Anomaly of Geomagnetic Variations in Japan

The anomalous features found in geomagnetic variations of various kinds, rapid and slow, are described on the basis of observations at well-distributed observatories in Japan. Since the anomalies are proved to be caused by the induced magnetic field arising from inside the Earth, the underground conditions responsible for the anomalies have been intensively examined. At the moment, the best way to account for the anomaly would be to assume that a conducting circuit, which comes up from the deep, forms a loop, about 1 000 km in length and 200 km in width, beneath Japan. It is also supposed that a wedge-shape penetration of the upper layer of the mantle must exist there. The depth of the bottom of the penetrating part is estimated at 700 km. These complicated structures seem to have an important bearing on the geophysical interpretation of the relationship between occurrence of deep-focus earthquakes, orogenic activity, trend of volcanic zones and so on.