A Two-Dimensional Conductivity Model across Central Japan

Abstract

Horizontal profiles of geomagnetic transfer function were obtained at the periods of 15, 30, 60 and 120 minutes by analysing 3-component geomagnetic variation data at 21 sites in the central part of Japan and 7 sites on the Philippine Sea floor. A two-dimensional conductivity model was constructed, by use of the finite element method, to represent the spatial and period dependence of these transfer functions.Characteristics of the observed transfer functions on the Pacific coast and continental shelf of the Philippine Sea were well explained by introducing a highly conductive layer beneath the Philippine Sea plate at a depth of about 30km in addition to a large conductivity contrast between land and seawater. In order to account for the anomalous phase variation observed at the northern half of the central Japan sites, conductive lower crust was required beneath this region. This lower crustal conductor also explained the reduction of the coast effect on the Japan sea coast, and the existence of free water in the lower crust was considered as its likely cause. Our result did not require the conductive upper mantle beneath the Japan Sea, at least 200km away from the coast, to explain the data, which may suggest that the plate motion is the possible cause of highly conductive asthenosphere as well as the partial melting.