Departmental Bulletin Paper 続発する日本の水蒸気噴火
Successive Occurrence of Phreatic Eruptions in Japan

井口, 正人  ,  中道, 治久

Eruptions occurred at the Kuchinoerabujima and Ontake volcano on August 3, and September 27, 2014, respectively. In addition, increase in volcanic activity, such as seismicity of volcanic earthquakes and inflation ground deformation are reported at many volcanoes in Japan. The eruptions at the Kuchinoerabujima and Ontake volcanoes are mostly phreatic types, which ejected fine ash and fragments generating pyroclastic flows. Alert levels were raised to 3 from 1 (normal level) by JMA, after occurrence of the eruptions; however significant precursory phenomena were detected in long-term and short-term immediately before. Multi-parameter precursors, such as increase in seismicity and gradual inflation of ground, heat supply rate and discharge rate of volcanic gas were reported at the Kuchinoerabujima volcano. Inflation of the ground were detected by GNSS at the late of 2006 at the Ontake volcano. Upward tilt changes of the crater sides were accelerated quite shortly at Kuchinoerabujima (1 hour before) and Ontake (10 min.) volcanoes. No transient phenomenon was recognized at Kuchinoerabujima, from long-term to short-term precursory periods, however increase in volcano-tectonic earthquakes and following low-frequency earthquakes are drastic as the transient at the Ontake volcano.

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