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Evidence for earthquake-induced subsidence about 1100 yr ago in coastal marshes of southern Puget Sound, Washington

¹ Department of Geological Sciences and U.S. Geological Survey, University of Washington, Box 351310, Seattle, Washington 98195, USA

Buried forest and high marsh soils indicate abrupt changes in relative sea level at four coastal localities in southern Puget Sound. At Little Skookum Inlet and Red Salmon Creek, Douglas fir stumps in growth position are buried by salt-marsh peat. At localities along McAllister Creek and the Nisqually River, high marsh soils are buried by tidal-flat mud. Localized liquefaction coincided with submergence of the high marsh soil at McAllister Creek.

Dramatic changes in seed and diatom assemblages across these contacts confirm rapid submergence. At Little Skookum Inlet and Red Salmon Creek, salt-marsh peat immediately above a buried forest soil contains diatoms indicative of low marsh and tidal-flat environments. At McAllister Creek and Nisqually River, low-marsh and tidal-flat diatoms are abundant in laminated mud directly over high marsh peat. Inferences from modern analogs indicate at least 1 m of subsidence at each site and possibly up to 3 m at Skookum Inlet.

Abrupt burial of lowland soils in southern Puget Sound is best explained by coseismic subsidence. Some of the submergence may be the result of coseismic compaction and postearthquake settlement. Widespread buried soils, large amounts of subsidence, coeval submergence across a wide area, and ground shaking at the time of subsidence all point to a large earthquake between 1150 and 1010 cal yr B.P. in southern Puget Sound as the most likely cause of subsidence.

Key Words: diatoms • earthquakes • paleoseismology • Puget Sound • subsidence

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