3D Broadband Ground-Motion Simulations of Cascadia Megathrust Earthquakes: Full-Margin Scenarios Incorporating Variable Magnitudes and Frontal Thrust Splay Faulting

Abstract

Great megathrust earthquakes (∼Mw 8–9) that rupture along the Cascadia Subduction Zone pose significant shaking and tsunami hazards to the U.S. Pacific Northwest. Although these large earthquakes are documented in the geologic record, none have been recorded with modern instrumentation, complicating regional seismic hazard assessments. To improve constraints on seismic hazard estimates for future earthquakes, we develop 3D broadband (0–10 Hz) ground-motion simulations for 36 full-margin megathrust earthquake scenarios through the Cascadia Coastlines and Peoples Hazards Research Hub. These simulations expand on the M9 Project (Frankel et al., 2018; Wirth, Frankel, et al., 2018) by incorporating a broader range of rupture parameters—variable magnitudes, splay faulting, and geodetically informed slip distributions. Benchmarking these simulations against the Next Generation Attenuation Subduction (NGA-Sub) ground-motion models shows reasonable fits for non-basin sites, although simulations predict stronger shaking at short (0–2 s) and long (6–10 s) periods. NGA-Sub basin terms improve fits to synthetic shaking within the Seattle Basin but underpredict amplitudes by a factor of 2 at periods of 2–5 s. We also evaluate how rupture characteristics influence regional inland shaking variability and basin response in the Puget Lowlands. Larger magnitude scenarios generally cause stronger inland shaking due to farther inland extents of slip and larger strong-motion-generating fault patches but show little difference in basin amplification within the Puget Lowlands compared to smaller magnitude events. Along-dip hypocenter location strongly influences overall inland shaking, with offshore (updip) hypocenters generating on average ∼45% higher shaking at long periods (>6 s) compared to onshore (downdip) hypocenters due to rupture directivity effects. Offshore splay faults minimally affect inland shaking but alter offshore deformation important for coupled ground-shaking and tsunami scenarios. Finally, we show that rupture directivity increases amplification within the Seattle Basin through the generation and interaction of higher order mode surface waves, an effect of 3D basin geometry that cannot be captured using 1D approximations.