Coastal marsh stratigraphy as an indicator of past earthquakes, Puget Lowland, Washington State

In coastal wetlands fringing Puget Sound lies evidence for large earthquakes 1,100 years ago and subsequent subsidence. This study uses sedimentologic, stratigraphic, and geomorphic tools, as well as tsunami modeling, to determine the impact of these earthquakes and attendant processes. In five shoreline locations along the shores of Puget Sound and Lake Sammamish, sandy deposits record Holocene earthquake events. These records, deposited by tsunamis, sand blows, and debris flows, look similar in many areas but are distinguishable using criteria developed in this study by comparing modem analogs to ancient examples. Accompanying evidence for liquefaction, tsunami, and debris flows are sharp stratigraphic facies changes interpreted as coseismic land-level change. Detailed studies at Gorst and the Skokomish delta reveal over 3 m and 1 m respectively of uplift about 1,100 years ago. At Gorst, land-level change preceded a tsunami and a sandy debris flow, both of which left deposits in the wetlands surrounding Sinclair Inlet, an arm of Puget Sound. These events coincide with an AD 900-930 Seattle fault-zone earthquake. At the Skokomish delta, uplift accompanies evidence for liquefaction and unusual delta morphology. The delta is tilted toward its eastern side, where the Skokomish River channel has remained for at least the last thousand years. Deposition from the stable river mouth has resulted in an eastern intertidal mudflat 2-km wider than the western mudflat. At the Skokomish delta it remains unclear which fault is responsible for abrupt stratigraphic changes and for tilting, but the most likely candidate is the Saddle Mountain fault zone. However there is also a hypothesized fault underlying the delta, expressed by a ridge; in this study, the ridge is interpreted as a paleoshoreline. Both Gorst and the Skokomish delta have experienced at least 1.5 m of submergence since the time of uplift. Eustatic sea-level rise, regional subsidence and possibly subduction-zone earthquakes may be underlying causes of the submergence. The newly mapped paleoseismologic features formed 1,100 years ago and the documented more recent submergence have implications for mapping the extent and estimating the magnitude of earthquakes on shallow crustal faults in this densely populated region.