Invar-wire Extensometer Observation in Izu-Oshima Volcano

Abstract

Izu-Oshima Volcano erupted on Nov.15, 1986. Many open ground cracks were formed in a north-northwest trending broad zone across the island nearly concurrently with the fissure eruption which began at 16 : 15 Nov. 21. The formation of the open cracks indicates that extentional horizontal displacement in the NE-SW direction occurred near ground surface. After the eruption, we commenced the observation of ground surface deformation mainly across the cracks by many invar-wire extensometers. The extensometer is useful for rapid setting up. This paper reports the result of the extensometer observation until Nov. 1987. It is noted that remarkable ground surface deformation was observed from Nov. 18, 1987 and extentional step occurred on Nov. 21 in the southeastern part of the island. The ground surface deformation suggests that the activity of dike-form magma under the area is still continuing.