Local galvanic distortions in the central part of north-eastern Japan. (Part 1)

Abstract

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.