Positioning of Magma Reservoir at Unzen Volcano by GPS Survey

DOI Web Site 4 Citations 18 References
  • NISHI Kiyoshi
    Sakurajima Volcanological Observatory, Disaster Prevention Research Institute, Kyoto University,
  • ISHIHARA Kazuhiro
    Sakurajima Volcanological Observatory, Disaster Prevention Research Institute, Kyoto University,
  • KAMO Kosuke
  • ONO Hiroyasu
    Aso Volcanological Laboratory, Faculty of Science, Kyoto University
  • MORI Hitoshi
    Usu Volcano Observatory, Faculty of Science, Hokkaido University

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Other Title
  • GPS測量による雲仙火山のマグマ溜りの推定
  • GPS ソクリョウ ニヨル ウンゼン カザン ノ マグマ タマリ ノ スイテイ

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Description

Unzen volcano located at Shimabara Peninsula, Kyushu, Japan, has been erupted at the summit since November 1990 after 198 years dormant period. Phreatic eruption continued until middle of May 1991, and the appearance of a lava dome was confirmed on May 20, 1991. Since the end of May 1991, the lava dome has been growing, and pyroclastic flows have been frequently generated by collapses of the dome. Total volume of discharged lava as of February 1994 is estimated to be 1.86×10^8m3 by Geographical Survey Institute. GPS surveys have been repeated at several sites around the volcano since January 1991. Until the middle of 1991, the ground around the volcano indicated expansion and then turned into contraction. Applying a point source model to the data on the change in length of GPS baselines, the location of a pressure source is estimated to be approximately 5 km west from the summit and approximately 11 km in depth. Dilatation of the ground in the area which includes the pressure source region closely relates to changes in extrusion rate of lava. While the rate exceeded about 1×105 m3/day, the ground indicated deflation, and in contrast, the contractive movement came to stay when the rate decreased to less than the value. This suggests continuous supply of magma through the lower crust to the zone where the pressure source is located. These results are consistent with those derived from levelling survey. It is concluded that the location of the pressure source estimated here indicates that of the magma reservoir.

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