M9 Cascadia Earthquake Scenarios
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Description
Overview
This catalog contains “ensemble ShakeMaps” based on the 2nd, 16th, 50th (median), 84th, and 98th percentile ground motions from a set of thirty magnitude 9 (M9) Cascadia earthquake rupture scenarios. All maps are based on the geometric mean of two horizontal components. These ensemble ShakeMaps are published in Wirth et al. (2020).
The ensemble ShakeMap that should be used by most users is the “median” map. This map supersedes the legacy M9.0 Cascadia earthquake scenario .
M9 Cascadia Earthquake Scenarios
The ensemble ShakeMaps were derived from the set of thirty M9 Cascadia earthquake scenarios presented in Frankel et al. (2018). These thirty rupture scenarios were developed using a logic tree approach that varied the hypocenter location, down-dip rupture limit, slip distribution, and location of strong-motion-generating (high-stress-drop) subevents.
Ground Motion Simulations
Frankel et al. (2018) estimated ground motions from the thirty M9 Cascadia earthquake scenarios using a kinematic source model and hybrid simulation approach. Low frequency (≤ 1 Hz) deterministic seismograms were calculated using a 3-D finite difference code that propagates seismic waves through a 3-D seismic velocity model for Cascadia (Stephenson et al., 2017). High frequency (≥ 1 Hz) seismograms were generated using a stochastic approach. The stochastic seismograms did not include differences in high frequency site response that would be dictated by surficial geology.
Empirical ground motion models were used to expand the ground shaking estimates beyond the original model extent of Frankel et al. (2018) to cover all of Washington State, Oregon, northern California, and southern British Columbia.
Site Response
We computed and applied site response adjustments to the ground motion simulations of Frankel et al. (2018). Site response was computed for generalized Pacific Northwest shear wave velocity profiles developed from Ahdi et al. (2017) and based on the mapped Vs30 estimates from the global hybrid model of Heath et al. (2020). Details of site response adjustments made to the Frankel et al. (2018) Cascadia earthquake scenarios are given in Wirth et al. (2020).
Ensemble ShakeMaps
The ensemble ShakeMaps presented here are based on the median, median ±1σ (16th and 86th percentile), and median ±2σ (2nd and 98th percentile) ground motion intensity measures from the set of thirty M9 Cascadia earthquakes. We emphasize that it is unlikely that a single M9 Cascadia earthquake scenario will produce ±1 or 2σ ground motions everywhere throughout the Pacific Northwest. However, the ±1 or 2σ ensemble ShakeMaps could be used to evaluate the impact of more extreme ground shaking scenarios on localized scales (e.g., the county- or state-level).
References
- Frankel, A., Wirth, E., Marafi, N., Vidale, J., and Stephenson, W. (2018). Broadband synthetic seismograms for magnitude 9 earthquakes on the Cascadia megathrust based on 3D simulations and stochastic synthetics, part 1: Methodology and overall results, Bulletin of the Seismological Society of America, vol. 108, no. 5A, pp. 2347-2369, https://doi.org/10.1785/0120180034 .
- Heath, D.C., Wald, D.J., Worden, C.B., Thompson, E.M., and Smoczyk, G.M. (2020). A global hybrid Vs30 map with a topographic slope-based default and regional map insets, Earthquake Spectra, https://doi.org/10.1177/87552930209 .
- Stephenson, W. J., Reitmann, N. G., and Angster, S. J. (2017). P- and S-wave velocity models incorporating the Cascadia subduction zone for 3D earthquake ground motion simulations, Update for OFR 2007-1348: U.S. Geological Survey Open-File Report 2017-1152, 17pp, https://doi.org/10.3133/ofr20171152 .
- Wirth, E. A., Grant, A., Marafi, N. A., and Frankel, A. D. (2021). Ensemble ShakeMaps for magnitude 9 earthquakes on the Cascadia subduction zone, Seismological Research Letters, https://doi.org/10.1785/0220200240 .