Nonlinear Broadband Simulations of M9 Megathrust Earthquakes in the Cascadia Subduction Zone
Prof. Dr. Olsen, Kim Bak
San Diego State University
We predict broadband (BB) ground motions in the Pacific Northwest (PN) urban areas during large earthquakes on the Cascadia megathrust using a hybrid approach which combines deterministic low-frequency (f < 1 Hz) signals obtained by 3-D wave propagation simulations with stochastic synthetics and 1-D simulations of nonlinear sites response. Velocities and densities in our computational mesh are derived by extending the regional Cascadia Community Velocity model (V1.6, Stephenson et al., 2017) to 150 km depth and embedding a more detailed model of shallow velocities (Molnar et al. 2011) in the Georgia basin. In the near surface part, we used a recent empirical correlation between geomorphic terrain classes and measurement-based VS30 values for the PN (Ahdi et al., 2017) to define a geotechnical layer. Wave propagation simulations were carried out with the discontinuous mesh version of the AWP finite difference code (AWP-GPU-DM). Our 3-D simulations use a spatial discretization of 66⅔ m in the shallow crust, with a minimum shear-wave velocity of 350 m/s and a maximum frequency of 1 Hz. The grid spacing was increased to 200 m at depths between 5 and 30 km, and increased again to 600 m at depths of 30 km or more. The ensemble of kinematic source realizations consists of 8 M9.0, two M8.8 and one M9.2 scenario, each consisting of a background slip distribution with superimposed high stress-drop subevents (Frankel, 2016) to mimic observations made during previous megathrust earthquakes (2011 M9 Tohoku and 2010 M8.8 Maule). An updated version of the San Diego State University broadband generation module, which includes frequency-dependent spatial correlation for risk analysis purposes, was used to compute BB synthetics. Site response simulations carried out using Noah1D show that nonlinear effects would not be significant for glacial deposits in Seattle or Vancouver, but potentially important for deep Quaternary deposits in the Fraser River delta. We obtain favorable comparisons between spectral accelerations for BB synthetics, the BC Hydro GMPE, and the M9 Tohoku event.