Estimation of regional resistivity structure beneath the Kyushu, southwestern Japan, as inferred from the Network-MT survey

Network-MT observations, which use telephone line networks as long baseline telluric measurements (Uyeshima, 1990), were carried out in the Kyushu district, southwestern Japan, from 1993 to 1998. The Kyushu district is the typical high angle subduction zone in Japan, which the Philippine sea plate subducts beneath the Eurasian plate, and some active volcanoes (for example, the Aso volcano, the Kirishima volcano group and Sakurajima volcano) are located along the volcanic front. We reanalyzed these data sets to determine regional scale deep electrical conductivity structure. In this reanalysis, we tried to choose triangular elements of the Network-MT again in order to obtain independency of each triangular element, and calculated magnetotelluric responses for each triangular element showing more suitable spatial distribution in survey area. Furthermore, comparing the geology and tectonics, we estimated electrical tendency to by the phase tensors analysis (Caldwell et al., 2004). As a preparatory step for imaging three-dimensional modeling, we carried out several tow dimensional inversion analyses to the Network-MT impedance responses across the characteristic geology, tectonics and volcanoes. In these two-dimensional inversions, we used the REBOCC code (Siripunvaraporn and Egbert, 1999), and reconsidered the horizontal and vertical smoothing factors while considering the intervals of the observation sites along each model profile. Then we obtained the final resistivity model of each profile which was judged expressing well the information of the MT responses. As a preliminary result, one of the resistivity models, whose profile goes along around the Kirishima volcano group, we obtained a remarkable conductor beneath the Kirishima volcano which shows a good agreement with the previous result of ULF MT survey (Ichiki et al., 2000). Further, we found that the bottom of this conductor extends to the subducting Philippine Sea Plate. However, at the present stage, we could not treat measured voltage differences for several kilometers electrode spacings in the inversions. For deducing detail electrical structure, we should make an improvement the inversion code for Network-MT. In this presentation, we would like to explain details of our reanalysis and obtained two-dimensional models, and introduce the future direction of this study.