Coupling of simulated directivity and basin effects during large earthquakes in Washington State’s Puget Sound region

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

Published September 8, 2024, SCEC Contribution #13997, 2024 SCEC Annual Meeting Poster #174

Sedimentary basins in the Puget Sound have shown a propensity for strong shaking amplification during large earthquakes. Using waveform simulations that account for 3-D basin structure, we can explore these effects, as well as their interactions with other source and path related processes. To this end, we use simulations of earthquakes on regional crustal faults to investigate the impact of rupture directivity on basin amplification of ground motions. Simulated M6.5+ earthquakes on the Southern Whidbey Island Fault (SWIF) generate much stronger long period (>4s) shaking in the Tacoma basin in scenarios with eastward rupture (forward directivity) than those with westward rupture (backward directivity). This is despite the fact that the Tacoma basin is not ideally located to receive a forward directivity pulse from the SWIF. We find that the contact between sediments and crystalline basement beneath the Puget Lowlands refracts long period surface waves to the southwest, preferentially channeling the directivity pulse through the region’s sedimentary basins. Further, comparison of the wavefield at basin and adjacent hard-rock sites reveals that peak shaking at periods of interest is controlled by surface waves of differing wavelengths between scenarios, accounting for the disagreement in estimated basin amplification. These results demonstrate the importance of accounting for 3-D seismic velocity structure when estimating region-specific directivity effects and basin amplification factors.

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

Citation
Stone, I., Moriarity, E. A., Grant, A., & Frankel, A. D. (2024, 09). Coupling of simulated directivity and basin effects during large earthquakes in Washington State’s Puget Sound region. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions (GM)