Abstract#
The Cascadia Subduction Zone (CSZ) poses significant hazards to the Pacific Northwest U.S. and Canada. Mantle flow patterns around the slab and stress transmitted to the shallow megathrust are governed by the 3D slab geometry. However, existing studies disagree on many aspects of Cascadia slab geometry, including the presence of a large gap in the Gorda segment beneath Oregon, the slab’s depth extent, and the location of the northern boundary of the Explorer segment. This disagreement is due to the low rate of background seismicity relative to other subduction zones, which makes it difficult to construct seismic tomographic imagery of the slab geometry. While seismic images of the slab are poorly constrained, it may be possible to constrain aspects of slab geometry by linking predictions of mantle flow and seismic anisotropy from geodynamic models to observations of shear wave splitting. Here we aim to compare predicted anisotropy and tomography from existing 3D geodynamic models of various CSZ slab geometries with SWS observations and seismic tomography models of Cascadia. Our approach leverages the ECOMAN software, which uses a modified D-REX method to calculate lattice-preferred orientation due to shear deformation, the SKS-SPLIT package to model synthetic SKS splitting at the surface, and the PSI-D package to forward-model synthetic seismic observables, to predict seismic anisotropy, and to predict isotropic and anisotropic seismic tomography. We anticipate that our results will help refine future slab models by providing a new constraint on slab geometry and flow in Cascadia. On a global scale, our proposed methods could improve understanding of mantle flow and upper mantle structure in other regions with complex flow and subduction-related hazards.
Plain-language Summary#
Segments of oceanic plates are being driven beneath the North American plate of the Pacific Northwest U.S. and Canada in the Cascadia Subduction Zone (CSZ). The most destructive earthquakes occur along this interface where the down-going and overlying plate meet. The pattern of mantle flow around the slab and the stress transmitted to the interface are controlled by the shape of the down-going slab, which is poorly constrained in the CSZ. It may be possible to improve slab shape models by linking predictions of the mantle flow around the slab from geodynamic models to seismic observations. We will use a computer software to generate synthetic seismic observations as if the earthquake waves had passed through each proposed model. Therefore, we can make direct comparisons between what we observe in the CSZ and what we would expect to see for a given model of the slab shape to determine the best match. We anticipate that our results will help improve future slab models by providing a new constraint on slab shape and mantle flow in Cascadia. On a global scale, our proposed methods could improve understanding of mantle flow and mantle structure in other regions at risk for subduction-related hazards.
References#
Faccenda, M., & Capitanio, F. A. (2013). Seismic anisotropy around subduction zones: Insights from three-dimensional modeling of upper mantle deformation and SKS splitting calculations. Geochemistry, Geophysics, Geosystems, 14(1), 243–262. https://doi.org/10.1002/ggge.20055
Faccenda, M., & VanderBeek, B. P. (2023). On constraining 3D seismic anisotropy in subduction, mid-ocean-ridge, and plume environments with teleseismic body wave data. Journal of Geodynamics, 158, 102003. https://doi.org/10.1016/j.jog.2023.102003
Faccenda, M., VanderBeek, B. P., de Montserrat, A., Yang, J., Rappisi, F., & Ribe, N. (2024). ECOMAN: An open-source package for geodynamic and seismological modelling of mechanical anisotropy. Solid Earth, 15(10), 1241–1264. https://doi.org/10.5194/se-15-1241-2024
Fraters, M., Billen, M., Naliboff, J., Staisch, L., Watt, J., & Li, H. (2025). Using Subducting Plate Motion to Constrain Cascadia Slab Geometry and Interface Strength. Geochemistry, Geophysics, Geosystems, 26(5), e2024GC011895. https://doi.org/10.1029/2024GC011895
Roy, P. (2024). Modeling of Lower Mantle Anisotropy Due to Plume Generation from Large Low-Shear-Velocity Provinces. https://doi.org/10.5281/ZENODO.13241048
VanderBeek, B. P., Bue, R. L., Rappisi, F., & Faccenda, M. (2023). Imaging upper mantle anisotropy with traveltime and splitting intensity observations from teleseismic shear waves: Insights from tomographic reconstructions of subduction simulations. Geophysical Journal International, 235(3), 2640–2670. https://doi.org/10.1093/gji/ggad389