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

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 pla nt rhizomes. The most distinctive stratigraphic unit is a couplet comprisin g a 0.5-3-cm-thick, laminated, fining-upward, tsunami-laid sand bed overlai n by 2-10 cm of gray clay. We correlated the couplet, which is generally si milar to2 m below the modern marsh surface, across an similar to 20 km(2) a rea. 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 r ecord liquefaction at the same time as couplet deposition. Differences in t he type and abundance of marsh plant rhizomes across the couplet horizon, a s well as the gray clay layer, suggest that compaction during this liquefac tion 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 fa ult, 50 km to the south. We have found evidence for at least two, and possibly as many as five, othe r earthquakes in the measured sections. At two or more stratigraphic levels above the couplet, sand dikes locally feed sand volcanoes. Radiocarbon age s 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 maximu m age of A.D. 1400-1640. We also interpret a sharp lithologic change, from olive-gray, rhizome-rich mud to grayer, rhizome-poor mud, similar to1 m abo ve the couplet, to indicate a second abrupt lowering of the marsh surface d uring an earthquake ca. A.D. 1040-1400, but no conclusive liquefaction stru ctures have been identified at this horizon. Two distinctive coarse-sand la minae, 30-80 cm below the couplet, may record tsunamis older than A.D. 800. Thus, study shows that in the past similar to 1200 yr, this part of Washing ton's Puget Lowland has been subjected to stronger ground shaking than in h istoric times, since ca. 1870.