Region-scale, 3-D wave propagation simulations of M6.5+ earthquakes on the southern Whidbey Island fault, WA, considering surface topography and a soil velocity profile

Ian Stone, Erin A. Wirth, Alex Grant, & Arthur D. Frankel

Published September 10, 2023, SCEC Contribution #13185, 2023 SCEC Annual Meeting Poster #178

The southern Whidbey Island fault (SWIF) is a large crustal fault in Washington State capable of producing large (M6.5-7.2) earthquakes. Despite its proximity to the Seattle, Everett, and Bellevue metropolitan regions, past simulations of earthquakes on this fault have focused only on a small number of rupture scenarios, resulting in a limited understanding of the ground shaking potential from a large SWIF event. Here, we present directly simulated ground motions from SWIF earthquakes to expand on the set of previous simulations and better characterize regional seismic hazard. We conduct a suite of high-frequency (<=3 Hz), local-scale simulations, as well as lower frequency (~1 Hz), region-scale simulations of large magnitude earthquakes on the SWIF. Simulations are run in 3-D using a spectral element method code (SPECFEM3D) on a mesh with a 30m-sampled topographic surface. The seismic velocity structure is constrained using a 3-D velocity model considering local geology and implementing a region-specific soil velocity profile in the upper ~100m. We test a variety of kinematic source scenarios considering different hypocenter locations and slip distributions. Additionally, we compare simulated ground motions between different iterations of the model (topography/flat, local-scale/region-scale) and validate them against the empirically predicted peak ground motions from the 2014 NGA-West2 ground motion models (GMMs). These simulated ground motions allow us to characterize complex source, path, and site effects that could only be approximated using empirical GMMs, including basin effects, rupture directivity, and topographic amplification. The results from this work provide a framework for updated seismic hazard analysis and direct earthquake simulation in the Pacific Northwest.

Key Words
ground motion modeling, waveform simulation, high frequency, velocity model, seismic hazard, Washington, Whidbey Island Fault, Seattle Fault, soil velocity profile, geotechnical gradient, topography, site effects, basin amplification

Citation
Stone, I., Wirth, E. A., Grant, A., & Frankel, A. D. (2023, 09). Region-scale, 3-D wave propagation simulations of M6.5+ earthquakes on the southern Whidbey Island fault, WA, considering surface topography and a soil velocity profile. Poster Presentation at 2023 SCEC Annual Meeting.

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Ground Motions