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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)

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Abstract
Large earthquakes along the Cascadia subduction zone and Benioff zone, as well as crustal faults, pose hazards to greater Vancouver and Seattle, whose ports are built on the Fraser and Duwamish River deltas, respectively. Although located more than 100 km from the outer coast, the cities remain at risk due to local conditions that increase the potential of earthquake-induced ground failure. The accumulations of loosely consolidated delta sediment are up to 300 m thick and can significantly amplify ground motions. Dynamic motions with site periods vary from 0.2 to 0.5 seconds on the delta margins to 4 seconds in the delta center.

Extensive drilling programs by the USGS, GSC, and other agencies recently have examined the sedimentary framework of these urbanized deltas. The primary objectives were to determine the stratigraphic and geographic distribution of sandy sediment, that is susceptible to liquefaction, and to evaluate the potential impact of earthquakes on the region's industrial and transportation infrastructure. Sand-filled dikes, sills and other paleo-liquefaction features are commonly observed and suggest the occurrence of strong ground shaking in the past. Cone-penetration tests (CPT) and standard penetration tests (SPT) were used to characterize potentially liquefiable deposits. The sandy unit of principal concern on the Fraser delta is 8-30 m thick and interpreted to represent a complex of distributary channel sands. The youngest and most liquefiable deposits are located adjacent to the present Main Channel and delta front. Buried channels and laterally continuous layers of sand, probably derived from lahars (volcanic debris flows), also occur on the Duwamish delta. Based on modeled earthquakes where ground acceleration significantly exceed 0.2 g, there is a high potential for large strain disintegrative flow failure on the delta front and slope, potentially including the port facilities.


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