Characterization of Long-Period Ground Motions in the Georgia Basin Region, British Columbia, Canada

Long-period (> 2 s) ground motions in the Georgia basin region of southwestern British Columbia (BC) are investigated for Pacific Northwest scenario earthquakes using 3D finite-difference simulations of viscoelastic wave propagation. The simulations are validated by comparing synthetic surface waveforms with 36 selected strong- and weak-motion recordings of the 2001 MW 6.8 Nisqually earthquake at sites spanning from Puget Sound, Washington, to southern BC. This is the first opportunity to validate the 3D geologic model of the Georgia basin region, such that waveform agreement improves when the upper 1 km structure is updated. Solely varying the degree of anelastic attenuation within geological reasonable bounds does not significantly improve agreement of the long-period waveforms. Subducting Juan de Fuca plate scenario earthquakes are investigated by initiating the Nisqually-model source in eleven different locations beneath Georgia basin. Source depths range from 35-77 km, with the deepest events occurring beneath Vancouver, BC. For all deep earthquake simulations, ground motion is predominantly amplified in the Georgia basin. The largest ground motions occur northwest of the source location, dramatically altering the amplitude and pattern of the simulated ground motion with source location. Hence, the most hazardous scenario to Vancouver occurs when a deep in-slab event occurs southeast of the city. Shallow crustal North America plate scenario earthquakes are explored using a slip distribution solution of the 1994 MW 6.7 Northridge, California, thrust event.