Dr. Kim Bak Olsen has two new papers published in the Bulletin of the Seismological Society of America where the authors model ground motions for large scenario earthquakes to investigate the effects of the Georgia basin structure on ground shaking in Greater Vancouver, British Columbia, Canada. The two papers simulate two scenario earthquakes; shallow blind‐thrust North America plate earthquakes and deep (>40 km) subducting Juan de Fuca plate earthquakes
Earthquake Ground Motion and 3D Georgia Basin Amplification in Southwest British Columbia:
Sheri Molnara, John F. Cassidyb,†, Kim B. Olsenc, Stan E. Dossoa, and Jiangheng Heb
aUniversity of Victoria, School of Earth and Ocean Sciences, 3800 Finnerty Road , Victoria, British Columbia V8P 5C2
bNatural Resources Canada, P.O. Box 6000, Sidney, British Columbia V8L 4B2
cSan Diego State University, Department of Geological Sciences, GMCS 231A, 5500 Campanile Dr, San Diego, California 92182‐1020
Shallow Blind‐Thrust Scenario EarthquakesAbstract Finite‐difference modeling of 3D long‐period (>2 s) ground motions for large (Mw 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 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 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. |
Deep Juan de Fuca Plate Scenario EarthquakesAbstract Finite‐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 deep (>40 km) subducting Juan de Fuca (JdF) plate earthquakes, simulated in locations congruent with known seismicity. Two sets of simulations are performed for a given scenario earthquake using models with and without Georgia basin sediments. The chosen peak motion metric is the geometric mean of the two orthogonal horizontal components of motion. The ratio between 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 10 deep subducting JdF plate earthquakes are simulated within 100 km of Greater Vancouver. Simulations are calibrated using records from the 2001 Mw 6.8 Nisqually earthquake. On average, the predicted level of average PGV at stiff soil sites across Greater Vancouver for an Mw 6.8 JdF plate earthquake is 3.2 cm/s (modified Mercalli intensity IV–V). The average increase in PGV due to basin structure across Greater Vancouver is 3.1. Focusing of north‐northeast‐propagating surface waves by shallow (<1 km) basin structure increases ground motion in a localized region of south Greater Vancouver; hence, scenario JdF plate earthquakes located ≥80 km south‐southwest of Vancouver are potentially the most hazardous. |
