抄録
Crustal tilt movements were observed before and during the 1986 eruption of Izu-Oshima Volcano by four tiltmeters installed in Izu-Oshima Island. A borehole-type tiltmeter settled at Habu area (HB), southern part of the island, recorded NNE ground-up tilt change up to 1 μ radian from November 4, 1986. The tilt change turned the direction on November 12. A bubble-type tiltmeter settled at Kita-Gairin (KG), located at the north apron, and a borehole-type tiltmeter settled at Seibu-Gairin (SG), located at the northwest apron, recorded tilt movements up to 10-20μ radian between November 14 and 19. The directions of movements seem to indicate the subsidence along the eruptive fissure on November 21. The movement turned around on November 19. The tiltmeters at Habu, Seibu-Gairin and Kita-Gairin indicated a precursory tilt change about 2 hours before the eruption on November 21. A bubble-type tiltmeter settled at Weather station (WS), located at the northern part of the island, also indicated an anomalous change 1.5 hours before the eruption. These tilt changes indicate a possibility to predict fissure eruption by tilt observations. The crustal tilt movements observed by the tiltmeters at Seibu-Gairin and Weather station turned the direction twice associated with the eruption activities before ceasing of the eruption on November 21. In the middle of this eruption, anomalous WSW ground-down tilt movements up to 20μ radian were observed by the tiltmeter at Habu. The tilt direction was changed twice clockwise at a right angle. The tilt movements towards the northeast ground-down direction attained more than 100μ radian in ten days. This crustal movement is attributed to a buried tensile movement in the southern part of the island.
