Research of Crustal Flow under Earthquake Swarm Area

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We make Observation of the crustal deformation by using GPS measurements and gravity changes at the Kozu-Niijima Islands. The earthquake swarms and remarkable ground deformation are observed in the region since 1990. The earthquake swarm happened at the Miyake Island on June 26, 2000 are shifted to the northwestern, and earthquakes more than M6 are observed at the swarm. We discuss the crustal movements observed at the Kozu, Niijima, Shikine and Miyake Islands, and the gravity changes at Kozu and Miyake Islands, and estimated the magma intrusion model in the region.A point pressure source with depth of 3-4 km under ground is estimated in the northeast coast off in the Kozu from the crustal deformation observed by GPS measurements. The same decrease pattern of the gravity changes are also observed by the precise gravity measurements. However the decrease rate of the gravity is not explained by the vertical ground deformation.A dike intrusion model with volume more than 1 km3 at Miyake-Kozu channel is estimated from the ground deformation observed at the Kozu, Shikine, Niijima and Miyake Islands, epicentral distribution and earthquake mechanism since June 26, 2000. The dike intrusion is just corresponded with the eruption at the Miyakejima Volcano. However the magma flow from the below of Miyake Volcano to the Miyaka-Kozu Channnel is not di stinguished.A point pressure source around the Kozu Island estimated from our research is also recognized from the seismic profile and the negative magnetism absolute in the area.