Some Phenomena before the Summit Eruptions at Sakura-zima Volcano

Abstract

In this report some phenomena observed in seismic events and ground deformation before the summit emptions which have been lasted since 1955 at Sakura-zima Volcano are introduced and discussed. The observational facts are the following: 1) The focus of seismic events migrate from deep to shallow part before eruptions, 2) the focus of summit explosion-quakes distribute beneath the active crater in depth 1 to 3km, 3) the focal mechanism occurred beneath the Volcano changes before and after eruptions, 4) the spectral structure of volcanic micro-tremores suggests that dimension and/or physical boundary conditions of the origin can change easily, 5) the ground deformation around the Volcano is closely related with the summit emptions and 6) the expansion rate of Aira caldera is evaluated as 10¹² cm³/year and the volume of ejected materials is estimated as 10¹³ cm³ in the active period, 1965-72, which corresponds to the volume of materials accumulated under the caldera in 10² years.