Discovery, Controls, and Hazards of Widespread Deep‐Seated Gravitational Slope Deformation in the Etsumi Mountains, Central Japan

Abstract 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 theM7.5 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 landslides are not different slope processes but represent different stages of the same process. Our results highlight limitations 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.