Abstract

Eleven plate-boundary earthquakes over the past 5500 yr have left a stratigraphic signature in coastal wetland sediments at the lower Sixes River valley in south coastal Oregon. Within a 1.8 km2 abandoned meander valley, 10 buried wetland soils record gradual and abrupt relative sea-level changes back in time to ∼6000 yr ago. An additional, youngest buried soil at the mouth of the Sixes River subsided during the A.D. 1700 Cascadia earthquake. Multiple lines of evidence indicate that tectonic subsidence caused soil burial, including permanent relative sea-level rise following burial, lateral continuity of buried soil horizons over hundreds of meters, diatom assemblages showing that sea level rose abruptly at least 0.5 m, and sand deposits on top of buried soils demonstrating coincidence of coseismic subsidence and tsunami inundation. For at least two of the buried soils, liquefaction of sediment accompanied subsidence.

The 11 soil-burial events took place between 300 and ∼5400 yr ago, yielding an average recurrence interval of plate- boundary earthquakes of ∼510 yr. Comparing paleoseismic sites in southern Washington and south coastal Oregon with the Sixes River site for the past 3500 yr indicates that the number and timing of recorded plate-boundary earthquakes are not the same at all sites. In particular, a Sixes earthquake at ∼2000 yr ago lacks a likely correlative in southern Washington. Therefore, unlike the A.D. 1700 Cascadia earthquake, some Cascadia plate-boundary earthquakes do not rupture the entire subduction zone from southern Oregon to southern Washington.

In the lower Sixes River valley, the upper- plate Cape Blanco anticline deforms sediment of late Pleistocene and Holocene age directly above the subduction zone. Differential tectonic subsidence occurred during two of the plate-boundary earthquakes when a blind, upper-plate reverse fault, for which the Cape Blanco anticline is the surface fold, slipped coseismically with rupture of the plate boundary. During these two earthquakes, sites ∼2 km from the anticline axis subsided ∼0.5 m more than sites ∼1 km from the axis.

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