Preliminary Report on a Magnetotelluric Array Study in the Norhtwest Pacific

Abstract

In order to investigate the electrical conductivity structure of a subduction zone, and to obtain the vertical profile of the conductivity beneath an old oceanic plate near the subduction zone, we observed electric and magnetic variations on the sea floor of the Northwest Pacific ocean over two months in the summer of 1981. Magnetometers and electric field recorders were installed at six sites across the Japan trench. The outermost station is located about 600km from the coast of Japan and 450km from the trench axis.
Short period variations in the geomagnetic field are greatly attenuated on the deep sea floor outside the trench, while they are significant in the records on the continental slope. Clear correlation is seen between the variations in the vertical and the east component on the slope, indicating that the geomagnetic variations are severely affected by the coast effect. Unusually large magnitudes of Parkinson vectors that exceed unity were obtained with their directions normal to the trench axis.
A three layer structure was tentatively assumed for inversion of magnetotelluric soundings. The result suggests the existence of an electrically conducting layer at depths of 150-170km beneath the oceanic plate of about 125 my of age. In comparison with the existing conductivity profiles in the Pacific, this seems to support the hypothesis that the conducting layer becomes deeper with the plate age, reflecting the evolution of the oceanic lithosphere.