Regional secular change in the geomagnetic field in the Oshima Island area during a tectonically active period.

Abstract

We have examined secular changes in the geomagnetic total intensity at four observatories distributed almost along the meridian 140°E from 1968 to 1986, and found that the total intensity at a site (NOM) in Oshima Island was subjected to an anomalous decrease until 1978 and recovered to the normal trend around 1980. The rate of the anomalous decrease has been estimated to be -1.3nT/year. Additional observations started in 1976 and 1978 at the south and the north of the volcano show similar secular changes. This seems to suggest that the anomalous behavior observed at NOM is not a local phenomenon in Oshima Island but a regional one occurring simultaneously over the entire island, except for a site very close to the central cone of the volcano.The time when the anomalous secular change returned to the normal variation coincides with the period when seismicity and crustal deformation such as shortening of the distance between Oshima Island and the Izu Peninsula changed its behavior. This implies that there was a change in the stress distribution over a large area between 1978 and 1980, which is supposed to have caused the anomalous secular change in Oshima Island. This regional stress change seems to have eventually led the volcano to erupt in 1986.