Megathrust Paleogeodesy at the Central Cascadia Subduction Zone

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The wetland sediments fringing estuaries at the Cascadia subduction zone harbor a unique record of plate-boundary earthquakes during the past 5,000 years. However, the precision of past measurements of land-level changes at Cascadia is low (errors of greater than plus or minus 0.5 meter), the measurements are spatially limited, and they span only fractions of complete cycles of subduction zone strain accumulation and release. This makes past measurements insufficient for determining which hypotheses of plate-boundary deformation are most valid. This deficiency will be redressed by applying recently developed statistical transfer functions to microfossils, such as foraminifera and diatoms, collected from Cascadia estuarine sediments. Similar studies of sea-level change on other continents have obtained an unprecedented vertical resolution of plus or minus 0.1-0.3 meter. Seasonal and spatial differences in modern foraminfera and diatoms from Cascadia estuaries are assessed and the results are used to improve our microfossil-based transfer functions. Transfer functions are applied to fossil foraminiferal and diatom data at several estuaries to reconstruct land-level changes spanning the four most recent great earthquake cycles in the central Cascadia subduction zone. Detailed lithostratigraphical and biostratigraphical descriptions and radiocarbon dating of estuarine sediment allow correlation of microfossil reconstructions among estuaries and help reconstruct a history of plate-boundary deformation in Oregon. The improved vertical resolution of our reconstructions will: (1) yield more precise measures of Cascadia deformation over time periods of 10 to 100 years; (2) provide critical tests for competing hypotheses of coastal uplift versus subsidence just prior to great earthquakes; (3) help constrain regional models of Cascadia plate-boundary deformation; and (4) directly test hypotheses of slip-predictable, time-predictable, and slip-time-unpredictable strain accumulation proposed for other subduction zones.