Topographic Effects of Shallow Crustal Earthquakes in the Greater Vancouver Region
Abstract
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.