Forecasting liquefaction hazard from Seattle fault zone earthquakes: impacts on a major metropolitan area

The Seattle Fault Zone (SFZ), a 70-km-long thrust fault system that passes directly beneath Seattle, Washington, poses a significant seismic hazard to the Puget Sound region due to its proximity to dense urban infrastructure. While other coseismic hazards, such as tsunamis, landslides, and structural damage, have been forecasted for SFZ earthquakes and garnered considerable attention, soil liquefaction—despite its potential for widespread disruption—remains understudied. This study forecasts the spatial distribution of liquefaction and its potential impacts on civil infrastructure from three Mw7.2 SFZ earthquake scenarios. Leveraging an ensemble of 18 popular cone-penetration-test based liquefaction models at 268 newly compiled study sites, 14,472 individual high-quality liquefaction predictions are made across the Puget Sound region. Results reveal that severe liquefaction is expected in Seattle and nearby suburbs, particularly within 20 km of the fault along the Interstate-5 corridor, where fill and alluvial soils are prevalent. Comparisons to the 2001 Mw6.8 Nisqually earthquake and potential Mw9.0 Cascadia Subduction Zone ruptures indicate that full-length SFZ earthquakes are expected to produce more severe liquefaction effects near Seattle due to their closer proximity and greater shaking intensities at short-to-moderate periods. Additionally, liquefaction-induced damage to shallow foundations is predicted to be widespread in Seattle and its suburbs but more limited in areas farther from the fault. These findings provide insights for government agencies and stakeholders to better prepare for future SFZ ruptures.