Abstract

Stratigraphic evidence for coseismic uplift, a tsunami, and a sandy debris flow linked to the A.D. 900–930 Seattle‐fault‐zone earthquake and subsequent submergence is present at Gorst at the terminus of Sinclair Inlet, Washington. This study indicates that at least 3 m of uplift preceded a tsunami followed by a sandy debris flow. Though the Seattle and Tacoma fault zones ruptured within the error of ages of the tsunami deposit, model simulations indicate the Seattle fault generates an order of magnitude larger tsunami in the vicinity of the field area than the Tacoma fault and is the more likely generator of the tsunami. Simulations indicate amplitude from a Seattle‐fault‐zone‐generated wave train was 4–5 m, and stratigraphy shows the tsunami left a sandy deposit. Soft‐sediment deformation narrows the time window for the debris flow to the hours to months after the tsunami. Slope failure in unconsolidated glacial sands likely initiated a debris flow that surged at least 2 km down Gorst Creek valley and left a more than 40‐cm‐thick deposit along the creek and in the wetlands. Submergence of a Thuja plicata (western red cedar) swamp into the intertidal zone indicates at least 1.5 m of submergence in the last 1100 years. This submergence indicates uplift on nearby marine terraces associated with the Seattle fault zone was likely higher, and the A.D. 900–930 earthquake was potentially larger, than previously recognized.

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