CONSTRAINTS ON EARTHQUAKE SOURCE MODELS OF THE 1700 CE TSUNAMI: MAPPING AND SEDIMENT TRANSPORT MODELING OF TSUNAMI DEPOSITS IMPLY >1 M OF MEGATHRUST COSEISMIC SUBSIDENCE AT THE SALMON RIVER, OR

The last great earthquake on the Cascadia subduction zone (1700 CE) ruptured much of the plate boundary and generated a high tsunami that deposited sand in coastal marshes from Vancouver Island to northern California. Although the geologic record of tsunami inundation is extensive in some of these marshes, few sites have been cored extensively enough to accurately determine the inland limit of sand deposition and capture variability in tsunami deposit thickness. We collected 105 cores in marshes of the Salmon River Estuary, Oregon, and reanalyzed 114 core logs from a 1987-88 study that delineates the inland extent and thickness variation (0.1–19.5 cm) of beach, dune, and channel sand deposited by the 1700 CE tsunami. The 1700 CE tsunami deposit is easily recognized in cores <1 m deep: a buried high-to-middle-marsh peat is typically sharply overlain by tsunami sand and tidal mud. Overall, the deposit thins and fines inland, and thins away from the main river channel. We use these data to validate models of tsunami sediment transport (using Delft3D-FLOW) that, in turn, can be used to test rupture models of the 1700 earthquake. Based on the 1700 deposit’s inland extent, model results suggest >15 m of megathrust slip offshore of the Salmon River, a result that requires >1 m of coastal coseismic subsidence. Such slip is consistent with recent models of heterogeneous megathrust slip in 1700. The inland extent of modeled tsunami deposition is sensitive to choices of surface roughness and river mouth topography, but rupture models resulting in <1 m of subsidence are ruled out as potential sources for the tsunami that deposited sediment in 1700 CE deposit at the Salmon River. Extensive coring and tsunami sediment transport modeling at other coastal sites in Cascadia, where the 1700 and older tsunami deposits are preserved, will help develop more detailed reconstructions of the size, inundation extent of past tsunamis and amounts of coastal deformation during megathrust earthquakes and improve earthquake and tsunami hazard assessment in Cascadia.