伊豆大島1986年,1987年噴火に伴い愛知県刈谷市で観測されたインフラソニック波

抄録

Remarkable infrasonic waves from the eruptions of Izu-Oshima Volcano in 1986 and 1987 were recorded by a tripartite array of low frequency capacitor microphones installed in Kariya, 215 km west of the volcano. The signals from the 1986 eruptions were recorded intermittently from Nov. 17 through Nov. 23, despite somewhat unfavorable propagation conditions, the infrasonic rays being reflected only in the thermosphere. The dominant frequency of the observed waves was around 0.5 Hz until Nov. 19, but it was as low as 0.1 Hz by the evening of Nov. 21. Also, the signal amplitude became considerably larger on Nov. 21 and the strongest signals were recorded from the evening of Nov. 21 through 0100 JST on Nov. 22. These facts are in accordance with the change in the mode of eruption at the summit crater from the early stage to eject lava spring continuously to a highly explosive stage. Although intensive fissure eruptions took place simultaneously on the evening of Nov. 21, the observed strong signals are mostly attributable to the eruptions at the summit crater. The infrasonic waves from the 1987 eruption arrived as two distinct wave packets with duration of 1 to 2 minutes, and this fact indicates that the eruption was of a nature of single intensive explosion, rather than a continuous or repetitive ones as in the case of 1986 eruptions. The separation into two packets is explained by the difference in the reflection heights in the atmosphere, the former being in the upper stratosphere and the latter in the thermosphere. Judging from the signal amplitude, the strength of the 1987 explosion is estimated to be comparable to the most intensive one that took place in the series of 1986 eruptions.