A Model for the 1989 Seismo-Volcanic Activity off Ito, Central Japan, Derived from Crustal Movement Data.

In July 1989, an extensive earthquake swarm took place off Ito City, eastern Izu Peninsula, central Japan. It was nearest to land and shallowest among the persistent swarms repeated in this region in the last 10 years. This activity eventually became a weak submarine eruption, the first of its kind in this region. Associated with this event, plenty of crustal movement data were obtained by various institutions. Based on these data combined with seismic data, we constructed a dislocation model to interpret the seismo-volcanic activity. The model consists of two tensile faults, each of which corresponds to magma intrusions in May and July, and a right-lateral reverse fault representing the largest seismic event (M5.5) among the swarm. An amount of opening was estimated to be about lm for the July event. From this model, we conclude that the collision of the shear fault with the tensile fault caused magma to come up into soft sedimentary layers at the seabottom, forming a sill and creating a knoll. Later, an instability brought about a magma-phreatic explosion which did not generate any substantial crustal deformations at deeper levels.