Memoirs of the Kakioka Magnetic Observatory Vol.15 No.01， p.1, December, 1972
Secular Variation of the Electrical Conductivity Anomaly in the Central Part of Japan
Vectors of geomagnetic short period variation are confined in a plane at a given station generally by a conductivity anomaly within the earth. Secular variation of the inclination of the plane deduced from geomagnetic variations observed at Tokyo and Kakioka is given for the period from 1897 to 1970. The range of the variation amounts to more than 30% of the present value. The smallest inclination coincided with the occurrence of the great Kanto earthquake in 1923. Initially the inclination was decreasing, and it increased rapidly after 1923, coming to the maximum around 1940. Since then it has decreased gradually. It suggests a cyclic change with a period of 70-100 years which may be explained by a tectonic motion of the floor of the Pacific Ocean.