箱根火山の東傾斜と丹那断層

Abstract

It is demonstrated that the eastern side of the Tanna fault has subsided relative to the western side using digital mesh data of geographic altitudes. The estimated subsidence is concordant with accumulated displacements of the Tanna fault and the fault slip at the 1930 Kita-Izu earthquake. Having noted that the eastern flank of Hakone volcano is steeper than the western flank, Suzuki (1971) concluded that Hakone volcano inclined to the east. This inclination of Hakone volcano is understandable if we note that it may be a manifestation of subsidence at the eastern side of the Tanna fault and its northern extension. We consider that subsidence at the eastern side of the Tanna fault represents a tectonic movement whereby the Manazuru block squared by the Tanna fault, Hirayama fault, Kannawa fault, and Kozu-Matsuda fault performs buoyant subduction. Based on the characteristics of crustal deformation at the Kita-Izu earthquake and aftershock distribution, it has been suggested that the western side of the Tanna fault moved south. Tectonically, this can be interpreted from the thesis that the western side of the Tanna fault, which had been dragged north by the subducting Manazuru block, rebounded at the Kita-Izu earthquake.