Perturbation of induced electric currents by surface conductivity inhomogeneity with special reference to anomalous behavior of short-period geomagnetic variations in the Kanto Plain, Japan.

Abstract

It has long been considered that anomalous behavior of short-period geomagnetic variations in the Kanto Plain (the Kanto anomaly) reflects a mantle conductivity anomaly and the sedimentary layer as thick as 3, 000m or more plays a rather minor role, only as local perturbation superposed on a regional anomaly. The complicated surface conductivity structure in this region, characterized by the sea existing to the east and the south and the thick sedimentary layer in the central part, has hampered quantitative discussion on the effects of seawater and sediments on short-period geomagnetic variation anomalies. It is shown, however, that such surface effects can be estimated approximately by considering two different polarization cases separately; one for a northwestward geomagnetic variation and the other for a northeastward geomagnetic variation. In the former case, the sea effect is striking as disclosed by numerical calculations of the peninsula effect. In the latter case, the so-called channeling effect due to the presence of the sedimentary layer seems to be a predominant factor. It is concluded that the Kanto anomaly is primarily accounted for by these surface effects, although no significant information on the crustal and mantle conductivity structure could be derived.