Uplift of the overriding plate at a subduction zone denotes interseismic strain accumulation, which is subsequently released during a megathrust earthquake. Although most interseismic strain is thought to be released elastically, observations of uplifted coastal regions at subduction zones worldwide indicate that some strain may result in permanent uplift. The Grays Harbor and Willapa Bay (Washington, USA) coastal region of the Cascadia subduction zone hosts flights of marine terraces testifying to late Pleistocene rock uplift. Our new detailed mapping of the marine terraces recognizes nine new units, including estuarine and fluvial sediments. Luminescence dating, relative age based on soil maturity and terrace elevation, and an evaluation of previous ages from fossil shells collectively constrain the probable ages of three estuarine units to sea-level high stands during Marine Isotope Stages 5a, 5c, and 5e. We estimate an average uplift rate of 0.4 ± 0.1 mm/yr for the terraced estuarine units, consistent with other Pleistocene uplift and incision rates in Cascadia. When compared with observed interseismic vertical deformation, these rates suggest that about one-tenth of interseismic strain may become permanent. The values are permissible within the uncertainties of uplift based on regional estimates of interseismic vertical strain rates and of coseismic subsidence.

Four muddy sand sheets occur within a tidal marsh peat at Swantown on the west coast of Whidbey Island, Washington. The two largest sand sheets pinch out about 100 m inland and became thinner and finer-grained landward. All four sand sheets contain marine microfossils and have internal stratification. They record repeated inundation of the marsh over a short time period by distinct pulses of sediment-laden ocean water, consistent with deposition by a tsunami wave train. The layers have been radiocarbon-dated to 1160–1350, 1400–1700, 1810–2060, and 1830–2120 cal yr B.P. The overlap in age between the two youngest layers and inferred great earthquake events at the Cascadia plate boundary, some 250 km to the west, suggests they were emplaced by tsunamis from this source area. The two older layers do not correlate with plate-boundary events. They may be products of tsunamis caused by earthquakes on local faults in the Strait of Juan de Fuca or by submarine landslides in this area.