Some Results from Measurements of the Geomagnetic Field and the Electrical Resistivity in the Izu-Tokai Region, Japan

Abstract

Repeated surveys of the geomagnetic total intensity, which have been carried out in the northeastern part of the Izu Peninsula, disclosed anomalous secular changes in the total intensity. The spatial distribution of the anomalous changes is in good agreement with that expected from piezomagnetic fields associated with stress accumulation in the crust which is inferred from crustal uplift in the survey area. Measurements of geomagnetic variations of short period have also been carried out at a station located in the uplift region, in order to detect crustal resistivity changes by analyzing inductive response of the crust to short-period geomagnetic variations. The observed anomalous secular changes in the amplitudes of horizontal components of short-period variations were found to be well correlated with time-dependent nature of the crustal uplift, whereas preliminary results of transfer function analysis reveal no anomalous change in the vertical component. It is concluded, therefore, that the resistivity underwent a change in the crust near the station, presumably in the uplift area.

An attempt of measuring absolute values of the three components of the geomagnetic field has been made in the Tokai region, using a new magnetometer designed for tectonomagnetic observations. Although the most important objective, that is measurements of absolute values of the three components, has not been attained yet, the observation system is useful for studies of tectonomagnetic changes and crustal resistivity changes. In view of an experimental evidence for rock samples showing an unusually large resistivity change associated with strain, measurements of the ground resistivity using a sensitive resistivity variometer have been attempted at Omaezaki in the Tokai region. A preliminary result shows a resistivity variation seemingly correlated with ground tilt due to ocean-tide loading.