Master of Science
Vancouver has a high seismic risk due to the proximity of the city to the Cascadia subduction zone offshore, the soft sediments the metropolitan region overlies, and the mountainous topography nearby. The soft sediments and mountainous topography could cause an increase in shaking due to the sedimentary basin, topographic resonance and constructive/destructive interference, respectively. Numerical three-dimensional (3D) wave propagation simulations (WPS) were run using a finite difference scheme developed by Peterson and Sjogreen (2012), Seismic Waves, 4th order, to perform simulations of large shallow crustal rupture scenario earthquakes using the 1994 M6.7 Northridge, California earthquake slip distribution. The 3D WPS quantify the effects that topography and higher frequency waves can have on ground motions by running simulations with and without topography, with maximum resolvable frequencies increased from 0.5 Hz to 2 Hz in the simulation with topography. The Peak Ground Velocity (PGV), relative percentage change in PGV between simulations with and without topography (TE), and velocity amplitude maps were computed. Results showed significant variability in amplification due to rupture characteristics and location, and PGV increased with topography when higher frequencies up to 2 Hz were included in simulations for most sources, particularly at mountainous sites. The effect of harder ground conditions at these sites was not counterbalanced by topographic effects for lower frequency (< 0.5 Hz) simulations. The mountainous sites showed de-amplification at lower frequencies and more amplification at higher frequencies, while low elevation sites showed more amplification at low frequencies and less at higher frequencies.
Summary for Lay Audience
Vancouver is surrounded by mountainous terrain and is underlain by soft soils, both of which could increase shaking during large earthquakes and cause additional damage to what is currently modelled and expected. The effect of these soft soils and the underlying Georgia sedimentary rock basin to earthquake ground motions has been studied, but the effect of surface topography (changes in elevation) in this region has not been. This study conducts numerical simulations that can model the ground motions from these large earthquakes and includes topography in the model for the GVA for the first time and quantifies the increase in shaking that was experienced for multiple different earthquake rupture scenarios at different locations throughout Vancouver. It was found that increased shaking from including topography in the models was dependent on the source that generates the earthquake and the earthquake location. It was also found that for most scenarios, mountainous regions experience more increase in shaking than areas that are at lower elevations when higher frequencies were incorporated into the simulation, while there was less shaking at lower frequencies.
Vanderhoeff, Alex, "Topographic Effects of Shallow Crustal Earthquakes in the Greater Vancouver Region" (2023). Electronic Thesis and Dissertation Repository. 9191.