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The regional subsidence database for Oregon shows an overall record of multiple burial events in all bays of Oregon during the late-Holocene using stratigraphic records
Estimates of the amount of subsidence observed in the most recent buried wetland are based on multiple paleotidal level indicators at Grays Harbor, Necanicum Estuary, Tillamook Bay, and Siletz Bay. Estimated subsidence at these localities is consistent with published subsidence data from adjacent bays.
Comparisons of tidal level indicators using modern Cascadia wetlands indicate that paleosubsidence of 0, 1, and 2 m. are justified based on a regional database of estimated paleosubsidence from 83 sites in southern Washington and northern Oregon demonstrates a regional trend of decreasing subsidence from north to south. These data indicate that vertical tectonic displacement for the most recently buried wetland was forced by regional strain release from megathrust earthquake, rather than from multiple independent ruptures along local faults.
The addition of paleosubsidence estimates to the current pre-subsidence conditions of flood elevations demonstrate increased flood elevations for Aberdeen, Grays Harbor, Washington, and Seaside, Necanicum Estuary, Oregon. In addition, the 10 yr. flood elevations with added paleosubsidence are greater than the 500 yr. flood elevations in the two study areas.
The potential hazards of post-subsidence flooding from rivers and storm surges represent a post-earthquake hazard that might persist for decades.