Large earthquakes can cause devastating damage, particularly in regions where soft sediments amplify shaking. Many major cities around the world, including those in the Pacific Northwest of the United States, are built on sedimentary basins that can trap seismic waves and greatly increase both their intensity and duration of shaking. Over 12 million people live in this region, which has the potential to produce both a megathrust great earthquake (potentially M9+) and also smaller crustal earthquakes that occur closer to population centers. This project aims to improve our understanding of how local geological structures affect earthquake ground motion amplification in the Pacific Northwest, particularly in densely populated sedimentary basins. The researchers will develop new ways to constrain the subsurface structure of Cascadia forearc basins to provide better information that can guide estimates of ground motions from seismic hazards. All data, methods, and results will be openly shared to support the broader scientific community and regional velocity model-building efforts, including collaboration with the Cascadia Region Earthquake Science Center (CRESCENT). This study will develop and apply two complementary passive seismic techniques to characterize the shallow subsurface structure of the Cascadia forearc basin, where most of the population in the Pacific Northwest lives. The research team will use particle motion analysis from teleseismic earthquakes to help define