DIFFERENCES IN CASCADIA GREAT-EARTHQUAKE MAGNITUDES INFERRED FROM TSUNAMI-LAID SAND AND MICROFOSSIL ASSEMBLAGES AT ALSEA BAY, OREGON
Alsea Bay, on the Oregon coast about midway between Washington and California, is an important site for reconstructing Cascadia's earthquake history because the nearest comparable stratigraphic records of subsided wetlands and tsunami-laid sand lie >60 km to the north and south. In a 2-km-long marsh on the eastern edge of Alsea Bay, four widespread sheets of sand, dated at about 0.3, 0.8, 1.3, and 1.6 ka, cover peaty middle- and high-marsh deposits. The thickness and lateral extent of the four sand sheets suggest that they were deposited by Cascadia-generated tsunamis. Three of the sheets are overlain by muddy deposits like those of the low marsh or tide flat, but the lithology of peat above and below the 0.8-ka sheet suggests little subsidence or uplift at the time of the tsunami. Transfer functions derived from changes in the proportions of the diatoms and foraminifers in samples from above and below the sand sheets suggest about 0.4 m of sudden marsh subsidence coincident with the AD 1700 tsunami, but indicate little permanent land-level change at about the times of at least two of the other three tsunamis. The minimal land-level changes may be due to segmented plate-boundary ruptures that were largely north, south, or seaward of Alsea Bay.