伊豆大島噴火活動期における火山性微動の挙動

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The 1986 eruptions of Izu-Oshima volcano were accompanied by volcanic tremors with large amplitudes. The tremors were observed by the 59-type seismometer with a proper period of 5 sec. at the Oshima Weather Station, situated at NNW 4.8 km distant from the central crater. The tremor occurrences were continuous or sometimes intermittent corresponding to eruptive durations. The amplitude variations and wave properties of the tremors were investigated by analyzing drum records of the seismometer. The amplitudes of the tremors did not always show a dependence on the discharge rates of magma, and increased from the morning of Nov. 17 when continuous lava fountain with rumbling sounds changed into that with explosive sounds. The tremors with lower frequency (ca. 0.5 Hz) were predominant from the evening of Nov. 17 to the morning of Nov. 18, and the tremors with higher frequency (ca. 0.8 and 1.0 Hz) were predominant since the morning of Nov. 18. As the result from particle motion diagrams, the tremors with lower frequency show the prevailing directions of vibration from NNE-SSW to NW-SE in horizontal plane, and seem to be composed of Rayleigh and SV types of wave motion. The wave frequencies of the tremors during the 1986 eruptions were lower than those during the 1950-51 eruptions. Released energy of the volcanic tremors during the 1986 eruptions were estimated to be 6×10^<19>erg. This is three times of energy of the volcanic tremors during the 1950-51 eruptions, though the eruptive duration of the 1986 eruptions were very shorter than that of 1950-51 eruptions.
特定非営利活動法人日本火山学会の論文
1988-06-30

伊豆大島噴火活動期における火山性微動の挙動

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