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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 Past

What are the Paleo- and Historic-Seismic Ground Deformation Histories of the Deltas?

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Photo of dike swarms.(68K) Photo and figure of dikes deforming estuarine deposits.(32K) Photo of recent sand boil.(36K)
Photo of recent sand boils.(48K) Evidence of the affects of ancient and historic ground deformation are abundant. (A) At the Duwamish river, swarms of liquefaction dikes(1100 ybp?) intrude one of the only remaining unfilled stretches of the waterway at Kellog Island. (B) Late Holocene liquefaction dikes deform fine grained estuarine deposits at the Fraser River. (C, D, E) Liquefaction and deformation of native laharic sand deposits was extensive during the 1949 and 1965 Olympia and Seattle earthquakes. Map of ground failures in the Duwamish River Delta.(76K)

Delta Architecture

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Cross section of Fraser River Delta.

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The Fraser and Duwamish are flat topped prograding deltas (A). Forsets are composed primarily of silt, silty sand, and sand-sized particles in a gradation range known to be susceptible to liquefaction. Liquefiable materials likely extend through the entire Holocene section of the deltas, a thickness in excess of 100 m along the axis of the valleys. (B) Geotechnical stratigraphic sections from multiple borehole investigations show liquefiable sand units that are laterally extensive. Surface bathymetric data (C) collected on the subaqueous Fraser Delta by EM-1000 multibeam clearly shows extensive mass wasting features along the entire flank of the delta front, and debris flow lobes on the floor of the Strait of Georgia. Image of surface bathymetry with Multibeam data off the Fraser River Delta.(40K)


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