Simulation-based Scenario ShakeMaps for crustal fault earthquakes in western Washington State
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Description
Overview
This catalog contains ensemble ShakeMaps based on the 50th percentile (median) ground motions from earthquake rupture scenarios on crustal faults in western Washington State. These include magnitude 6.5-7.5 earthquakes on the Seattle, Tacoma, and southern Whidbey Island faults. Maps are based on the maximum horizontal shaking intensity between the two horizontal components. The development of these maps is described in Stone et al. (2026).
Crustal Earthquake Scenarios
The ensemble ShakeMaps were derived from sets of earthquake scenarios on individual crustal faults in western Washington. We present ShakeMaps for M6.5 and M7.0 rupture scenarios on the Seattle fault; M6.5 and M7.0 scenarios on Tacoma fault; and M7.0, M7.2, and M7.5 rupture scenarios on the southern Whidbey Island fault. For each fault and magnitude combination, shaking intensities were derived by taking the median ground motion values across suites of three to nine simulated earthquakes, each with a different hypocenter location and slip distribution. Because these ShakeMaps are constructed from multiple individual rupture scenarios, we refer to them as ‘ensemble ShakeMaps’ to distinguish them from ShakeMaps representing single rupture scenarios.
Ground Motion Simulations
Unlike typical ground motion model (GMM) based ShakeMaps, the maps presented here are based on wave propagation simulations, which directly model the influence of source complexity, 3-D velocity structure, and site conditions on shaking intensity. Stone et al. (2026) estimated ground motions in northwest Washington State using a kinematic source model and hybrid simulation approach. Low frequency (≤ 1 Hz) deterministic seismograms were calculated using a 3-D spectral finite element method that propagates seismic waves through a 3-D seismic velocity model for Cascadia (Wirth et al., 2025). High frequency (≥ 1 Hz) seismograms were generated using a stochastic approach. The stochastic seismograms were calculated considering the site- and region-specific soil velocity profiles of Grant et al. (2025) and the Vs30 map compilation of Heath et al. (2020).
Beyond northwest Washington, empirical GMMs were used to expand the shaking estimates to include areas likely to experience Modified Mercalli Intensity (MMI) 4 or stronger shaking, including eastern Washington, northwest Oregon, and southwest British Columbia.
Median Ensemble ShakeMaps
Rather than showing peak shaking intensities for a single scenario earthquake, these maps display the median shaking intensity across multiple rupture scenarios. As such, they represent the generalized shaking pattern for a given source and magnitude, similar to empirical GMM-based ShakeMaps. However, these maps differ from GMM-based ShakeMaps in that they directly model shaking from complex source, path, and site effects (e.g., body and surface wave radiation patterns, basin amplification).
We emphasize that the peak shaking intensity during any one scenario may be higher or lower at a given site due to variability in rupture parameters (e.g., hypocenter location, slip distribution). Data from individual rupture scenarios, including broadband waveforms and spectral accelerations, are available in an associated data release: https://doi.org/10.5066/P1LOPVED.
References
- Grant, A.R., Wirth, E.A., Stone, I.P., 2025, A soil velocity model for improved ground motion simulations in the U. S. Pacific Northwest: Seismica, v. 4, no. 2, 16 p., https://doi.org/10.26443/seismica.v4i2.1672.
- Heath, D., Wald, D. J., Worden, C. B., Thompson, E. M., and Scmocyk, G. (2020). A Global Hybrid VS30 Map with a Topographic-Slope-Based Default and Regional Map Insets”, Earthquake Spectra, v. 36, no. 3, 1570-1584. https://doi.org/10.1177/875529302091113
- Stone, I.P., Wirth, E.A., Grant, A.R., Grossman, J., Frankel, A.D., 2026, Simulation-based scenario ShakeMaps for large magnitude (Mw6.5+) crustal earthquakes on the Seattle, Tacoma, and southern Whidbey Island faults, Washington, USA: Article In Review at Bulletin of the Seismological Society of America (January 2026).
- Stone, I.P., Wirth, E.A., Grant, A.R., Grossman, J., Frankel, A.D., 2026, Simulation-based scenario ShakeMaps for large magnitude (Mw6.5+) crustal earthquakes on the Seattle, Tacoma, and southern Whidbey Island faults, Washington, USA: U.S. Geological Survey data release, https://doi.org/10.5066/P1LOPVED.