Relative Locations of the Eruption Vent(Teishi Knoll) and the Hypocenter of the Largest Earthquake (M5.5) in the 1989 Earthquake Swarm Activities in the Offshore Region to the Northeast of the Izu Peninsula

Earthquake swarm activities have occasionally occurred in the offshore region to the northeast of the Izu Peninsula since 1978. Anomalous crustal deformation has also continued around the northeast part of the Izu Peninsula since 1978. Earthquake swarm activities started on June 30 in 1989, and peaked on July 4. A volcanic tremor started on July 11, and a submarine eruption followed on July 13. The largest earthquake (M5.5) in the 1989 swarm activities occurred on July 9. It was the second largest earthquake among the swarm activities since 1978. Tensile fault models have been proposed to interpret the ground deformation and the earthquake swarm activities in 1989. It has been estimated that magma intrusion had been most active from July 4 till July 11. The largest earthquake took place when magma was intruding, and it was located near the site of the submarine eruption. We checked up on the location of the earthquake to estimate the effect of the intrusion on earthquake initiation. We compared observed strong motion records with synthetics which were computed for various starting points of rupture oh the assumed fault, and checked the initial motion polarities observed with seismographs around the inferred earthquake fault. It was estimated that the epicenter of the earthquake was apart from the vent of the eruption, and that the fracture started at the bottom of the fault.