BROAD SCALE PROBABILISTIC SCREENING FOR LANDSLIDE HAZARD IN WESTERN OREGON

We present a simple procedure to screen large areas for hazard due to earthquake-induced landsliding. This screening is based on broad based GIS data sets freely available to the public, but also has a framework that facilitates incorporation of more detailed data that may be available, or data collected for specific sites, or regions.

The landslide hazard model presented here contains two separate elements used to assess the potential for landsliding. The first element, susceptibility a site to landsliding, is calculated using a weighted combination of the following parameters: the slope of the ground surface, the presence or absence of an identified landslide, and the relative susceptibility to landsliding of the geologic unit. We calculate slope from the 10 meter USGS digital elevation models. We use the Statewide Landslide Information Database of Oregon (SLIDO) to test for the presence of an identified landslide. For certain regions, such as within a large part of the Oregon Coast Range, the SLIDO database is supplemented with local landslide maps or other assessments. We estimate geologic unit susceptibility based on spatial correlation between mapped geologic units and regional landslides.

The second element is an assessment of whether a landslide will initiate under seismic loading during an earthquake. We use probabilistic peak ground acceleration (PGA) for return periods of 475 and 2475 years predicted by the USGS in their 2008 National Seismic Hazard Maps as a basis for seismic loading.

The model is a large scale tool providing regional hazard assessment for both landslide susceptibility and probabilistic likelihood of landsliding failure during an earthquake. It is useful for developing relatively quick estimates of levels of landslide risk to lifelines, municipalities, and other large-scale developments. It also may be used for prioritization plans and planning for more thorough local assessments of landslide hazard, site-specific studies, and mitigation efforts.