抄録
An earthquake swarm started in the north-eastern part of the Izu Peninsula at the end of June 1989. The swarm was the most active during the period from July 4 to 10, during which the largest earthquake of M5.5 occurred. Then a sea-bottom eruption occurred east off the coast of Ito City on July 13. Magnetometer array observation in this region was intensified immediately after the sea-bottom eruption to monitor changes in thermal and/or stress state of the crust due to deep-seated magma.
During the period from October to December, 1989, remarkable changes in the total intensity were observed at several stations. Those magnetic changes happened to occur in the same period during when the distance between Ito and Hatsushima showed abrupt extension amounting to 15mm.
