Imaging the Seattle Basin to Improve Seismic Hazard Assessments

Imaging the Seattle Basin to Improve Seismic Hazard Assessments Shown are Rayleigh wave phase velocities for periods between 2 and 10 seconds.
Much of Seattle lies atop a deep sedimentary basin. The Seattle Basin amplifies and distorts the seismic waves from nearby moderate and large earthquakes in ways that modulate the hazard from earthquakes. Seismic hazard assessments heavily depend upon upper crustal and near-surface S-wave velocity models, which have traditionally been constructed from P-wave models using an empirical relationship between P-wave and S-wave velocity or by interpolating across widely spaced observations of shallow geologic structures. Improving the accuracy and resolution of basin S-wave models is key to improving seismic hazard assessments and predictions for ground shaking. Tomography, with short-period Rayleigh waves extracted using noise interferometry, can refine S-wave velocity 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 S-wave model for use in hazard assessment. Continuous data from the Seismic Hazards in Puget Sound (SHIPS) array as well as permanent stations from the Pacific Northwest Seismograph Network (PNSN) and Earthscope’s Transportable Array (TA) have inter-station distances are as short as a few kilometers. This allows us to extract Rayleigh waves between 2 and 10 seconds period that are sensitive to shallow basin structure. Our results show that shear wave velocities are about 25% lower in some regions in the upper 3 km than previous estimates. Using the new velocity model we run earthquake simulations using a finite difference code to validate the model, and then to make predictions on the level of ground-shaking for various realistic earthquake scenarios at various locations around the urban area.
</p><p>Acknowledgements: This study is funded by the National Earthquake Hazards Program (NEHRP).</p>


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