Geomorphology and potential slope instability on the Fraser River Delta foreslope, Vancouver, British Columbia

The Fraser River delta lies in one of the highest-risk earthquake regions in Canada and is composed largely of loose fluvial sands, widely recognized as possessing a high seismic liquefaction potential. Recent investigations of the western margin of the Fraser delta indicate that cyclic liquefaction during a large earthquake is possible, potentially giving rise to deformations including lateral spreading cyclic softening, and possibly flow sliding. A preliminary evaluation of postshaking. static slope stability is reported, wherein analyses were constrained by estimated ultimate or steady-state shear strengths inferred from correlations with field data and by shear stresses calculated from existing seabed bathymetry. The resulting postliquefaction factor of safety against sliding was found to be less than unity for upper Roberts Bank slope, assuming a low bound ultimate shear strength equivalent to 0.06 P-o'. Insufficient data exists to fully evaluate the validity of this assumption; however, extensive failure deposits exist along the base of the delta slope.