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TitleEffects of 3D basin structure on long-period ground motions in SW British Columbia, Canada, for large scenario earthquakes
AuthorMolnar, S; Cassidy, J F; He, J; Olsen, K B; Dosso, S E
SourceProceedings of the 15th world conference on earthquake engineering; 2012 p. 1-11
Alt SeriesEarth Sciences Sector, Contribution Series 20120064
Meeting15th World Conference on Earthquake Engineering; Lisbon; PT; September 24-28, 2012
ProvinceBritish Columbia
NTS92B; 92C/01; 92C/08; 92C/09; 92C/10; 92F/01; 92F/08; 92F/09; 92F/10; 92G
AreaVancouver; Georgia Basin; Vancouver Island; Victoria
Lat/Long WENS-124.5000 -122.0000 49.0000 48.2500
Subjectsgeophysics; earthquakes; earthquake studies; earthquake magnitudes; earthquake mechanisms; earthquake damage; basins; basin analyses; strong motion seismology; models; modelling; seismic models; Cascadia subduction zone
Illustrationslocation maps
ProgramTargeted Hazard Assessments in Western Canada, Public Safety Geoscience
AbstractFinite-difference modeling of 3D viscoelastic wave propagation for large (MW 6.8) scenario earthquakes is conducted to investigate effects of the Georgia basin structure on long-period (> 2 s) ground shaking in Greater Vancouver, British Columbia, Canada. This research provides the first detailed investigation of 3D earthquake ground motion for a sedimentary basin in Canada. The region of greatest seismic risk in the country is the Greater Vancouver area with critical infrastructure and a population of >2 million located at the northern end of the Cascadia subduction zone. The Georgia basin is a northwest-oriented structural depression that extends from southwestern British Columbia southward into Washington State, with Tertiary dimensions of 130 by 70 by 5 km. Scenario earthquakes include shallow (5 km) and deep (> 40 km) events within the over-riding North America (NA) and subducting Juan de Fuca (JdF) plates, respectively, simulated in locations congruent with known seismicity within 100 km of Greater Vancouver. Two sets of simulations are performed for a given scenario earthquake using 3D physical-structure models with and without basin sediments. The ratio between predicted peak ground velocity for the two simulations is applied here as a quantitative measure of amplification due to 3D basin structure. Simulations are calibrated using records from the 2001 MW 6.8 Nisqually earthquake. For all simulations, some general effects are observed consistently when Georgia basin sediments are included in the 3D structure model: the symmetry of the seismic radiation pattern is distorted, the area of higher ground motions is increased, and the highest onshore basin amplification occurs in a localized region of southern Greater Vancouver from the focusing of surface waves due to shallow (< 1 km) basin structure. Deep JdF plate scenario earthquakes are simulated in 10 different locations; the predicted average maximum peak ground velocity and basin amplification at stiff soil sites across Greater Vancouver is ~6 cm/s (intensity V) and a factor of ~2.5, respectively. From simulations of shallow NA plate scenario earthquakes in 9 different locations, the corresponding average values are ~30 cm/s (intensity VII-VIII) and a factor of ~3, respectively. Overall, this study shows that the presence of 3D Georgia basin structure increases the level of predicted long-period ground shaking.