Wide-band (20,000 Hz to 0.002 Hz) magnetotelluric (MT) observations have been conducted since 1990 in the central part of the Tohoku district in the northeastern part of Japan to investigate the deep electrical resistivity structure. MT data were obtained on three east-west-trending MT transects about 140 km long and separated by about 20 km. In this paper we investigate three-dimensional galvanic distortions on the three transects using the Groom-Bailey tensor decomposition technique. We have focused on the regional strike direction and are not considering static shifts in the data. The decomposition method worked well at most sites but failed at some sites suggesting considerable regional three-dimensionality there. We found that the north-northwest to south-southeast direction is the predominant regional strike. This direction coincides with the regional geological strike being nearly perpendicular to the subduction direction of the Pacific plate. The induction vectors are generally in agreement with the regional strike direction deduced by the decomposition method on the southernmost line, but they deviate systematically on northernmost and middle lines suggesting conductivity contrasts in these areas.
Wide-band (0.002∼20,000 Hz) magnetotelluric measurements (MT) observations have been conducted along three traverses in the central Tohoku district of the northeastern Japan arc at 86 observation sites since 1990 in order to image the electrical resistivity structure. We used the impedance tensors fully corrected for the three-dimensional galvanic distortion effects including static shift effects in order to refine previous 2-D models (discussed in Fujinawa et al., 1997). The subsurface 3-D effects are found to be generally small with the result of a slight difference between the present model and the previously reported one.The modeling results indicate that the crust is homogeneous without an enhanced conductivity zone in the lower crust, in general agreement with results in the northern part of the Tohoku district. The refined resistivity profiles delineate more clearly two near-surface conductive anomalies located in the fracture zone between the Dewa Hill and the Central Basin Range, and in the zone between the Kitakami and Abukuma River regions. Conductors in the crust west of the Sekiryo Mountain Range generally correlate well with mapped faults or pre-Tertiary tectonic lines. Several buried faults are also suggested from the conductivity data. The electrical resistivity distribution and known active faults are integrated to better understand the seismo-tectonics and geologic regime associated with the subduction processes in the Japan arc region.
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