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Preliminary report on regional resistivity variation inferred from the Network MT investigation in the Shikoku district, southwestern Japan


The Network MT method was used in the eastern part of the Shikoku district, southwestern Japan, and a total of thirty-nine MT impedances (64 to 2560 sec) were obtained. These MT impedances had their values averaged over a triangular element, whose sides were a few kilometers long with geomagnetic observatory data from the Kakioka Geomagnetic Observatory. Well-determined MT impedances varied from north to south with the greatest differences being at the Median Tectonic Line, which is consistent with the surface geology in the area. In addition, very large or very small values of apparent resistivity were obtained in some triangular elements. These triangles were located on a cape or near an estuary, with effects of three-dimensionality clearly apparent. Stable MT impedances were not obtained for three groups of triangular elements: (1) those where one or two sides of the triangular element cross the coast; (2) those where the electric field was contaminated by severe artificial noise, these were mainly in the northwestern part of the survey area; (3) those where the triangles had an extremely acute- or obtuse-angle.

A resistivity cross section was derived from the TM-mode data for a profile crossing the eastern part of the area. The shallower layer, which approximately corresponds to the crust, was divided into three blocks. Two resistive boundaries coincide with the geological tectonic lines and the strong horizontal contrast found at the Median Tectonic Line. The northernmost block is the most resistive, and the block to the south is the most conductive. Beneath these blocks, the subducting Philippine Sea plate was represented by a thick and highly resistive north-dipping layer. A highly conductive thin layer was found above the resistive layer on the southern side of the Median Tectonic Line. This layer is only found beneath the southern side of the Median Tectonic Line and is probably caused by pore water and/or sediment at the upper plane of the subducting Philippine Sea plate. Another conductive layer was found under the highly resistive north-dipping layer.

The resistivity structure from the lower crust to the upper mantle is firstly obtained using the Network-MT method. However, further developments are needed in methods of data analysis, which are robust to artificial electric noise, in order to clarify the spatial distribution of MT impedances in the complete study area.


  • Bendat, J. S. and A. G. Piersol, Random Data: Analysis and Measurement Procedures, 407 pp., John Wiley and Sons, Inc., New York, 1971.

    Google Scholar 

  • Filloux, J. H., Magnetotelluric sounding over the northeast Pacific may reveal spatial dependence and conductance of the asthenosphere, Earth Planet. Sci. Lett., 46, 244–252, 1980.

    Article  Google Scholar 

  • Filloux, J. H., Magnetotelluric exploration of the North Pacific: progress report and preliminary soundings near a spreading ridge, Phys. Earth Plant. Inter., 25, 187–195, 1981.

    Article  Google Scholar 

  • Fujita, K., Y. Ogawa, S. Yamaguchi, and K. Yaskawa, Magnetotelluric imaging of the SW Japan forearc-a lost paleoland revealed?, Phys. Earth Planet. Inter., 102, 231–238, 1997.

    Article  Google Scholar 

  • Furukawa, Y., Temperature structure in the crust of the Japan arc and the thermal effect of subduction, in Terrestrial Heat Flow and Geothermal Energy in Asia, edited by M. L. Gupta and M. Yamano, pp. 203–219, Oxford & IBH Publishing Co. Pvt. Ltd., New Delhi, 1995.

    Google Scholar 

  • Goto, T., S. Yamaguchi, N. Sumitomo, and K. Yaskawa, The electrical structure across the Median Tectonic Line in east Shikoku, southwest Japan, Earth Planet. Sci., 50, 405–415, 1998.

    Google Scholar 

  • Hada, S., T. Suzuki, K. Okano, and S. Kimura, Crustal section based on the geological and geophysical features in the Outer Zone of southwest Japan, Mem. Geol. Soc. Japan, 21, 197–211, 1982.

    Google Scholar 

  • Hilde, T. W. C., Sediment subduction versus accretion around the Pacific, Tectonophysics, 99, 381–397, 1983.

    Article  Google Scholar 

  • Hyndman, R. D., Dipping seismic reflectors, electrically conductive zones, and trapping water in the crust over a subducting plate, J. Geophys. Res., 93, 13391–13405, 1988.

    Article  Google Scholar 

  • Jones, A. G., Electrical conductivity of the continental lower crust, in Continental Lower Crust, edited by D. M. Fountain, R. Arculus, and R. W. Kay, pp. 81–143, Elsevier, Amsterdam, 1992.

    Google Scholar 

  • Karato, S., The role of hydrogen in the electrical conductivity of the upper mantle, Nature, 347, 272–273, 1990.

    Article  Google Scholar 

  • Kimura, S. and K. Okano, A seismological examination of the Median Tectonic Line and its surrounding area in Shikoku, Southwest Japan, Mem. Geol. Soc. Japan, 40, 187–195, 1992.

    Google Scholar 

  • Kurtz, R. D., J. M. DeLaurie, and J. C. Gupta, A magnetotelluric sounding across Vancouver Island detects the subducting Juan de Fuca plate, Nature, 321, 596–599, 1986.

    Article  Google Scholar 

  • Ogawa, Y., Two-dimensional resistivity modeling based on regional magne-totelluric survey in the northern Tohoku district, J. Geomag. Geoelectr., 39, 349–366, 1987.

    Article  Google Scholar 

  • Ogawa, Y., T. Yukutake, and H. Utada, Two-dimensional modeling of resistivity structure beneath the Tohoku district, northern Honshu of Japan, by a finite element method, J. Geomag. Geoelectr., 38, 45–79, 1986.

    Article  Google Scholar 

  • Oldenburg, D. W., Conductivity structure of oceanic upper mantle beneath the Pacific plate, Geophys. J. R. astr. Soc., 65, 359–394, 1981.

    Article  Google Scholar 

  • Poll, H. E., J. T. Weaver, and A. G. Jones, Calculations of voltages for magnetotelluric modeling of a region with near-surface inhomogeneities, Phys. Earth Planet. Inter., 53, 287–297, 1989.

    Article  Google Scholar 

  • Saito, M., Automatic design of recursive digital filters, Geoexploration, 31, 240–263, 1978.

    Google Scholar 

  • Shankland, T. J. and H. S. Waff, Partial melting and electrical conductivity anomalies in the Upper Mantle, J. Geophys. Res., 82, 5409–5417, 1977.

    Article  Google Scholar 

  • Shiozaki, I., Two-dimensional resistivity structure beneath Chugoku and Shikoku districts in Southwestern Honshu, Japan, Ph.D. Thesis, Kobe University, 276 pp., 1993 (in Japanese).

  • Sugimura, A., Chikyu-Kagaku, vol. 10, edited by K. Kasahara and A. Sugimura, pp. 159–181, Iwanamisyoten, Tokyo, 1978.

  • Suyari, K., M. Iwasaki, and T. Suzuki, Regional Geology of Japan, Part 8, SHIKOKU, edited by Editorial Committee of SHIKOKU, Part 8 of Regional Geology of Japan, Kyoritsu Shuppan, Tokyo, 1991.

  • Swift, C. M., Jr., A magnetotelluric investigation of an electrical conductivity anomaly in the southwestern United State, in Magnetotelluric Methods, Geophys. Reprint Series, vol. 5, edited by K. Vozoff, pp. 156–166, Soc. Explor. Geophys., Tusla, Okla., 1986 (extract from a thesis submitted to Dep. Geol. Geophys., M. I. T., 1967).

  • Taira, A., H. Tokuyama, and W. Soh, Accretion tectonic and evolution of Japan, in The Evolution of the Pacific Ocean Margins, edited by Z. Ben-Avraham, pp. 100–123, Oxford Univ. Press, New York, 1989.

    Google Scholar 

  • Uchida, T. and Y. Ogawa, Development of Fortran code for two-dimensional magnetotelluric inversion with smoothness constraint, Geological Survey of Japan Open-File Report, No. 205, 115 pp., 1993.

  • Utada, H., Y. Hamano, and J. Segawa, Conductivity anomaly around the Japanese Islands, in Geology and Geophysics of the Japan Sea (Japan-USSR Monograph Series, Vol. 1), edited by N. Isezaki, I. I. Bersenev, K. Tamaki, B. Ya. Karp, and E. P. Lelikov, pp. 103–149, Terra Sci. Pub., Tokyo, 1996.

    Google Scholar 

  • Uyeshima, M., Application of Network MT method to the study of electrical conductivity structure in the central and eastern part of Hokkaido, Dr. Sc. Thesis, Univ. Tokyo, 235 pp., 1990.

  • Wannamaker, P. E., J. R. Booker, A. G. Jones, A. D. Chave, J. H. Filloux, H. S. Waff, and L. K. Law, Resistivity cross section through the Juan de Fuca subduction system and its tectonic implications, J. Geophys. Res., 94, 14127–14144, 1989.

    Article  Google Scholar 

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Correspondence to Satoru Yamaguchi.

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Yamaguchi, S., Kobayashi, Y., Oshiman, N. et al. Preliminary report on regional resistivity variation inferred from the Network MT investigation in the Shikoku district, southwestern Japan. Earth Planet Sp 51, 193–203 (1999).

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