PALEOSEISMOLOGY ACROSS THE SHORELINE IN THE DUWAMISH WATERWAY AND ELLIOTT BAY, SEATTLE

The Duwamish Waterway and Elliott Bay, waterbodies adjacent to the city of Seattle, provide opportunities for onshore-offshore paleoseismic studies that may clarify earthquake hazards to the city. Three generalized earthquake sources dominate these hazards: the Cascadia subduction thrust, sources within the subducting Juan de Fuca slab, and crustal faults that include the Seattle Fault Zone, which is thought to cross the study area. A large Seattle Fault Zone earthquake in 900–930 cal yr CE produced a Puget Sound tsunami while elevating some of the shorelines by about 6 m. Here, we build on previous paleoseismic and geophysical studies to present new observations from the Duwamish Waterway and a planned seismic reflection survey through the waterway and Elliott Bay.

Muddy banks along the Duwamish Waterway expose evidence for at least two episodes of earthquake-induced liquefaction since the uplift in 900–930 cal yr CE. The evidence for liquefaction consists of sand dikes and lenses exposed in outcrops of peaty mud. One episode of liquefaction has been bracketed between 1010 and 1150 cal yr CE, and another to some time after 1250 cal yr CE, by radiocarbon analysis of herbaceous fossils in growth position. The ages may aid in determining which of the area’s three generalized earthquake sources best explains each episode of liquefaction, as a guide to shaking hazards within Seattle city limits.

Seismic reflection surveys planned for the summer of 2019 have three goals: (1) map strands of the Seattle Fault and related deformation that cross the Duwamish Waterway; (2) investigate a drowned shoreline as a potential uplift marker to assess whether the fault has produced more than one earthquake in the past 11,000 years; and (3) examine submarine landslides beneath Elliott Bay as potential tsunami sources. The planned profiles are intended to fill an urban gap in seismic reflection coverage between Puget Sound and Lake Washington. We hope to tie seismic reflectors to stratigraphic units observed onshore and in geotechnical borings.