抄録
Volcanoes are mechanical as well as thermal anomalies in the Earth's crust. Because of their weakness, volcanoes may begin to deform earlier than the surrounding region in both volumetric and shear strain. A possible example of volumetric strain is the 1974 eruption of the Izu-Oshima volcano, which started two days earlier than the 1974 Izu-Hanto-Oki earthquake (M6.9). The location of the volcanoes and the earthquake, and sense of the faulting permit this interpretaion. Shear strain may also be concentrated in volcanoes, deforming them aseismically. The aseismic deformation of volcanoes may create stress concentration in the surrounding more brittle region, where small earthquakes will occur. They occasionally trigger a larger earthquake farther away from the volcano and are called foreshocks. This probably was the case for the foreshocks and main shock (M7.0) of the 1978 Izu-Oshima-Kinkai earthquake. Active faults having an active volcano at one end may be result of the recurrence of the above mechanism. Such examples include the Tanna and the Hirayama faults to the south and north of Hakone volcano and the Atotsugawa fault to the west of Tateyama volcano. Earthquake epicenters during the 1986 summit eruption of the Izu-Oshima volcano had a similar distribution to those during the period prior to the eruption, in that the summit region was rather aseismic and the epicenters were concentrated to the north and west of the summit. This pattern may be understood in the same way as the 1978 foreshocks, considering that the ambient stress field is of strike-slip type with the P axis in a NW-SE direction. Following the summit eruption, a fissure eruption took place on the flank of the Izu-Oshima volcano. The hypocenter distribution dramatically changed from the pattern during the summit eruption. Earthquakes are now distributed in a linear zone trending NW-SE, the same direction as the flank fissures and the P axis of the ambient stress field. Taking the vertical and horizontal deformation associated with the flank eruption into consideration, the linear erathquake distribution may be interpreted as indicating dike intrusions at depth.
