Impacts of Simulated M9 Cascadia Subduction Zone Motions on Idealized Systems

Ground motions have been simulated for a magnitude 9 (M9) Cascadia Subduction Zone earthquake, which will affect the Puget Lowland region, including cities underlain by the Seattle, Everett, and Tacoma sedimentary basins. The current national seismic maps do not account for the effects of these basins on the risk-targeted Maximum Considered Earthquake (MCE_R). The simulated motions for Seattle had large spectral accelerations (at a period of 2 s, 43% of simulated M9 motions exceeded the MCE_R), damaging spectral shapes (particularly at periods near 1 s), and long durations (5%–95% significant durations near 110 s). For periods of 1 s or longer, the resulting deformation demands and collapse likelihood for four sets of single-degree-of-freedom systems exceeded the corresponding values for motions consistent with the conditional mean spectra at the MCE_R intensity (MCE_R). The regional variation of damage was estimated by combining probabilistic characterizations of the seismic resistance of structures and of the effective spectral acceleration, S_a,eff, which accounts for the effects of spectral acceleration, spectral shape, and ground-motion duration. For high-strength, low-ductility systems located above deep basins (Z_2.5 > 6 km), the likelihood of collapse during an M9 earthquake averaged 13% and 18% at 1.0 s and 2.0 s periods, respectively. For low-strength, high-ductility systems, the corresponding likelihoods of collapse averaged 18% and 7%.