Ground Tilt Changes Preceding the 1989 Submarine Eruption off Ito, Izu Peninsula

A submarine eruption took place about 3 km offshore of Ito City on July 13, 1989. Earthquake swarm activity started 2 weeks before the eruption. Associated with the swarm, ground tilt of more than 20 ad was detected at the Ito station, located about 5 km south of the eruption point. In late May, ground tilt of ∼ 3 μrad was also detected, associated with a small-scale swarm. The features of the observed tilt changes and the swarm events are presented.