Numerical Simulation of M9 Megathrust Earthquakes in the Cascadia Subduction Zone

Abstract

We estimate ground motions in the Pacific Northwest urban areas during M9 subduction scenario earthquakes on the Cascadia megathrust by simulating wave propagation from an ensemble of kinematic source descriptions. Velocities and densities in our computational mesh are defined by integrating the regional Cascadia Community Velocity Model (CVM) v1.6 (Stephenson et al. P-and S-wave velocity models incorporating the Cascadia subduction zone for 3D earthquake ground motion simulations—update for open-file report 2007–1348, US Geological Survey, 2017) including the ocean water layer with a local velocity model of the Georgia basin (Molnar, Predicting earthquake ground shaking due to 1D soil layering and 3D basin structure in SW British Columbia, Canada, 2011), including additional near-surface velocity information. We generate six source realizations, each consisting of a background slip distribution with correlation lengths, rise times and rupture velocities consistent with data from previous megathrust earthquakes (e.g., 2011 M 9 Tohoku or 2010 M 8.8 Maule). We then superimpose M ~ 8 subevents, characterized by short rise times and high stress drops on the background slip model to mimic high-frequency strong ground motion generation areas in the deeper portion of the rupture (Frankel, Bull Seismol Soc Am 107(1):372–386, 2017). The wave propagation is simulated using the discontinuous mesh (DM) version of the AWP finite difference code. We simulate frequencies up to 1.25 Hz, using a spatial discretization of 100 m in the fine grid, resulting in surface grid dimensions of 6540 × 10,728 mesh points. At depths below 8 km, the grid step increases to 300 m. We obtain stable and accurate results for the DM method throughout the simulation time of 7.5 min as verified against a solution obtained with a uniform 100 m grid spacing. Peak ground velocities (PGVs) range between 0.57 and 1.0 m/s in downtown Seattle and between 0.25 and 0.54 m/s in downtown Vancouver, while spectral accelerations at 2 s range between 1.7 and 3.6 m/s2 and 1.0 and 1.3 m/s2, respectively. These long-period ground motions are not significantly reduced if plastic Drucker-Prager yielding in shallow cohesionless sediments is taken into account. Effects of rupture directivity are significant at periods of ~ 10 s, but almost absent at shorter periods. We find that increasing the depth extent of the subducting slab from the truncation at 60 km in the Cascadia CVM version 1.6 to ~ 100 km increases the PGVs by 15% in Seattle and by 40% in Vancouver.