A Model of the Geoelectric Structure beneath Japan

Abstract

A quantitative evaluation of the geoelectric subterranean model beneath Japan is made on the basis of the analyses of short-period geomagnetic variations. For the model it has been surmised that the surface of a highly conducting mantle layer must steeply drop to a depth of 200km or so from a depth of 40-50km beneath the Pacific coast of central Japan. The depression seems likely to become shallower toward the Japan Sea underneath which the depth of the layer again reaches 40km.
As the above “depression model” was derived on the basis of a perfect conductor approximation, it is important to see whether the model can account for the frequency characteristics of geomagnetic variation as brought out by intensive observations in recent years.
The electromagnetic response of a conductive half-space having a triangular depression on its surface is estimated entirely numerically. Similar studies are also made for a case in which thin conductive sheets representing the sea are added. It turns out that the model seems likely to account for the characteristics of geomagnetic variations having a period range of a few tens of minutes. Although there is no guarantee for uniqueness, the depression model cannot be ruled out as a possible geoelectric structure beneath Japan.