Geomagnetic Secular Variation Anomalies in Relation to the Recent Crustal Movement in the Southwestern Region of Japan

Abstract

The tectonomagnetism in the southwestern region of Japan has been studied through the analysis of the geomagnetic data of two surveys carried out with an interval of about 8 years. From over 100 sites, the local secular variations in the total magnetic field have been examined. A characteristic pattern of the arrangement of anomalies of the secular variation has been revealed; a distinction between zones of the relatively positive and relatively negative anomalies is fairly clear in almost all of the region surveyed. Especially, in the Chugoku-Kinki District it was noticed that the positive and negative anomalies were alternately arranged in the east-west direction in which the tectonic force has prevailed. The arrangement of the anomalies was also closely correlated to a feature of the recent crustal movement derived from analyses of the geodetic data. Some interpretations of these local anomalies have been made on the basis of the piezomagnetic effect. It can be concluded that the anomalies are probably caused by some inhomogeneous stress changes in the upper layer of the earth's crust and that these stress changes have temporarily been induced in the folded layer by the horizontal compression related to the tectonic forces which have effected the upper crust in the region since about one million years ago.