1983 年三宅島火山の噴火に関連した電気比抵抗変化

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Electrical resistivity measurements by means of ELF and VLF MT (magnetotelluric) technique were carried out repeatedly after the eruption of volcano Miyake-jima on Oct. 3, 1983. On the scoria cones newly formed on the south-eastern side of eruptive fissures, VLF surveys were performed along four survey lines. Resistivity changes were detected, which relate to the cooling process of scoria. Cooling rate at each site has a good correlation with the volume of the erupted scoria. The VLF survey, at the north-eastern tip of the fissure along a line across the north-eastward extension of its trend, revealed no remarkable low resistivity structure even 3 days after the eruption. This implies non-existence of high temperature mass at a shallower depth within 10 m or so. One of the purposes of ELF・VLF measurements is to detect changes in the resistivity structure associated with the eruption by comparing the 1980 (YUKUTAKE et al., 1982) survey and the present one. Significant changes were found in the resistivity structure at two observation sites ; one was situated about 400 m north-west from the fissure, and another a fumarole on the central caldera floor. The former can be interpreted as caused by destruction of ground water system, and the latter as a result of hot water supply from the deeper part. The apparent resistivity at the fumarole site is still decreasing even 7 months after the eruption, suggesting the expansion of the hydrothermal system possibly established after the eruption. This low resistivity structure seems to elongate south-eastward from the fumarole site, i.e. in the direction of the eruptive fissure. A phreatic layer does exist beneath the other part of the central caldera, although its resistivity is an order of magnitude higher than the prescribed hydrothermal aquifer.
特定非営利活動法人日本火山学会の論文
1984-12-28

1983 年三宅島火山の噴火に関連した電気比抵抗変化

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