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Coastal marshes record a 6500 yr history of coseismic displacements in great earthquakes at the Cascadia subduction zone. We compiled estimates of coseismic displacement for past megathrust events based on correlations with megathrust-triggered turbidites, and estimated megathrust slip based on comparisons of marsh displacements with dislocation model predictions. Age-correlated marsh data are compatible with event rupture extents defined by the published turbidite record, and a 6500 yr mean recurrence interval that increases northward from ~230 to ~480 yr. Within the constraints of the marsh data, the width of the coseismic rupture zone generally agrees with the downdip width of the interseismic locked zone inferred from geodetic and thermal data. In southern most Cascadia, where the model does not include the complex deformation near the Mendocino triple junction, the coastal data may be better fit by a model with an ~25% narrower rupture than that inferred from regional geophysical data. At each coastal marsh site, coseismic displacements are roughly similar from event to event, independent of the time since the previous event. Slip in the A.D. 1700 earthquake was consistent with the preceding interval of strain accumulation (~200 yr) only at the northern and southern ends of the margin, but it was apparently much higher in southern Washington and northern Oregon, possibly indicating postseismic contamination and/or catch-up coseismic slip to make up for a deficit in the preceding event. Overall agreement between the dislocation models and the marsh data for most of the margin implies that such models can be usefully applied to rupture and ground shaking predictions. doi: 10.1130/B30108.1

Seismic hazard assessments for a Cascadia subduction zone earthquake are largely based on the rupture area predictions of dislocation models constrained by geodetic and geothermal data; this paper tests the consistency of the models for the 1700 great Cascadia earthquake with compiled coastal coseismic subsidence as estimated from paleoelevation studies. Coastal estimates have large uncertainties but show a consistent pattern. Greatest coseismic subsidence (~1-2 m) occurred in northern Oregon/ southern Washington; subsidence elsewhere was ~0-1 m. Elastic dislocation models constrained by interseismic geodetic and thermal data are used to predict the coseismic subsidence for two likely strain accumulation periods of (i) 800 and (ii) 550 yr of plate convergence and for uniform megathrust slip of 10, 20, 30, and 50 m. The former two models provide a better and equally good fit; predicted subsidence is in broad agreement with marsh estimates. Discrepancies exist, however, at the ends of the subduction zone. In the south, misfit may be due to breakup of the Gorda plate. The discrepancy in the north may be explained if the 1700 event released only part of the accumulated strain there, consistent with long-term net uplift in excess of eustatic sea-level rise. The coseismic slip magnitude, estimated by comparing uniform slip model predictions with marsh coseismic subsidence estimates, is consistent with the M 9 earthquake indicated by Japanese tsunami records. The coseismic slip was greatest in northern Oregon/southern Washington, declining to the north and south. Keywords: Cascadia subduction zone, earthquakes, modeling, Holocene, estuaries, paleoseismology.