← Back to Sessions

High Frequency (2+ Hz), 3D Wavefield Simulations of Large Earthquakes on the Southern Whidbey Island Fault, Washington State

Description: 

We directly simulate large magnitude crustal earthquakes (Mw7.0-7.5) on the southern Whidbey Island fault in Washington State to better constrain the possible extent and distribution of strong shaking in the Puget Sound region. Simulations employ a 3-D seismic velocity model and kinematic, finite-fault sources, and are run using a spectral element method code (SPECFEM3D) on a mesh with a 30m-sampled topographic surface. We also implement targeted updates to the seismic velocity model at shallow depths to improve accuracy of high-frequency shaking (i.e., modeled up to ~2.5 Hz). These include adding a region-specific, shallow (~100m-thick) soil velocity model and surface topography. Model quality is assessed by comparing results to empirical ground motion models (NGA-West2) and records from the 2001 Mw6.8 Nisqually earthquake. We test the sensitivity of ground motion estimates to a variety of source parameters, including hypocenter location, fault dip direction, and source location. We find that all Mw7.0 scenarios produce strong shaking in the city of Everett, Washington, and produce peak shaking greater >10 cm/s in the cities of Seattle, Bellevue, and Tacoma. For larger magnitude events, strong shaking can extend as far north as Victoria, British Columbia, and as far south as Olympia, Washington. In addition, the sedimentary structure of the nearby Everett Basin encourages the generation of high-amplitude, intermediate- and long-period surface waves. We also observe a strong dependence of long-period amplification in the Everett, Seattle, and Tacoma Basins on rupture directivity and source location. The inclusion of a shallow soil velocity model impacts site effects at periods of 1-2 s, with the greatest impact observed at sites with VS30 < 400 m/s. The results indicate that the potential strong shaking from large southern Whidbey Island fault earthquakes will be greatest in the central Puget Sound region, though the exact distribution and intensity of shaking will be highly dependent on local geology and earthquake source parameters, like hypocenter location and dip direction.

Session: 3D Wavefield Simulations: From Seismic Imaging to Ground Motion Modelling - I

Type: Oral

Date: 5/2/2024

Presentation Time: 08:00 AM (local time)

Presenting Author: Ian

Student Presenter: No

Invited Presentation: Yes

Authors