Detection of Deep-Seated Gravitational Slope Deformations by Aerial Photo Interpretation and Statistical Analysis in an Accretionary Complex in Japan

The objective of this study was to identify the locations of deep-seated gravitational slope deformations (DGSDs), which are precursors of future deep-seated landslides, and to characterize the numerical characteristics of these deformations in order to theoretically elucidate observed phenomena. To this end, we attempted to characterize quantitatively the topographic features of DGSDs, focusing on surface morphology. Signs of gravitational deformation that are frequently observed in pre-slide topographic maps include terminal cliff failures, irregular undulations, and gullies. The presence of these topographic features suggest that progressive deformation has occurred over a prolonged period of time. In order to track gravitational deformation over time, in this study, we interpreted aerial photographs of DGSDs that were associated with deep-seated landslides (DLs) on the Kii Peninsula that had been induced by Typhoon Talas, which occurred from 2 to 5 August 2011. We were able to observe the deformations that are considered to be caused by gravitational deformation over time. Numerical analysis using 1-m digital elevation models (DEM) revealed that the landslide areas exhibited characteristic eigenvalue ratios, slope angles, topographic wetness indices, and underground openness, demonstrating that characteristic morphologies exist in the areas that eventually experience landslides. One such morphological feature is the existence of a gentle sloping area in the upper section of the mass comprising a catchment basin, but without the corresponding valley and gully. The findings suggest that rainwater penetrates into the ground, degrading and deforming the rock within the landslide mass, and ultimately causes the slope to fail after torrential rainfall.