Coastal subsidence in Oregon,USA, during the giant Cascadia earthquake of AD 1700 |
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| Authors: | AD Hawkes BP Horton AR Nelson CH Vane Y Sawai |
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| Institution: | 1. Department of Geological Sciences, University of Canterbury, PB 4800, Christchurch 8140, New Zealand;2. United States National Park Service, Geologic Resources Division, P.O. Box 25287, Denver, CO, USA;3. Institute of Geological and Nuclear Sciences, PO Box 30368, Lower Hutt, New Zealand;1. International Research Institute of Disaster Science, Tohoku University, 980-8579, Japan;2. Institute of Geology and Geoinformation, Geological Survey of Japan, AIST, Tsukuba Central 7, Tsukuba, Ibaraki 305-8567, Japan;3. School of Biological, Earth and Environmental Sciences, UNSW Australia, Sydney, NSW 2052, Australia;4. Institute for Environmental Research, Australian Nuclear Science and Technology Organisation, Kirrawee DC, NSW 2232, Australia;5. US Geological Survey, Santa Cruz, CA 95060, USA |
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| Abstract: | Quantitative estimates of land-level change during the giant AD 1700 Cascadia earthquake along the Oregon coast are inferred from relative sea-level changes reconstructed from fossil foraminiferal assemblages preserved within the stratigraphic record. A transfer function, based upon a regional training set of modern sediment samples from Oregon estuaries, is calibrated to fossil assemblages in sequences of samples across buried peat-mud and peat-sand contacts marking the AD 1700 earthquake. Reconstructions of sample elevations with sample-specific errors estimate the amount of coastal subsidence during the earthquake at six sites along 400 km of coast. The elevation estimates are supported by lithological, carbon isotope, and faunal tidal zonation data. Coseismic subsidence at Nehalem River, Nestucca River, Salmon River, Alsea Bay, Siuslaw River and South Slough varies between 0.18 m and 0.85 m with errors between 0.18 m and 0.32 m. These subsidence estimates are more precise, consistent, and generally lower than previous semi-quantitative estimates. Following earlier comparisons of semi-quantitative subsidence estimates with elastic dislocation models of megathrust rupture during great earthquakes, our lower estimates for central and northern Oregon are consistent with modeled rates of strain accumulation and amounts of slip on the subduction megathrust, and thus, with a magnitude of 9 for the AD 1700 earthquake. |
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