Depth of rainfall induced landslides revealed by DEM of difference analysis using airborne LiDAR data in igneous terrains

Landslide areas and depths were investigated using a digital elevation model (DEM) of difference (DoD) analysis based on pre- and post-rainfall airborne laser survey data. Three regions, consisting predominantly of granite or granodiorite bedrock, were selected as study sites: (A) the Noborikawa River Basin, (B) the Serizawa and Iriyamazawa Basins, and (C) the Uchikawa River Basin. Site C features gentler slopes than A and B. Although, assuming scale-invariant geometry, landslide depth is theoretically expected to depend on area, this was not observed, especially for shallow landslides. However, in all three regions, the landslide area was not a dominant determining factor for the depth of particularly shallow landslides. At site C, the landslide depth did not correlate with area, while at sites A and B it showed only a weak dependence. In both regions, gentle slopes showed a decreasing lower envelope of the landslide depth with increasing slope angle, while steep slopes showed a decreasing upper envelope. The results align with infinite slope theory, suggesting that landslide depth is controlled by mechanical equilibrium and maximum soil depth on steep slopes, rather than landslide area.