Abstract
The theoretical island effect relevant to Miyake-jima Island as calculated on the basis of electromagnetic induction in a thin sheet representing the sea does not agree with observations when the coupling effect between the sea and an underground conducting layer is ignored. It is shown that the electrical conductivity structure beneath Miyake-jima Island can be estimated by examining the coupling effect in detail.
It is concluded that a highly conducting layer having a conductivity of 10-12emu or thereabout must lie close to the earth's surface beneath Miyake-jima Island and that this layer is likely to be underlain by a poorly conducting layer, 10-14emu extending from 50km to about 400km in depth. The highly conducting layer at the top seems to have an important bearing on the thermal state at the northern part of the Izu-Bonin arc.
