Electrical conductivity anomalies beneath the Japan arc.

Abstract

As a result of intensive observations of geomagnetic variations over many years, the overall distribution of anomalous Z fields has become clear in Japan. One of the anomalies, the central Japan anomaly, has been accounted for by a depression of a highly conducting layer in the mantle beneath Japan, although the effect of the sea on the anomaly has not been brought out quite clearly. The period dependence of the anomalies along lines across central and northeastern Japan is examined by making use of the transfer function technique. Anomalous Z fields can be traced on a few islands in the Pacific Ocean south of central Japan, although they are strongly contaminated by island effects there. It is found out that the central Japan anomaly has a strong period dependence. The anomaly along the line across northeastern Japan also has a strong period dependence which is slightly different in its characteristics from that for the central Japan anomaly. Numerical calculations of electromagnetic induction based on the method given by JONES and PASCOE (1971), have been made for two-dimensional models, and the calculated transfer functions are compared with the observed ones. As a result, it is found out that the sea surrounding the Japan Islands plays an important role on both the anomalies. The anomaly in the central part of Japan, however, cannot be accounted for by the sea only. On the basis of these calculations, possible electrical conductivity models beneath the central and northeastern parts of Japan are put forward. It is concluded that a highly conducting layer seems likely to lie at a depth of 30km beneath the Philippine Sea and the Japan Sea. Anomalous Z fields associated with ssc's, geomagnetic bays and similar changes have been found on Oshima, Miyake-jima and Hachijo-jima Islands. Such an island effect is certainly caused by the induced electric currents which are distorted by the low conductivity of the island. However, the calculated effect does not agree with the observed one when electromagnetic coupling between the sea and the conducting layer in the mantle is ignored. A detailed study of electromagnetic coupling suggests that a highly conducting layer lies close to the earth's surface beneath the northern part of the Izu-Bonin arc.