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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
Future

How Will the Deltas of the Vancouver and Seattle Urban Coastal Zone Respond During in Future Big Earthquakes?

Assessment of Liquefaction Potential: Example analyses from the Fraser delta (A), and Duwamish delta (B) indicate extremely high liquefaction hazard, consistent with paleo liquefaction and historic observations. For the Fraser delta (A), a broad zone in native sands has a liquefaction resisting strength (CRR) well below earthquake stresses (CSR) that are likely to be induced in a large magnitude event. For the Duwamish delta (B), SPT data are used to compute factor of safety against liquefaction occurrence (i.e., CRR/CSR). Factor of safety of less 1.0 predicts liquefaction. This analysis shows that for the same M=7.5 event, differences in the amplitude of peak accelerations controls how extensive the liquefaction damage will be. For local events with PGA~0.4g the damage will be severe.

Graph of cyclic stress/resistance ratio in the Fraser Delta sands.

Four figures showing peak accelerations from 0.1 to 0.4g. Peak acceleration 0.4.Peak acceleration 0.3.Peak acceleration 0.2.Peak acceleration 0.1.
Click on figure for a larger image (24K), (48K), (48K), (36K),(36K)

Assessment of Disintegrative Flow Slide Potential: An example analysis from the Fraser delta indicates a high flow slide hazard, consistent with multibeam and side scanning sonar imagery of debris flows and mass wasting features, and historic static failures. Examples of flow slides in Holocene deltas triggered during earthquakes are Port Valdez and Seward, Alaska. In the State analysis below, the liquefied soil mass is further analyzed to assess whether the remaining residual shear strength is greater or less than the slope gravitational shear stress. For the Fraser delta, there are continuous zones within the soil column that have strength below the slope stress. Even when the shaking stops, these zones, if they connect together, may allow the delta soil mass to continue to slide and disintegrate as it moves into deeper water. This sliding might include portions of the Port.
Soil State Concept. Plot of High and Low Disintegrative Flow Potential
Click on figure for a larger image (76K), (36K).

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