TANAKA Satoru Tohoku University, Graduate School of Science, Research Associate, 大学院・理学研究科, 助手 (60281961)
HAMAGUCHI Hiroyuki Tohoku University, Graduate School of Science, Professor, 大学院・理学研究科, 教授 (20004385)
|Budget Amount *help
¥2,500,000 (Direct Cost : ¥2,500,000)
Fiscal Year 1998 : ¥900,000 (Direct Cost : ¥900,000)
Fiscal Year 1997 : ¥1,600,000 (Direct Cost : ¥1,600,000)
Volcanic tremor was first observed at Iwate volcano on Sep.15, 1995. Since then, the activities of volcanic tremor and volcanic earthquakes have been at a high level. To investigate the seismic activity, We installed a dense seismic observation network. Precise observation revealed the temporal changes in mode of activity and in hypocenter distribution. The seismic activity started in 1995 as volcanic tremor consisting of successive low-frequency earthquake about 10 km beneath the eastern flank. But, a series of severe activity of high-frequency earthquakes took plane at West Iwate volcanoes in 1998. The most distinctive feature of the activity in 1998 is that the seismic source region expanded westward in the period from March to July. Finally the source region was formed beneath a narrow area elongated about 10 km in E-W direction from the summit to the west end of West Iwate volcano.
Significant volcanic ground deformations accompanying the seismic activity were observed by GPS obser
vations and also by strain and tilt observations. The horizontal displacements observed by GPS were analyzed using an isotropic pressure source model. The migration of the pressure source (increase in pressure) from a part about 10 km beneath the eastern region of West Iwate volcanoes to a part a few km beneath the west end of West Iwate volcanoes was revealed, It is noteworthy that the pressure source is located nearly beneath the west end of the expanding seismic region. That is to say the westward expansion of the seismic source region and the westward migration of the pressure source took place simultaneously.
We interpret the phenomenon means that the both of seismic activities and ground deformations were caused by an intrusion and migration of some magmatic liquid from a deeper part near the summit to a western shallower part.