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Basin Shear-Wave Velocities beneath Seattle, Washington, from Noise-Correlation Rayleigh Waves

Tomography with short-period Rayleigh waves, extracted using noise interferometry, can refine S -wave velocity ( V S ) models in urban areas with dense arrays of short-period and broadband instruments. We apply this technique to the Seattle area to develop a new shallow V S model for use in seismic-hazard assessment. Continuous data from the Seismic Hazards in Puget Sound (SHIPS) array and local broadband stations have interstation distances of 90 km or less. This spacing allows us to extract Rayleigh waves with periods between 2 and 10 s that are sensitive to shallow-basin structure.

This new V S model for the Seattle basin is constructed using direct observations rather than using P -wave velocity ( V P ) observations and a V P / V S ratio as all previous 3D models at this scale have been constructed. Our results reveal greater detail in the upper 3.5 km than previous models. Earthquake simulations calculated using our new model better predict peak ground velocities (PGV) at periods between 1 and 2 s for two local earthquakes than the previous model used to calculate Seattle’s seismic-hazard map (Frankel et al. , 2007).

We collected data from two local earthquakes and ran finite-difference simulations using our new velocity model as well as the previous velocity model used in development of the Seattle seismic-hazard maps to assess how well our model predicts ground motions relative to the previous model. With a recent deployment of Netquakes strong-motion stations by the Pacific Northwest Seismic Network (PNSN) and the U.S. Geological Survey, we are now able to make more comprehensive assessments of the predictions for recent events.