鳶崩れ (飛越地震, 1858) による天然ダムの形成と決壊に伴う土砂移動の実態

抄録

Hietu Earthquake of M 7.0-7.1 induced a large landslide so called as Tombi landslide on Mt. Tombi in the up-stream area of the Jouganji River in central Japan, on April 9, 1858. This study examines all the available informationon the Tombi landslide, especially its volume and movement, in order to make the whole aspect as clear as possible.
According to old documents, two peaks called as Oh (large)-Tombi and Ko (small)-Tombi collapsed and the land-slide debris dammed up mountain valleys to form two large lakes. The landslide dams then failed on April 23 after 14days, and a mud or debris flow rushed down along the Jouganji valley into the Jouganji fan. A mud or debris flow oc-curred again on June 7 after 59 days. The Tombi landslide scarp is composed of massive andesite lava on granodioritebase, which the uppermost part is fragile, and this geological setting was apparently one of the important causativefactors for the slope failure with the shock of the earthquake. The fades of the landslide debris indicated that a rapiddebris avalanche occurred.
The volume of landslide debris was estimated as 1.27×10⁸m³ from the distribution of remaining debris in the valley.The landform of Oh-Tombi and Ko-Tombi before the landslide was supposed referring to old document and drawings.The volume of landslide mass was estimated by comparing the old landform with the present landform. The result ofvolume 1.14×10⁸m³ corresponds well with the estimated volume of depositions.