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A Tale of Two Deltas: Potential Impacts of Cascadia-Margin Earthquakes on the Fraser River (Vancouver) and Duwamish River Deltas (Seattle)

Index Back to home page About these web pages Conclusions Future Present Past Holocene deltas Abstract
Present

What is at Risk in the Vancouver and Seattle Urban Coastal Zone?

Map of Duwamish DeltaClick on map for enlargement (88K). Extensive soil boring data collected along the Seattle and Vancouver waterfronts allow for the analysis of ground deformation potential.
Photo of the Port of Seattle.

Photo of the Port of Vancouver. Photo of the Seattle Waterfront.
Field SPT data.

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Field CPT data.

Click on figures to view larger images.

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Ground deformation and ground motion studies on the delta utilize data collected in the field. Liquefaction assessment and displacement potential can be analyzed with Standard Penetration Test (SPT), Cone Penetration Test (CPT), and Shear Wave (Vs) data. (A) SPT data along the West Seattle- Harbor Island delta front shows laterally continuous sand layers of low resistance. (B) Similar studies on the Fraser delta have primarily utilized the CPT. Spectral analysis of ground motion data and characterization of the acceleration response spectra at delta deposit sites (C) indicates that the natural resonant frequency of the Holocene soil is significantly lower than at adjacent upland sites. As a result, motions on the delta will likely be more damaging to tall engineered structures with similar response frequencies, than in the stiffer Pleistocene soils. Graph showing natural resonant frequency of buildings.

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