between long distant points. Owing to the long electrode spacings, Network-MT data are relatively free from the static shift and the method enables us to cover a target area with less efforts compared with the conventional MT method.
The regional resistivity structure was obtained through a series of steps. First, the data with low artificial noise were carefully selected. Then, we were processed using the robust BIRRP code to estimate the Network-MT response function between each voltage difference and the horizontal components of the magnetic field in the frequency domain. Finally, the FEMTIC inversion code was applied to these response functions to obtain a 3-D resistivity structure.
The final 3-D resistivity model revealed three main features in these districts. First, the subducting Philippine Sea Plate is identified as a high-resistivity body. Second, a low-resistivity body is detected in the back-arc region, providing evidence of fluid upwelling in the mantle wedge and potentially contributing to intense crustal activities. Finally, a low-resistivity body in the fore-arc is associated with activities of LFEs, but more detailed discussion on relationship between location and intensity of the low-resistivity body and LFE activities will be necessary.