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

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 buri ed by salt-marsh peat. At localities along McAllister Creek and the Nisqual ly River, high marsh soils are buried by tidal-flat mud. Localized liquefac tion coincided with submergence of the high marsh soil at McAllister Creek. Dramatic changes in seed and diatom assemblages across these contacts confi rm rapid submergence. At Little Skookum Inlet and Red Salmon Creek, salt-ma rsh peat immediately above a buried forest soil contains diatoms indicative of low marsh and tidal-flat environments. At McAllister Creek and Nisquall y River, low-marsh and tidal-flat diatoms are abundant in laminated mud dir ectly over high marsh peat. Inferences from modern analogs indicate at leas t 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 coseism ic compaction and postearthquake settlement. Widespread buried soils, large amounts of subsidence, coeval submergence across a wide area, and ground s haking at the time of subsidence all point to a large earthquake between 11 50 and 1010 cal yr B.P. in southern Puget Sound as the most likely cause of subsidence.