伊豆大島における重力変化 : 1985年11月〜1988年5月

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During the period from Nov. 1985 to May 1988, the Izu-Oshima Volcano erupted in Nov. 15-20, 1986 (called phase-I), Nov. 21, 1986 (phase-II), (Dec. 1986 (trivial)), Nov. 16, 1987, and Nov. 18, 1987. Phase-I was the summit eruption from the pit crater of Mt. Mihara and phase-II was the fissure eruption having the NW-SE strike. One year after the phase-II eruption, the Izu-Oshima Volcano erupted again, and the lava head of the pit crater fell down about 100 meters on the morning of Nov. 18, 1987. In this period, we carried out ten sets of precise gravity survey in Izu-Oshima Island using the LaCoste & Romberg Model D gravity meter No. 109. The measurements were made on the bench marks of the leveling route rounding the island which was established by the Geographical Survey Institute (GSI) in 1979 and on gravity points by the Earthquake Research Institute and GSI. We discuss the changes in gravity differences referred to the Oshima First Gravity Station (FGS). We observed the increase in gravity values which might be caused by the charge of the magma reservoir during the period from Nov. 1985 to Nov. 15, 1986 (The beginning of the phase-I eruption). We also observed the decrease in gravity values which was caused by the phase-II eruption. In this period, except for the north and southwest stations, the relation between elevation and gravity changes did not show Bouguer gradient nor free air gradient. This fact suggests the occurrence of mass defect under the ground with the phase-II eruption. After the end of phase-II eruption, the gravity values in Izu-Oshima Island decreased gradually, and increased again in November 18, 1987. We suppose that this step-like increase was caused by the drain back of lava from the pit crater to the magma reservoir. We estimated a center of gravity increase using a point source model. The estimated center is at northwest rim of the Izu-Oshima caldera, 0.5 kilometers below the sea level.
特定非営利活動法人日本火山学会の論文
1988-12-28

伊豆大島における重力変化 : 1985年11月〜1988年5月

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