Source Mechanism of Low-frequency Seismic Events at Unzen Volcano, Kyushu, Japan : Volcanic Tremor on May 10 and Explosion Earthquake on June 11, 1991

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  • 1991年に観測された雲仙岳の長周期地震波動の震源過程 : 5月10日の微動及び6月11日の爆発地震
  • Source Mechanism of Low-frequency Seismic Events at Unzen Volcano,Kyushu,Japan--Volcanic Tremor on May 10 and Explosion Earthquake of June 11,1991
  • Source Mechanism of Low-frequency Seism

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Various types of low-frequency seismic events with predominant periods longer than 2 s have been observed at Unzen volcano since 1991. These events are, in chronological order, low-frequency tremors, A-type volcanic earthquakes, seismic events triggered by pyroclastic flows, and explosion earthquakes. In this study, the source processes of a volcanic tremor episode on May 10 and an explosive eruption on June 11 were investigated by waveform inversion. A vertical single force or a moment tensor were used as equivalent body forces for those events. To determine the appropriate source model for the events, we performed two kinds of inversions : the first is based on assumed source time functions, and the second estimates the shape of the source time functions. The tremor, which is composed mainly of Rayleigh waves, was better explained by a moment tensor representing contraction of the source than a vertical single force. The explosive eruption was also better explained by the moment source. The source time functions we derived indicate that the source first contracted and subsequently expanded about 3-4 s later. This functional dependence may be related to the mass flux of the explosion which culminated about 4 s after the event onset. The volume change of the source region of the explosion earthquake represents only about 1 % of that estimated from measurements of tilt associated with magmatic activity.

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