Determining Hillslope-scale Material Strength from Seismically-triggered Landslide Events

Marin Clark, University of Michigan
Sean Gallen, University of Michigan

Natural hillslope strength or slope stability, relevant for landscape evolution and hazard assessment, falls far short of laboratory measurements of rock strength on hand-sized samples. This limitation stems from the fact that laboratory shear tests are performed on intact rock, yet it is fracture density, aperture and size that set the limit on hillslope-scale (10²-10³ m²) rock strength. In this study, we exploit large earthquakes in high relief settings to quantify hillslope strength because an earthquake imparts a measurable forcing (strong ground motion) and a quantifiable landscape response (landsliding). Here we apply an infinite-slope stability model developed by the hazard community from which we can assess slope stability given known topographic slope, as a function of landslide thickness and shear-strength properties (cohesion and internal angle of friction) for a particular seismic event given measured peak ground acceleration (PGA). Using the 2008 M7.9 Wenchuan earthquake in China as a test case, we demonstrate how PGA and observed landslides can be inverted to quantify hillslope-scape rock strength. Our preliminary results suggest that tectonic history and climate factors, rather than lithology, play a more fundamental role in rock strength at large spatial scales.