COASTAL SUBSIDENCE IN OREGON, USA, DURING THE GIANT CASCADIA EARTHQUAKE OF AD 1700

Quantitative estimates of land-level change during the AD1700 giant Cascadia earthquake along the Oregon coastline are inferred from relative sea-level changes reconstructed from fossil foraminiferal assemblages preserved within the stratigraphic record. We apply a transfer functions based upon a regional training set of surface sediment samples from estuaries along the Oregon coast. The transfer function is calibrated to fossil assemblages across sharp peat-mud and peat-sand contacts to provide paleomarsh elevations and associated sample-specific errors. The elevation estimates are supported by lithological, carbon isotope, and faunal and floral zonation data. Coseismic subsidence from Nehalem River, Nestucca River, Salmon River, Alsea Bay, Suislaw River and South Slough varies between 0.18 m and 0.85 m with associated errors of 0.18 m to 0.32 m. The quantitative estimates with increased precision compared to previous studies have implication for predictions of coseismic subsidence from elastic dislocation models, particularly in north-central Oregon. Our estimates constrain strain accumulation to 550 yrs or less with equivalent slip on the megathrust between 10 and 30 m, which is consistent with the M9 earthquake for the AD 1700 earthquake.