Earthquakes, ecological change, and human modification: records in sediments of the Duwamish delta at Seattle, Washington

Sediments at the Duwamish delta record natural hazards, ecological change, and human modification of the landscape. We examine these deposits in former tidelands and beneath Elliott Bay to study records of earthquakes and set the geologic backdrop from which other influences can be distinguished. The delta crosses the Seattle Fault Zone, which has produced large earthquakes in recent millennia. Despite extensive study and substantial seismic hazard to Seattle and the surrounding region, the location, structure, and frequency of earthquakes in the Seattle Fault Zone is poorly-known, in part because the faults are obscured by complicated overlying sediments. To study these sediments, we collected an ultra-high-resolution shallow seismic reflection survey in 2019, which images the top meters of sediment on the seafloor in Elliott Bay. We are pairing this new dataset with observations from mud on the banks of the Duwamish Waterway, topographic and bathymetric data from 19th-century maps created prior to extensive development, and geologic boreholes collected during modern excavations. Boreholes and prior studies show that delta and seafloor sediments record glaciation, impounded fresh- and salt-water lakes, and lahar runout from Mount Rainier. New results catalog wetland succession at the delta and liquefaction likely produced by strong ground shaking during earthquakes. These results contribute to descriptions of local geology and the interplay of sediments and tectonics amongst ecology and people