Two-dimensional modelling of resistivity structure beneath the Tohoku district, northern Honshu of Japan, by a finite element method.

Abstract

A finite element method was applied to study the resistivity structure below the Tohoku district, the northern Honshu of Japan. An east-west profile across the island axis was determined by two dimensional modelling. Mesh elements were designed so that the main features of geophysical structure so far known were smoothly incorporated into the model. Resistivities of the crust, the upper and lower, and the upper mantle were determined by solving an electromagnetic induction problem for E-polarization, where the electric field changes in parallel with the coast line and the magnetic field perpendicular to it.In this area, geomagnetic variations in the vertical component are coherent to those in the east component. Transfer functions were computed for the data acquired on land and on the ocean floor. Resistivity of this model was adjusted to explain the observed B-values of the transfer functions for the periods of 15 and 60 minutes. It has resulted in that large B-values observed on the Pacific side of the Tohoku district and on the continental slope are explained by a resistivity contrast between the land and the Pacific Ocean, but that the distribution of the B-values from the volcanic front near the central part of the area to the Japan Sea coast is difficult to interpret simply by a coast effect due to the Japan Sea. Low resistivity (5ohm·m) of the lower crust is necessary to consider extending from the volcanic front to below the Japan Sea coast.