2003 Volume 40 Issue 1 Pages 10-21
The 1997 Sumikawa landslide suggested that the pre-Yakeyama caldera covered by the younger Yakeyama volcano had played an essential role for the landslide to take place. Recently, about eighty calderas of late Miocene to late Pleistocene in age have been found along the Ohu Backborn Mountains in Tohoku district, northern Japan.
Our studies have revealed that those calderas are responsible for landslides of extremely large-scale in this district. The dissection stage of caldera volcanoes can be divided into three substages, 1) the post-caldera volcano edifice and exotic volcanics erosion substage, 2) caldera lacustrine-deposit erosion substage and 3) caldera-fill pyroclastic (ash-flow) deposit erosion substage. At the first substage, landslides medium to small in scale often observed on post-caldera volcanoes but sagging can be found in some of them. At the second substage, extremely large-scale landslides (-5km wide and-6km long) develop in caller deposits, because of their cap-rock structure composed of volcanic rocks and underlying lacustrine deposits. Combination of grabens and radial landslides of large-scale can be identified in some volcanoes. So many landslides of medium to small scale can be usually found on the surface of the lacustrine deposits that the topography of the lacustrine deposits sometimes becomes extremely irregular. At the third subtage, rockfalls, rockslide-avalanches and debris flows occur, sometimes causing severe disasters. In co-operation with hydrothermal activity play the lacustrine deposits an important role for occurrence of large-scale landslides especially.
