Trends in Lithology Revealed through Three-Dimensional Mapping of Rupture Surface Geometry and Shear Strength Properties using Landslide Inventories

Abstract:

Landslide inventories are a key source of information towards evaluating hazard and activity at a regional scale. These inventories have also produced critical datasets for understanding how landscapes respond to disturbance and insight towards geologic controls on landslide activity, particularly with advance of new mapping and remote sensing techniques. However, these datasets still only “scrape the surface” – that is, they do not always yield subsurface information critical to understanding landslide processes or the material properties associated with their activity. Using high-quality landslide inventories and high-resolution topographic data, we establish a process by which we map (1) the estimated three-dimensional geometry of inventoried landslide rupture surfaces and (2) their associated shear strength properties through three-dimensional back-analyses. Performed on over 800 landslides at three sites of diverse geologic conditions in Oregon, both the distribution in mapped rupture surfaces (e.g. volumes, geometry, mechanism) and associated shear strength properties reveal different trends in the range of lithologies. Smaller, but steeper rotational slides exhibit higher shear strengths that are associated with potential presence of cementation, while larger compound features and earthflows exhibit lower shear strengths that reflect a residual, sheared state. While difficult to isolate causation and correlation, these trends in observed shear strength properties may indirectly reflect the controls on observed landslide volumes. Finally, the proposed workflow serves as a technique to expand the utility of landslide inventories as a means of isolating mechanical parameters critical towards understanding a range of earth surface processes.