Earthquake Ground Motion and 3D Georgia Basin Amplification in Southwest British Columbia: Shallow Blind-Thrust Scenario Earthquakes

Abstract

Abstract Finite‐difference modeling of 3D long‐period (>2 s) ground motions for large ( M w 6.8) scenario earthquakes is conducted to investigate the effects of the Georgia basin structure on ground shaking in Greater Vancouver, British Columbia, Canada. Scenario earthquakes include shallow blind‐thrust North America (NA) plate earthquakes, simulated in locations congruent with linear clusters of shallow seismicity, that is, potential active faults. A slip distribution model of the M w 6.7 Northridge, California, blind‐thrust earthquake, with the hypocenter modified to 5 km depth, is used to characterize the source rupture process. Two sets of simulations are performed for a given scenario earthquake using models with and without Georgia basin sediments. The ratio of predicted peak ground velocity (PGV) for the two simulations is applied here as a quantitative measure of amplification due to 3D basin structure. A total of eight shallow blind‐thrust NA plate scenario earthquakes are simulated within 100 km of Greater Vancouver. Overall, predicted ground motions are higher in the down‐dip direction of each epicenter due to the source radiation pattern; hence, scenario earthquakes located south of Vancouver produce the highest motions in the city. The average maximum PGV at stiff soil sites across Greater Vancouver considering all eight scenario earthquakes is 17.8 cm/s (modified Mercalli intensity VII); the average increase in peak motion due to the presence of Georgia basin sediments is a factor of 4.1. The effective duration of moderate‐level (≥3.4 cm/s) shaking within Greater Vancouver is an average of 22 s longer when Georgia basin sediments are included in the 3D structure model. Online Material: Snapshots and videos of wave propagation, and peak ground velocity maps.