Coastal marsh stratigraphy as an indicator of past earthquakes, Puget Lowland, Washington State
Abstract
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 modern 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.
Collections
- Tsunami Modeling [24]
- Earth and Space Sciences [111]