Two Seattle Earthquakes: Evidence From the Duwamish Waterway

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

The Duwamish Waterway in the city of Seattle provides clues to the city’s earthquake hazards. The industrialized banks of the waterway locally expose intertidal mud of the former Duwamish River estuary. These deposits contain evidence for two earthquakes in the centuries since the large Seattle Fault earthquake of 900–930. (All age ranges are in calibrated years CE at two standard deviations).

The evidence consists of intrusions and extrusions of andesitic sand within the mud. These sand bodies were likely produced by earthquake-induced liquefaction of the Mount Rainier lahar-runout deposits that underlie the mud. The earlier of the two inferred earthquakes is evidenced by a sand layer that extends discontinuously for at least 50 m. This layer consists of coalesced sand lenses up to 12 cm thick. The layer coincides with the upper limit of parallel, mostly vertical sand intrusions interpreted as evidence for lateral spreading. The implied earthquake likely dates to 1010–1150, as judged from radiocarbon ages of growth-position Triglochin maritima leaf bases that one of the sand lenses drapes. At least one later earthquake is evidenced by dikes in younger mud. One of these dikes approaches the stratigraphic level of T. maritima leaf bases from 1250–1290. Other dikes nearby were traced up to Bolboschoenus maritimus corms from 1470–1640. None of the injected or erupted sand bodies observed thus far reach the likely stratigraphic level of the 1700 Cascadia earthquake or the Puget Sound earthquakes of 1949, 1965, and 2001.

I am grateful for guidance by Brian Atwater, Juliet Crider, and Carrie Garrison-Laney; and for diatom analysis by Eileen Hemphill-Haley.

Presenting Author: Elizabeth J. Davis

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