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Models of rapid relative sea-level change in Washington and Oregon, USA

I. Shennan

Environmental Research Centre, Department of Geography, University of Durham, Science Site, South Road, Durham DHJ 3LE, UK

Much geological evidence points to repeated plate-boundary earthquakes on the Cas-cadia subduction zone, Pacific Northwest USA. Current land motions, calculated. using shortterm (<100 yr) tide gauge, repeat levelling and other geodetic data, record deformation during the present interseismic period. These short-term data form the basis of efforts to model longer-term plate-boundary deformation and assess seismic hazard in this region. In this paper we use relative sea-level (RSL) data from the last 4000 cal. yr BP to examine four aspects of these plate-boundary deformation models over several earthquake cycles: rates of isostatic rebound, the spatial pattern and magnitude of coseismic deformation, and rates of interseismic strain accumulation. Age/altitude plots of RSL data from a 300-km-long string of sites stretching from Copalis River (central Washington) to Alsea Bay (central Oregon) reveal a systematic northsouth decline in the rate of isostatic rebound equal to c. 0.25 ± 0.02 mm yr^-1 100 krn’. High precision microfossil and RSL data from Johns River (Washington) and Netarts Bay (Oregon) register repeated episodes of late-Holocene coseismic submergence. This is contrary to the coseismic uplift expected, based on differences in uplift rates calculated using late-Quaternary marine terrace data from the 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 Netarts Bay for pre-seismic RSL rise of up to 0.4 mm yr’ in the decades immediately preceding each episode of marsh burial. Together, our findings shine new light on models of rapid relative sea-level rise in the Pacific Northwest, and highlight the need to strengthen further the dialogue between long-term geological and short-term geodetic studies in this region.

Key Words: Earthquake • palaeoseismicity • isostasy • Cascadia subduction zone • biostratigraphy • pollen • diatoms • tide gauge • seismic hazard • sea-level change • Holocene

The Holocene, Vol. 8, No. 2, 129-142 (1998)
DOI: 10.1191/095968398666306493


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