The USGS has produced a new series of earthquake hazard maps for the City of Seattle. These 'urban seismic hazard' maps provide a much higher-resolution view of the potential for strong earthquake shaking than previously available. This new view is particularly important for Seattle, which sits atop a sedimentary basin that strongly affects the patterns of earthquake ground shaking and therefore, of potential damage. These new hazards maps incorporate shaking effects not captured in the National Seismic Hazard Maps, which include:

• The subsurface geologic structure of the Seattle basin and its environs can amplify and lengthen the duration of strong shaking in some places. The seismic waves that shake the ground may be focused and diffused by the shape of materials within subsurface geologic structures.
• Surficial and shallow deposits of artificial fill and young alluvium (river deposits) may strongly amplify earthquake waves.
• The earthquake rupture process can also cause higher ground shaking in certain directions from a fault. A large earthquake grows like a propagating crack, radiating seismic waves along the way. This can lead to a pile-up of wave energy in front of the fault and spread it out behind.