DIFFERENCES IN CASCADIA GREAT-EARTHQUAKE MAGNITUDES INFERRED FROM TSUNAMI-LAID SAND AND MICROFOSSIL ASSEMBLAGES AT ALSEA BAY, OREGON

Despite its importance to seismic hazard assessment in central western North America, the history of plate-boundary earthquakes at the Cascadia subduction zone is sketchy. Is most plate-boundary slip accommodated during magnitude-9 earthquakes that rupture much of the 1200-km-long plate boundary, or does slip during magnitude-8 earthquakes create a more complex, segmented pattern of plate-boundary rupture? Widespread sheets of sand in stratigraphic sequences of intertidal wetland sediment mark tsunamis generated by Cascadia plate-boundary earthquakes. Assemblages of fossil diatoms and foraminifers above and below the sand sheets may yield measures of the amount of regional coseismic subsidence coincident with each tsunami. Greater subsidence implies larger plate-boundary rupture areas during greater earthquakes.

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.