Constraining landslide frequency across the United States to inform county-level risk reduction

Abstract. Informative landslide hazard estimates are needed to support landslide mitigation strategies to reduce landslide risk across the United States. Whereas existing national-scale landslide susceptibility products assess where landslides are likely to occur, they do not address how often, which is a critical element of landslide hazard and risk assessments. In particular, the U.S. Federal Emergency Management Agency’s National Risk Index (NRI) requires landslide frequency estimates to inform expected annual loss estimates. In this study, we present county-level landslide frequency (landslides area^-1 y^-1) estimates for the 50 U.S. states. We applied Bayesian negative binomial regression to estimate both the expected (average) reported landslide frequency and full distribution of annual landslide counts for each county. We compared a suite of models that used combinations of landslide susceptible area, probability of potentially triggering earthquakes, frequency of potentially triggering precipitation, and ecological region as predictors. We trained our models with landslide inventory data from counties with the most comprehensive records available nationwide and used zero-inflated negative binomial distributions as an incompleteness model to correct for temporal reporting gaps. We selected a preferred model based on information criteria and physically plausible parameter estimates. Our preferred model showed that average annual reported landslide frequencies vary by five orders of magnitude across U.S. counties, ranging from 0.002 (0.00015–0.05) landslides 1000 km^-2y^-1 in Kusilvak Census Area, Alaska to 29 (19–46) landslides 1000 km^-2 y^-1 in Lake County, California, reflecting the country’s strong variations in landslide susceptibility, earthquake probability, and other factors for which ecological region serves as a proxy. Counties with estimated frequencies in the top 20 % of all counties are predominately along the West Coast of the continental United States, in mountainous regions of the Pacific Northwest and Intermountain West, in locally steep or earthquake prone regions of the Midwest and Southeast, along the Appalachians, in southern Alaska, and on some Hawaiian Islands. By examining the number of landslides predicted in 99^th percentile years for each county, we identified that 26 % of U.S. counties likely have potential for widespread landsliding with more than 10 landslides 1000 km^-2 y^-1, even when such large events have not been reported in the training data for that county. Overall, our results better represent the range of possible landslide frequencies and spatial variations than previous national-scale estimates reported in the NRI and can inform other risk reduction and loss mitigation efforts across the United States.