MODELS OF RAPID RELATIVE SEA-LEVEL CHANGE IN WASHINGTON AND OREGON, USA

Much geological evidence points to repeated plate-boundary earthquakes on the Cascadia subduction zone, Pacific Northwest USA. Current land motions, calculated using short-term (<100 yr) tide gauge, repeat leve lling and other geodetic data, record deformation during the present i nterseismic period. These short-term data form the basis of efforts to model longer-term plate-boundary deformation and assess seismic hazar d in this region. In this paper we use relative sea-level (RSL) data f rom the last 4000 cal. yr BP to examine four aspects of these plate-bo undary deformation models over several earthquake cycles: rates of iso static rebound, the spatial pattern and magnitude of coseismic deforma tion, and rates of interseismic strain accumulation. Age/altitude plot s of RSL data from a 300-km-long string of sites stretching from Copal is River (central Washington) to Alsea Bay (central Oregon) reveal a s ystematic north-south decline in the rate of isostatic rebound equal t o c. 0.25 +/- 0.02 mm yr(-1) 100 km(-1). High precision microfossil an d RSL data from Johns River (Washington) and Netarts Bay (Oregon) regi ster repeated episodes of late-Holocene coseismic submergence. This is contrary to the coseismic uplift expected, based on differences in up lift:rates calculated using late-Quaternary marine terrace data from t he region. Moreover, the magnitudes of coseismic submergence inferred from these RSL data are typically half the modelled predictions based on current deformation rates. Lastly, we identify evidence from Netart s Bay for pre-seismic RSL rise of up to 0.4 mm yr(-1) in the decades i mmediately preceding each episode of marsh burial. Together, our findi ngs shine new light on models of rapid relative sea-level rise in the Pacific Northwest, and highlight the need to strengthen further the di alogue between long-term geological and short-term geodetic studies in this region.