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TitleEarthquake ground motion and 3D Georgia Basin amplification in southwest British Columbia: shallow blind-thrust scenario earthquakes
AuthorMolnar, S; Cassidy, J FORCID logo; Olsen, K B; Dosso, S E; He, J
SourceBulletin of the Seismological Society of America vol. 104, no. 1, 2014 p. 321-335,
LinksOnline supplement / supplément en ligne
Alt SeriesEarth Sciences Sector, Contribution Series 20130063
PublisherSeismological Society of America (SSA)
Mediapaper; on-line; digital
File formatpdf
ProvinceBritish Columbia; Western offshore region
NTS92B; 92C; 92F; 92G
AreaStrait of Georgia; Juan de Fuca Strait; Vancouver; Vancouver Island
Lat/Long WENS-126.0000 -122.0000 50.0000 48.0000
Subjectsgeophysics; tectonics; tectonic setting; earthquakes; modelling; seismicity; faults; Georgia Basin
ProgramPublic Safety Geoscience Targeted Hazard Assessments in Western Canada
Released2014 01 21
AbstractFinite-difference modeling of 3D long-period (> 2 s) ground motions for large (MW 6.8) scenario earthquakes is conducted to investigate 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, i.e. potential active faults. A slip distribution model of the MW 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 8 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 8 scenario earthquakes is 17.8 cm/s (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.
Summary(Plain Language Summary, not published)
Building code provisions currently use near-surface geological conditions as a proxy for estimating variations in earthquake ground shaking. However, in areas of large sedimentary basins (such as Greater Vancouver) deep 'basin structure' can play a very significant role in altering ground shaking and the resulting damage patterns from earthquakes. For example, in the 1995 M 6.9 Kobe Japan earthquake, damage was concentrated along the edge of a basin. In this study we use detailed basin models, realistic shallow earthquake scenarios, and advanced computer modelling to examine the variation in earthquake shaking across southwest BC associated with shallow earthquakes. Our study shows that sedimentary basin significantly alter the ground shaking (much more than near-surface geology currently used in building codes) with 'deep basin' amplification factors of up to 8 times and much longer duration shaking (moderate shaking increasing from 3 s to 25 s duration). Studies such as this one will contribute to improving future versions of codes and standards for buildings and infrastructure.

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