SUBSIDENCE WITHIN THE PAST 600 YEARS AT PUGET SOUND, WASHINGTON

Four intertidal marshes at southern Puget Sound show evidence for subsidence that postdates the large earthquakes about 1100 years ago on the Seattle, Tacoma, and Olympia faults.

Three of the subsided sites, southwest of Bremerton, coincide with previously identified areas of coseismic uplift from 1100 years ago (Burley, Lynch Cove, and Gorst). The fourth subsided site, near Olympia, may have subsided 1100 years ago as well (Mud Bay).

The subsidence common to all four sites changed a freshwater marsh or forest into a tidal marsh. The change is marked by a sharp contact between peat or upland soil and an overlying unit of peaty mud or muddy peat. This contact typically spans less than 1 mm.

The environmental change is further marked by snags of western redcedar (Thuja plicata). At all four sites, snags with diameters of 1-2 m stand in growth position and are rooted in the buried soil that marks the subsidence. The snags differ distinctly from a fringe of smaller dead and dying trees at the modern forest edge. Unlike the buried soil and the large snags, this fringe records submergence that is gradual and ongoing.

The subsidence at the four sites predates 19th-century maps that show tidal marsh in the areas of the large redcedar snags. The subsidence may have occurred around AD 1700 because, in preservation, the snags resemble ones along the Pacific coast that died in the first years after the AD 1700 earthquake. However, at Lynch Cove the subsidence probably predates AD 1420-1640. This age is a two-sigma range corresponding to a radiocarbon age of rhizomes of the salt-marsh grass Distichlis spicata from above the contact that marks the subsidence.

As potential causes of this subsidence, I am considering isostatic loading from uplift 1100 years ago, seismic slip on crustal faults, and seismic or postseismic slip from the 1700 Cascadia earthquake. My questions include: Can gradual isostatic depression produce the abrupt contact that marks the subsidence, and could it extend to Mud Bay? Which crustal faults are capable of reversing uplift documented at Burley and Lynch Cove (and possibly at Gorst)? Do the dated rhizomes at Lynch Cove rule out correlation with the 1700 Cascadia earthquake?