We tracked the chronological distribution of the active landslides geomorphologically defined by the interpretation of the aerial photographs taken from 1952 to 2007. We chose some landslide slopes and ranges where the reactivated landslides concentrated, based on the distributive characteristics of the active landslides for 55 years. In some of these areas, the landslide disasters have also occurred actually. Therefore, it can be estimated that these areas are under the conditions moved easily. These areas are shown as ”active area I“, and we can give information on the most-likely place to slide in a distribution map. Furthermore, although they are not landslide slopes, we also sampled places where many first-activated landslides are distributed. It is useful for the prevention of landslide disasters to rank these areas as slopes that must need appropriate observations of the transition in the future. We specify these as ”active area II“.
One ancient large-scale landslide, which is tentatively named as the Honki Slide, can be classified as a DZL (diagenetic zone landslide) occurred on a diagenetic zone hard shale of the Lower Miocene Tsubetsu Formation from the Honki landslide-prone area in the Non-Green Tuff region, Hokkaido, Japan, characterized by small amounts of authigenic interstratified illite/smectite minerals based on the bedrock geology. The Honki Slide is a reverse-dip slope type and can also be sorted as a weathered rockslide occurred on a moderate or strong weathering zone soft shale characterized by relatively large amounts of weathering smectite based on the landslide body. The occurrence of this DZL is closely related to the softening from hard shale to soft shale resulting from a decrease of bulk density, an increase of porosity, a decrease of uniaxial compressive strength and smectitization of chlorite by weathering, and the weathering zone nearly parallel to the surface of the slide body, as a whole, formed obliquely against the geologic structure.