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Great Earthquakes and Tsunamis of the Past 2000 Years at the Salmon River Estuary, Central Oregon Coast, USA

Alan R. Nelson, Andrew C. Asquith^* and Wendy C. Grant

U.S. Geological Survey, MS 966
Geologic Hazards Team
PO Box 25046
Denver, Colorado 80225
(A.R.N.)
Department of Geography
University of Southampton
Southampton, SO14 3ZH United Kingdom
(A.C.A.)
U.S. Geological Survey at Department of Earth and Space Sciences
University of Washington
Seattle, Washington 98195
(W.C.G.)

Manuscript received 10 October 2003.

Four buried tidal marsh soils at a protected inlet near the mouth of the Salmon River yield definitive to equivocal evidence for coseismic subsidence and burial by tsunami-deposited sand during great earthquakes at the Cascadia subduction zone. An extensive, landward-tapering sheet of sand overlies a peaty tidal-marsh soil over much of the lower estuary. Limited pollen and macrofossil data suggest that the soil suddenly subsided 0.3-1.0 m shortly before burial. Regional correlation of similar soils at tens of estuaries to the north and south and precise ¹⁴C ages from one Salmon River site imply that the youngest soil subsided during the great earthquake of 26 January A.D. 1700. Evidence for sudden subsidence of three older soils during great earthquakes is more equivocal because older-soil stratigraphy can be explained by local hydrographic changes in the estuary. Regional ¹⁴C correlation of two of the three older soils with soils at sites that better meet criteria for a great-earthquake origin is consistent with the older soils recording subsidence and tsunamis during at least two great earthquakes. Pollen evidence of sudden coseismic subsidence from the older soils is inconclusive, probably because the amount of subsidence was small (<0.5 m). The shallow depths of the older soils yield rates of relative sea-level rise substantially less than rates previously calculated for Oregon estuaries.