Recent progress and future perspective of research on deep-seated gravitational slope deformation in Japan from the engineering geological viewpoint

Deep-seated gravitational slope deformation (DSGSD) creates characteristic geomorphological features, such as double (or multiple) ridges, uphill-facing scarps, downhill-facing scarps, and ridge-top depressions, which had been considered periglacial landforms until the 1970s. Research on such topographical features is important for landslide disaster mitigation, because they might develop into deep-seated, large-scale, and catastrophic landslides. This paper reviews the terminology, research history and perspective, as well as formation factors of DSGSD and related geomorphic features in Japan, mainly from the viewpoint of engineering geology.The first paper to clearly propose that geomorphological features such as linear depressions were formed by DSGSD was published in 1980 in a study of the Hida Mountains. Many subsequent examples of DSGSD-related topographies have been reported, not only in alpine zones above the timberline, but also under forest canopy on low mountains. Some authors have successfully estimated the formation ages of these topographical features using radiocarbon dating and tephrochronology. Recent development of LiDAR (Light Detection and Ranging or Laser Imaging Detection and Ranging) surveying has revealed that uncountable DSGSD topographic features are present under forest canopy in low-altitude, high-relief mountainous areas that are difficult to access. The distribution of DSGSD topographic features is not homogeneous: their development is closely related to geomorphological and geological factors such as the relief of mountains, the amount of Quaternary uplift, and the orientation of foliation planes in basement rocks. The formation of some DSGSD topographic features might be triggered by earthquakes. In the future, we need clear criteria to differentiate and classify DSGSD-related topographic features when assessing the risk of landslide disasters.