Alsea Bay, Oregon Paleoseismology

Alan R. Nelson (U.S. Geological Survey, MS 966, Box 25046, Denver CO 80225),
Anne E. Jennings, Linda D. Gerson (INSTAAR, University of Colorado, Boulder CO 80309), and
Brian L. Sherrod (U.S. Geological Survey at Dept. of Geological Sciences, University of Washington, Seattle WA 98195)

Reference:
Nelson, Alan R., Jennings, Anne E., Gerson, Linda D., Sherrod, Brian L., 2000, Differences in great-earthquake rupture extent inferred from tsunami-laid sand and foraminiferal assemblages beneath tidal marshes at Alsea Bay, Oregon: Geological Society of America**Abstracts with Programs, v. 32, no. 7.

ABSTRACT

Despite general agreement that the Cascadia subduction zone has generated earthquakes as large as magnitude 9, the recurrence and magnitude of great plate-boundary earthquakes remain important issues in seismic hazard assessment in central western North America. Is all plate-boundary slip accommodated during M9 earthquakes that rupture much of the 1200-km-long plate boundary, or do earthquakes closer to M8 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 and assemblages of foraminfers above and below the sheets yield measures of the amount of regional coseismic subsidence coincident with each tsunami. The larger the earthquake, the greater the rupture area and amount of coseismic subsidence.

Alsea Bay, on the Oregon coast about midway between Washington and California, is a critical site for reconstructing the earthquake history of the Cascadia plate boundary because the nearest comparable stratigraphic records of subsided wetlands and tsunami-laid sands 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, 1.0, 1.0-1.5, and 1.6 ka, cover peaty middle and high marsh deposits. The thickness and lateral extent of all four sand sheets strongly suggest Cascadia-generated tsunamis. Three of the sheets are overlain by muddy deposits like those of the low marsh or mudflat, but peat above and below the 1.0-ka sand sheet suggests little change in intertidal environment at the time of the tsunami. In contrast, transfer functions derived from changes in the proportions of the foraminifers in samples from above and below the sand sheets suggest about 0.7+/-0.2 m of sudden marsh subsidence coincident with the 1.0-ka tsunami, but little land-level change about the times of two of the other three tsunamis. The minimal land-level changes may be due to plate-boundary ruptures that were largely north, south, or seaward of Alsea Bay.

Cascadia subduction zone

Cascadia subduction zone (from Atwater et al., 1995 and Clague, 1997). Pink dots show sites with tsunami deposits that are probably correlative with some of those at Alsea Bay.

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