The analysis of marsh cores from the tidal zones of the Siuslaw, Umpqua, and Coos River systems on the south-central Oregon coast provides supporting evidence of coseismic subsidence resulting from megathrust earthquakes and reveals the landward extent of the zero-isobase. The analysis is based on lithostratigraphy, paleotidal indicators, microfossil paleotidal indicators, and radiocarbon age. Coseismic activity is further supported by the presence of anomalous thin sand layers present in certain cores. The analysis of diatom assemblages provides evidence of relative sea-level displacement on the order of 1 to 2 m. The historic quiescence of local synclinal structures in the Coos Bay area together with the evidence of prehistoric episodic burial of wetland sequences suggests that the activity of these structures is linked to megathrust releases. The distribution of cores containing non-episodically buried marshes and cores that show episodically buried wetlands within this area suggests that the landward extent of the zero-isobase is between 100 km and 120 km from the trench. The zero-isobase has a minimum width of 10 to 15 km. Radiocarbon dating of selected buried peat sequences yields an estimated recurrence interval on the order of 400 years. The apparent overlapping of the landward margin of both the upperplate deformation zone (fold and/or thrust fault belt) and the landward extent of the zero-isobase is interpreted to represent the landward limit of the locked zone. The earthquake magnitude is estimated to be 8.5 based on an arbitrary rupture length of 200 km and a locked zone width of 105 km. The identification of the zero-isobase on the southcentral Oregon coast is crucial to the prediction of regional coseismic subsidence and tsunami hazards, the testing of megathrust dislocation models, and the estimation of megathrust rupture areas and corresponding earthquake magnitudes in the Cascadia Margin.

Beach shoreline retreat induced by coseismic subsidence in the Cascadia subduction zone is an important post-earthquake hazard. Sand on a beach acts as a buffer to wave attack, protecting dunes, bluffs and terraces. The loss of sand from a beach could promote critical erosion of the shoreline. This study was initiated in order to estimate the potential amount of post subsidence shoreline retreat on a regional scale in the Central Cascadia Margin. The study area is a 331 km stretch of coastline from Copalis, Washington to Florence, Oregon. Several erosion models were evaluated, and the Bruun model was selected as the most useful to model shoreline retreat on a regional scale in the Central Cascadia Margin. There are some factors that this model does not address, such as longshore transport of sediment and offshore bottom shape, but for this preliminary study it is useful for estimating regional retreat. The range of parameter input values for the Bruun model include: the depth of closure (h) range from 15 m to 20 m water depth; the cross-shore distance (L) range from 846 m to 5975 m; and the estimated subsidence amount (S) range from O m to 1.5 m. The minimum to maximum range of post-subsidence shoreline retreat is 142 to 531 m in the Columbia River cell, 56 to 128 m in the Cannon Beach cell, 38 to 149 m in the Tillamook cell, 25 to 91 m in the Pacific City cell, 11 to 126 m in the Lincoln City cell, 30 to 147 m in the Otter Rock cell, 0 to 165 m in the Newport cell, 0 to 76 m in the Waldport cell, and 0 m in the Winchester cell. Results of the study suggest that many of the beaches in the study area are at risk of beach and personal property loss. Beach communities could limit the amount of potential damage in these areas through coastal zone planning.