LAUSR

Deep-seated gravitational slope deformation (DSGSD) is a largely unnoticed but important long-term mass wasting process that may result in catastrophic failure of mountain slopes. Manifested by small topographic irregularities such as ridge-parallel scarps and linear depressions, it has been predominantly reported in alpine landscapes above timber lines. On the basis of area-wide high-resolution topographic data acquired by light detection and ranging (LiDAR) surveys, we here show that ~96 % of existing gravitational scarps have been hidden under forest canopies in the Etsumi Mountains, central Japan. The scarps are surprisingly widespread over the mountains with a mean line density of as large as 0.87 km/km2. Our analyses of the scarp distribution suggest that uphill-facing scarps are primary geomorphic signals of DSGSD with a destabilized rock mass larger than ~105 m2, whereas downhill-facing scarps principally occur in response to more localized slope deformation. In terms of controls, topography is by far the most influential factor in triggering and promoting DSGSD. Despite the M 7.5 earthquake in 1891, impact of large local earthquakes proves to be not very strong. Comparison with preexisting landslide maps further suggests that DSGSD and large-scale landslide are not different slope processes but represent different stages of the same process. Our results highlight limitation of aerial-photograph interpretation in forest-covered mountains and the need for LiDAR-assisted mapping for deeper understanding of this long-term process and interactions between surface and tectonic processes.