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Geologic evidence of earthquakes at the Snohomish delta, Washington, in the past 1200 yr

Joanne Bourgeois^*,1 and Samuel Y. Johnson^,2

¹ Department of Geological Sciences, University of Washington, Seattle, Washington 98195, USA
² U.S. Geological Survey, M.S. 966, Box 25046, Denver Federal Center, Denver, Colorado 80225, USA

Exposed channel banks along distributaries of the lower Snohomish delta in the Puget Lowland of Washington reveal evidence of at least three episodes of liquefaction, at least one event of abrupt subsidence, and at least one tsunami since ca. A.D. 800. The 45 measured stratigraphic sections consist mostly of 2–4 m of olive- gray, intertidal mud containing abundant marsh plant rhizomes. The most distinctive stratigraphic unit is a couplet comprising a 0.5–3-cm-thick, laminated, fining-upward, tsunami-laid sand bed overlain by 2–10 cm of gray clay. We correlated the couplet, which is generally 2 m below the modern marsh surface, across an 20 km² area. Sand dikes and sand-filled cracks to 1 m wide, which terminate upward at the couplet, and sand volcanoes preserved at the level of the sand bed record liquefaction at the same time as couplet deposition. Differences in the type and abundance of marsh plant rhizomes across the couplet horizon, as well as the gray clay layer, suggest that compaction during this liquefaction led to abrupt, local lowering of the marsh surface by as much as 50–75 cm. Radiocarbon ages show that the tsunami and liquefaction date from ca. A.D. 800 to 980, similar to the age of a large earthquake on the Seattle fault, 50 km to the south.

We have found evidence for at least two, and possibly as many as five, other earthquakes in the measured sections. At two or more stratigraphic levels above the couplet, sand dikes locally feed sand volcanoes. Radiocarbon ages and stratigraphic position suggest that one set of these dikes formed ca. A.D. 910–990; radiocarbon ages on a younger set indicate a limiting maximum age of A.D. 1400–1640. We also interpret a sharp lithologic change, from olive-gray, rhizome-rich mud to grayer, rhizome-poor mud, 1 m above the couplet, to indicate a second abrupt lowering of the marsh surface during an earthquake ca. A.D. 1040– 1400, but no conclusive liquefaction structures have been identified at this horizon. Two distinctive coarse-sand laminae, 30–80 cm below the couplet, may record tsunamis older than A.D. 800.

Thus, study shows that in the past 1200 yr, this part of Washington's Puget Lowland has been subjected to stronger ground shaking than in historic times, since ca. 1870.

Key Words: earthquakes • paleoseismicity • Puget Lowland • sedimentology • stratigraphy

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