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

Informative landslide hazard estimates are needed to support landslide mitigation strategies to reduce landslide risk across the United States (U.S.). While 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 by county, which are U.S. administrative regions ranging from 120 km² to 377,055 km² in size, 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) landslide frequency and full distribution of annual landslide counts for each county as a function of landslide susceptible area, frequency of potentially triggering precipitation, and propensity for triggering earthquakes. We trained our model with 62,720 reported landslides from 316 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 found that average annual landslide frequencies vary by nearly three orders of magnitude across U.S. counties, ranging from 0.05 (0.04–0.07) landslides 1000 km^-2y^-1 in Midland County, Texas to 31 (21–43) landslides 1000 km^-2y^-1 in Lake County, California and reflecting the country’s strong variations in landslide susceptibility, earthquake probability, and precipitation frequency. 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 South, along the Appalachians, in southern Alaska, and on the big island of Hawaii. By examining the number of landslides predicted in 99^th percentile years for each county, we identified that 31% of U.S. counties have potential for widespread landsliding, 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 across the entire United States than previous national-scale estimates reported in the NRI and can inform other risk reduction and loss mitigation efforts across the United States.