High-resolution CHIRP survey offshore Seattle, Washington

Abstract:

A new CHIRP seismic reflection survey images the skin of deposits offshore Seattle’s urban waterfront. The survey, collected in October 2019, consists of about 180 km of lines within a 22 sq km area, capturing the top meters of sediments on the floor of Elliott Bay and urbanized remnants of the Duwamish River delta. Several processes influenced deposition of these sediments, including multiple glaciations, active tectonics, volcanism, river delta processes, and extensive human modification. Local geologic history provides context for these processes: Repeated ice cover during Pleistocene glaciations deposited a drift plain composed of lenses of tills, sands, and silts, which was dissected by sub-glacial meltwater. Resulting incisions hold Elliott Bay and the Duwamish River valley. Lahar sediment from Mount Rainier prograded the Duwamish delta through the valley to its current position, which crosses an active crustal fault. This fault, now called the Seattle Fault Zone, produced a M ~7–7.5 earthquake 900-930 CE that raised the delta front ~6m and triggered landslides. Most recently, human activity dramatically altered the landscape. Colonists sluiced hillsides around the Seattle waterfront into Elliott Bay to accommodate urban development and dredged a straight channel through the delta to form the Duwamish Waterway. The new survey is a high-resolution sample of the shallowest of these deposits, imaging delta sediments atop the fault zone, clinoforms at the delta front, landslide scarps around bay margins, and features with glacial morphology on the bay floor.