DYKE INTRUSION MODEL FOR THE 1989 SEISMOVOLCANIC ACTIVITY OFF ITO, CENTRAL JAPAN

In the last 10 years, extensive earthquake swarms have occurred intermittently in the region east of Izu Peninsula, central Japan. In July 1989 an active swarm again took place off Ito City, and among all past activities, it was nearest to land and shallowest. This activity eventually became a weak submarine eruption, the first of its kind in this region. Associated with this event, a borehole tiltmeter at 92 m depth, installed several kilometers from the source area, detected distinct continuous tilt change with the following features: (1) Tilt of 20-mu-rad was observed within 1 week preceding the eruption. (2) Tilt change showed a good correlation to swarm activity. (3) No obvious coeruption tilt change was observed. On the basis of the tilt record and swarm distribution, we constructed a dislocation model to interpret the seismovolcanic 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 earthquake (M5.5) at 1109 LT on July 9. This model can simulate not only the above tilt data but also the record of a nearby dilatometer, the results from daily laser distance measurements and daily Global Positioning System monitoring, as well as results from other conventional distance surveys and leveling surveys in the surrounding region. From this model, we conclude that the intersection of the shear fault with the tensile fault caused magma to come up into soft sedimentary layers at the seabottom, creating a lava dome. Later an instability brought about a magma-phreatic explosion which did not generate any substantial crustal deformations at deeper levels.