LAUSR

Abstract In rapidly uplifting regions, deep seated landslides are a prominent natural hazard that influence landscape evolution and are an important source of sediment. They pose a threat to infrastructure, and their failure can deliver sediment that overwhelms or blocks river channels. Consequently, a better understanding of why and where deep-seated landslides occur would help us reduce hillslope erosion, improve water quality, and allow us to better understand how landscapes evolve. Although the slope threshold model provides a framework for understanding how these landslides form, it has several shortcomings. Specifically, it fails to predict the spatial organization of landslides within a region, and it does not explain why landslide-prone portions of a region can exhibit relatively low slopes. To explore these issues, we have used a landslide susceptibility analysis within the Whanganui Basin of the North Island of New Zealand to explore the role that multiple susceptibility factors play in the occurrence of both translational and rotational deep-seated rock slides. Our findings contribute to a growing view of the relative importance of fluvial incision and the more subordinate role that earthquakes play in the development of large, deep-seated landslides. Additionally, our analyses identify the potential influence that widespread deforestation may have had on promoting instability, despite the deep-seated nature of the landslides studied. Most importantly however, we find that where slopes and weak geological structures align, a universal threshold slope does not exist, and many landslides develop on hillsides with below-average slope angles. This suggests that the presence of structural controls on landslide occurrence create a more complex landscape, where hillsides structurally predisposed to landslide occurrence have a much lower threshold slope angle.