Volcanic and Seismic Activities Observed around Iwate Volcano in 1998

Abstract

The relationship between volcanic and seismic activities observed around Iwate volcano in 1998 are discussed. The first subject is the relation between the seismic activity and the ground deformation, which were simultaneously observed in the period from February to August, 1998. The seismic activity is a characterized by westward extension of the focal region in the western part of the volcano. The ground deformation is explained by moving sources of tensile faults and pressure sources located near the west end of the focal region at each twomonth period considered. The synchronism of these phenomena is explained by the idea that both of the activities are two different aspects of volcanic activity caused by the westward migration of magma in the western part of the volcano. The idea is confirmed by the examination using the temporal change of the Coulomb failure function (ΔCFF). The second subject is the relation between the volcanic activity mentioned above and a moderate earthquake (M6.1) of Sept. 3, 1998, that took place at 10 km SW from the summit of the volcano. The ΔCFF calculated from the source model for the volcanic ground deformation shows no evidence that the volcanic activity triggered the earthquake. On the other hand, temporal changes in seismicity in the volcanic area are explained as the effects of the stress changes caused by the earthquake. Nevertheless, some relations between the seismic activities and the volcanic activity are explained by changes in static stress, but there remain other phenomena that were observed at the same time and whose relationships are not yet understood.