Geology along the Duwamish waterway, south of downtown Seattle, provides preliminary clues to the city’s earthquake and tsunami hazards. The banks of the dredged waterway expose the muddy deposits of an estuary that formerly drained Mount Rainier. Different outcrops expose evidence for former events. So far, inferred events include two episodes of liquefaction and two unusual floods from land or sea. All events postdate the large Seattle fault earthquake of 900—930 CE.
Evidence for liquefaction consists of dikes and extrusive lenses of andesitic sand among muddy tidal deposits. This sand was likely vented from lahar runout deposits that underlie the tidal mud. One prominent outcrop on the east side of the waterway displays coalesced sand lenses, which are up to 12cm thick, bulbous, and intersected by parallel, mostly vertical sand dikes. One sand lens drapes Triglochin maritima leaf bases in growth position, which dates to 1010—1150 CE. Later liquefaction is evidenced by a dike which approached the stratigraphic level of T. maritima leaf bases dated to 1250—1290 CE. Two dikes at a site on Kellogg Island were observed no higher than Bolboschoenus sp. corms dated to 1470—1640 CE. Though additional sand dikes have yet to be dated, none of the dikes observed are likely to be as young as the 1700 Cascadia earthquake or any of the historical Puget Sound earthquakes of 1949, 1965, or 2001.
Two persistent, horizontal silt layers observed in one outcrop suggest unusual flooding, either from Puget Sound or from upriver. Radiocarbon ages of T. maritima and Bolboschoenus sp. limit the times of this flooding to 1030–1180 CE and 1320–1400 CE.
These preliminary findings may yield insights into Seattle’s earthquake and tsunami hazards, after further work including comparison with histories of earthquakes and tsunamis elsewhere in the region.||en_US