One ancient large-scale landslide, which is tentatively named as the Oshinkoshin Slide, and some more ancient and recent landslides within the Oshinkoshin landslide-prone area in a dominant hard shale-bearing formation from the Middle to Upper Miocene Onnebetsugawa Formation in the Green Tuff region can be classified as an HAZL (hydrothermal alteration zone landslide) based on the bedrock geology. These HAZLs are dip slope type and can also be sorted as large-scale hydrothermally altered rockslides such as the Oshinkoshin Slide, middle-scale debris and earth slides such as the 2006 Oshinkoshin Slide, small-scale debris and earth slides such as the 1999 Oshinkoshin Slide, etc. based on the landslide body. The occurrence of these HAZLs is closely related to the thin beds of large amounts of smectite-bearing soft tuff within the thick hard shale.
To clarify the characteristics of scale and occurring position of earthquake-induced landslides within pre-existing landslide topography, we investigate landslides induced by the Mid-Niigata Prefecture earthquake and pre-existing landslide topographies. The results show that the scale of earthquake-induced landslides tends to smaller than that of pre-existing landslide topography ; the scale of earthquake-induced landslides were likely to be influenced by basic geology and geological structure of the sites ; the landslides occurred inside pre-existing landslide topography tend to larger than that occurred outside of pre-existing landslide topography ; much of the earthquake-induced landslides occurred as partial area within pre-existing landslide topography, mostly at the lower part of pre-existing landslide topography, due to considerable influence by river erosion.