HR: 0800h
AN: G51A-0062    [Abstracts]
TI: The Pressure Sources Beneath Unzen Volcano Inferred From Geodesic Survey
AU: * Kohno, Y
EM: yuhki-k@sevo.kyushu-u.ac.jp
AF: Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, 2-5643-29 Shin'yama, Shimabara, 855-0843 Japan
AU: Matsushima, T
EM: mat@sevo.kyushu-u.ac.jp
AF: Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, 2-5643-29 Shin'yama, Shimabara, 855-0843 Japan
AU: Shimizu, H
EM: shimizu@sevo.kyushu-u.ac.jp
AF: Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, 2-5643-29 Shin'yama, Shimabara, 855-0843 Japan
AB: Unzen Volcano, which is located on Shimabara Peninsula, west Kyushu Island, Japan, erupted from 1990 to 1995. The ground deformations caused by volcanic activity were observed by several methods, such as Leveling survey, GPS and Tilt meters. In particular, it turned out from leveling data that the west coast area of Shimabara Peninsula sank about 8 cm since eruption had started. Joint Research Team of the national Universities suggested in 1992 that the model which has three pressure sources, could explain the ground deformation data in those days. But this model couldn_ft explain the leveling data which was observed after the eruption had stopped. In order to explain this latest leveling data, we had to add the fourth deeper pressure source beneath the Chidiwa Bay, confronted in the west seashore of the Shimabara peninsula ({\it Matsushima et al}., 2003; {\it Kohno et al}., 2003). In this study, we re-consider the source model beneath Unzen Volcano, using 1991-2001 and 2004 Leveling data along the northern flank of Unzen Volcano and the western coast of Shimabara peninsula. Also we use GPS data monitored by Kyushu University and Geographical Survey Institute. In calculation, both vertical and horizontal displacement was calculated applying the point source model ({\it e.g. Mogi}, 1958), and we get the best-fit source parameters. Parameters of the pressure sources are the location and the volume changes of pressure sources. Through the model calculation, the half-infinite surface was made for every height of each observation point, and geographical feature was reproduced in approximation. Analysis showed that after 1995 shallower source had started to deflate, on the other hand, two deeper sources still had kept expanding caused by intrusion of magma. After 1999, three shallower sources had begun to contract, and the only deepest (a depth of 15 km) source had expanded. But it is inferred from the 2004 Leveling that the deepest source turned to contract since 2001, and the all sources have started to contract.
DE: 8434 Magma migration
DE: 1709 Geodesy
DE: 1749 Volcanology, geochemistry, and petrology
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting