| Budget Amount *help |
¥1,800,000 (Direct Cost: ¥1,800,000)
Fiscal Year 2004: ¥700,000 (Direct Cost: ¥700,000)
Fiscal Year 2003: ¥1,100,000 (Direct Cost: ¥1,100,000)
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| Research Abstract |
From June 26, 2000, an intensive earthquake swarm started under the Miyake-jima Island, 180km south off Japan. This swarm was closely related to the eruption of the Miyake-jima Island, probably dominated by the underground magmatic activity. The swarm spread toward northwestern ocean region from Miyake-jima Island, in which a huge number of earthquakes (over about 100,000) including five large events with M>6.0 were detected for about 2 months. This earthquake swarm was the most active one since we started the seismic observation in 1970's.
Although some telemetered observation stations exist on the Izu volcanic islands, no offshore instruments were operated in the area of this earthquake swarm. To understand both the spatial and temporal changes of this activity, we conducted a series of ocean bottom seismometer observations. According to the variation in the seismic activity with time, we changed the array configuration of OBSs six times. Furthermore, a real time seismic observation was undertaken using a buoy-telemetering OBS system.
Combining the OBS data with those of the island stations, very precise earthquake locations were determined. The epicenter distribution obtained strongly indicates a northwest-southeastern lineament. The dike intrusion was a vertical thin plane by the distribution of the earthquake. But the distribution was not uniform and a gathering of some long tablets or columns. They consist of some burst activities, of which the hypocenters migrated upward. In view of the migrated hypocenter, we investigate the spread of the dike and the migration of the magma.
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