伊豆大島の火山活動(1986-1987 年)に伴う傾斜変動 : 御神火及び波浮における地殻傾斜変動連続観測結果

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During the period from Nov., 1986 to Nov., 1987, the Izu-Oshima volcano erupted five times. The National Research Center for Disaster Prevention had been conducting continuous crustal tilt observations using the borehole-type tiltmeter. These observations were being made at Gojinka and Habu, located at the northwest caldera rim and the southeast part of the island, respectively. Tilt changes associated with a train of eruptions were observed at both stations and results were as follows. 1) Precursory changes just before the eruption were observed at Gojinka four times, except for the eruption which occurred on Nov.15, 1986. Also at Habu precursory changes just before the eruption were observed prior to the eruptions on Nov.21, 1986 and Nov.18, 1987. 2) At both stations, the directions of these precursory changes were mostly ground-up, towards the north-northwest. On the other hand, the direction of changes observed after the eruptions were ground-down, again towards the north-northwest. These tilt changes before and after the eruptions indicate an inflation-deflation process in north-northwest direction. 3) Utilizing these tilt data and the volumetric strain data we have made an estimation on the location of the pressure source based on the Mogi's model. It was determined that the pressure source was located approximately at a depth of about 2 kilometers, less than 1 kilometer away from Gojinka towards north-northwest. 4) Most of the precursory changes are attributed to the inflation-deflation process of the magma reservoir, which we estimated above. However, direction of the precursory change about two hours prior to the fissure eruption on Nov.21, which was observed at Gojinka, was not north-northwest but east-northeast (towards the fissure). We presumed that this change was caused by the magma intruding on to the surface of the earth. 5) From the results of earthquake observations and precise levelling, it has been conjectured that after the eruptions on Nov.21, the magma intruded in the southeastern direction of the island (YAMAOKA, 1987 ; TADA and HASIMOTO, 1987). The tilt changes at Habu corresponded to the intrusion. From the results of continuous tilt observations, it can be conjectured that the greater part of the intrusion took place by Nov.28.
特定非営利活動法人日本火山学会の論文
1988-06-30

伊豆大島の火山活動(1986-1987 年)に伴う傾斜変動 : 御神火及び波浮における地殻傾斜変動連続観測結果

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