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| Subduction of the Juan de Fuca Plate beneath North America has formed the active volcanoes of the Cascade Mountains, and the broad structural basin that underlies the Puget Lowland. Pleistocene glaciations sculpted deep, linear troughs that comprise the numerous arms of Puget Sound, and covered the landscape with thick glacigenic deposits. Relative sea level has fluctuated in a complex pattern, largely due to the effects of glacial-isostatic rebound. Holocene sea-level rise has flooded the glacially carved valleys and transformed them into deep, elongate marine embayments that extended more than 50 km upstream of the present position of deltas in this study.
The ports of Seattle and Tacoma, two of the busiest ports in North America, are built on deltas and were damaged by moderate earthquakes that struck in 1949 and 1965. These events caused liquefaction, lateral spreads, and subsidence in loosely consolidated landfills. The deltas may be subject to more intense shaking from recently discovered sources of earthquakes at Puget Sound. Chief among these additional sources are the Seattle fault, which directly underlies the Duwamish River delta.
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 Click on map to view larger image (56K) |
Paleotopographic map of the lower Puyallup and Duwamish River valleys in the middle Holocene (Dragovich et al., 1994). The ancient deltas, shaded in yellow, were graded to a sea level of about -8 m below present. Since 5700 cal yr B.P, the Puyallup and Duwamish deltas have prograded 14 and 51 km, respectively. Confined in narrow valleys, the deltas have grown at average rates up to 9 m/yr! |
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Index Map of the Study Area
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