11:45 〜 12:00
[SEM14-11] Subsurface electrical resistivity structure beneath the southern part of Tohoku, NE Japan, revealed by magnetotelluric and geomagnetic transfer functions
キーワード：Resistivity, Magnetotelluric, Tohoku
Significant volcanic activities have been observed at the southern part of Tohoku as an implication of the subduction system. Knowing that fluids have an essential role in arc magmatism and the associated seismicity, electromagnetic methods that probe the Earth's electrical resistivity structure are useful for delineating the fluid-rich area. Time-varying electric and magnetic field data were recorded at 15 stations along a profile line across the southern Tohoku. The frequency-domain response functions of the measured fields were interpreted to yield an electrical resistivity structure using a newly developed joint inversion code. In addition to the commonly used response functions, the horizontal magnetic field transfer function (HMTF) was also considered to determine the two-dimensional (2D) regional trend and estimate the electrical resistivity structure. A numerical experiment of synthetic inversion and the sensitivity tests of the final model demonstrated the usefulness of HMTF in enhancing the resolution and sensitivity of the inverted resistivity structure.
Three conductive zones match the in-land volcanic and seismic activities throughout the profile. First, a sub-vertical conductor is detected beneath the central part in the depth range of the middle crust to the uppermost mantle. A good spatial correlation with deep low-frequency earthquakes indicates that partial melting and/or aqueous fluids within host rocks with less than 5% porosity may explain the high conductivity. This conductor explained the presence of Mt. Numazawa in the back-arc side, hypocenters of DLFE, and the vacancy of the volcanic front throughout the survey line. Second, a conductive upper crust is found in the back-arc side just outside the profile. The geometry is consistent with the eastward dipping reverse fault of the Tsukioka fault zone revealed by a seismic reflection study. The conductive content can be attributed to clay minerals and/or fluids. Third, there is a highly conductive middle crust beneath the fore-arc side of the volcanic front. This conductive zone might be related to seismogenic process of swarm earthquakes in the upper crust beneath the volcanic front. A Fluid-rich area is a possible candidate to explain the high conductivity, but it requires further investigation. In addition, although the 2D assumption of the resistivity structure is valid to some extent, a three-dimensional analysis will be desirable for future studies in the southern Tohoku area.
Three conductive zones match the in-land volcanic and seismic activities throughout the profile. First, a sub-vertical conductor is detected beneath the central part in the depth range of the middle crust to the uppermost mantle. A good spatial correlation with deep low-frequency earthquakes indicates that partial melting and/or aqueous fluids within host rocks with less than 5% porosity may explain the high conductivity. This conductor explained the presence of Mt. Numazawa in the back-arc side, hypocenters of DLFE, and the vacancy of the volcanic front throughout the survey line. Second, a conductive upper crust is found in the back-arc side just outside the profile. The geometry is consistent with the eastward dipping reverse fault of the Tsukioka fault zone revealed by a seismic reflection study. The conductive content can be attributed to clay minerals and/or fluids. Third, there is a highly conductive middle crust beneath the fore-arc side of the volcanic front. This conductive zone might be related to seismogenic process of swarm earthquakes in the upper crust beneath the volcanic front. A Fluid-rich area is a possible candidate to explain the high conductivity, but it requires further investigation. In addition, although the 2D assumption of the resistivity structure is valid to some extent, a three-dimensional analysis will be desirable for future studies in the southern Tohoku area.