Determining hillslope-scale material strength from seismically-triggered landslide events

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A non-technical description of the project, which explains the project's significance and importanceLandslides are a continual threat in many mountainous regions of the world and the danger is increased in areas prone to large earthquakes. In spite of this association, knowledge concerning the extent to which seismic events can trigger landslides is rudimentary. This project will examine data from the 1994 Northridge, California earthquake in order to evaluate how lithology, tectonics, rock strength, and fracture density influence the susceptibility of hillslopes to landslides. Data will be entered into a model that was developed as a pilot test for the 2008 Wenchuan earthquake in China. The more detailed and field-checked results from the California earthquake will permit refinement of the model and engender a wider applicability for hazard assessments in other areas. In particular, the data will be incorporated into a program of the U.S. Geological Survey, which is developing an automated system that produces estimates of impact and losses for earthquakes around the world for emergency response, government and aid agencies, and the media. Advances resulting from this project will aid in their implementation.A technical description of the projectNatural hillslope strength, which is a major controlling factor for landscape evolution and hazard assessment, is usually quite different than measurements of rock strength on hand-sized samples done in the laboratory. Laboratory tests are performed on intact rock, yet it is fracture density, aperture, and size that determine hillslope-scale rock strength. The strength of hillslopes can be quantified by using large seismic events in high relief settings, where an earthquake causes strong ground motion (peak ground acceleration) and a quantifiable landscape response (landsliding). This project assesses slope stability as a function of landslide thickness and shear-strength properties (cohesion and internal angle of friction) for a particular seismic event using modeled peak ground acceleration. The research focuses on landslides caused by the 1994 magnitude 6.7 Northridge earthquake, where it is hypothesized that tectonic deformation causes a strength reduction of hillslopes that can be isolated from lithologic and climatic factors. The project will integrate modeling and fieldwork to test this hypothesis and the results will improve knowledge of tectonic processes as a governor of rock weakening. The findings are relevant to earth surface processes, landscape evolution, tectonics, and natural-hazard assessment.