Vertical Ground Deformation in the Summit Caldera Related to the 1986 Eruption of Oshima Volcano, Izu

Abstract

Repeated precise levelling has been carried out annually since August 1982, on the summit caldera area of Oshima volcano, Izu. From May 1986, levelling has been performed with a few months interval. In Novemver 1986, the summit as well as the flank-fissure eruptions occurred in Oshima volcano. The levelling in the summit caldera revealed some unusual relations between the vertical ground deformation and the eruption. Typical features of the summit caldera deformation are summarized as follows : 1. The inside of the caldera is continuously subsiding with respect to the outer somma of the caldera. The center of subsidence is situated at the central cone Mihara-yama. It is presumed that the subsidence is caused by compaction mainly due to the load of material which piled up in the past eruption. 2. Unlike the traditional idea of inflation-deflation process, the central cone did not inflate before the summit eruption. Following the summit eruption, subsidence was suddenly accelerated by the increasing load of newly erupted material. Subsidence rate was decreasing exponentially with time after the eruption. 3. At the margin of the caldera, a considerable amount of subsidence occurred during the eruption. Therefore, the summit caldera rim of Oshima volcano can be regarded as an active fault.