USGS PCMSC

The Effects of Volcanic Debris Flows (Lahars), Earthquakes and
Landslides on Holocene Deltas at Puget Sound, Washington

Index Abstract Objectives Geologic Setting Nisqually River Delta Puyallup River Delta Duwamish River Delta Delta Model Conclusions References About these Web Pages Back to Home Page Duwamish River Delta
Bathymetric map of Elliot Bay The Duwamish River discharges into Elliott Bay and forms a delta that is largely occupied by the Port of Seattle and other industry. Landfill and stream channelization have destroyed over 99% of the marshes and tidal flats that originally existed on the delta. The Seattle fault zone passes directly beneath the delta. Sandy artificial landfills and natural sand deposits on the delta are susceptible to liquefaction and landslides during earthquakes.
GPR image and interpretation of lahar deposit GPR image of a sandy lahar deposit on Duwamish River delta. The sand varies in thickness from 1-2 m and overlies a low-relief contact on top of shell-rich estuarine sediment. The sand probably represents widespread sedimentation that followed the 1,100 yr B.P. laharic flood upstream by "hours, days, or weeks" (Pringle et al., 1997).
Photography of dike in tidal-marsh deposit

Evidence of earthquake-induced liquefaction is provided by sand-filled dikes at Kellogg Island on the Duwamish River delta. The dark, fine-medium sand was probably derived from Mount Rainier.

The dikes intrude tidal-marsh deposits that postdate the A.D. 900-930 uplift along the Seattle fault. Some of the dikes can be traced upward to lenses of silty sand that are probably the flanks of vented-sand volcanoes (sand boils) erupted from the dikes during earthquakes. (Photograph by Brian Atwater, 2000)

For locations of GPR profile and photograph, see map above

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contact: Robert Kayen
last modified 2018