TSUNAMI DEPOSITS AND TSUNAMI MODELING OF THE 900 AD SEATTLE FAULT EVENT IN NORTHERN PUGET SOUND

About 1,100 years ago, an earthquake on the Seattle fault generated a tsunami that flooded low-lying areas in at least six known sites in eastern Puget Sound, as far north as Livingston Bay on Camano Island. These few paleotsunami field sites begin to illustrate the hazard posed by a rupture of the Seattle fault, yet they do not define the extent of coastline inundated by that tsunami. Understanding the behavior of the 900 AD tsunami will provide significant knowledge that can be used in hazard planning throughout Puget Sound.

New investigations of paleotsunami deposits were made at Elger Bay, a location lying between sites with known Seattle fault tsunami deposits and sites without deposits. A sand layer near the inlet of the bay is interpreted as being from a tsunami, although a calibrated radiocarbon date of 1478-1664 AD from charcoal below the deposit rules out the Seattle fault as the depositing event. Modeling of tsunamis from published source models of the Seattle fault earthquake using GeoClaw agrees with conclusions that this event likely did not flood Elger Bay unless the earthquake occurred at or near high tide. The Seattle fault tsunami was also simulated to other northern Puget Sound sites where deposits are known to exist in (West Point, Deer Lagoon, Snohomish Delta, Cultus Bay, Priest Point, and Livingston Bay) to validate the models. The date from the sand layer could indicate 1700 Cascadia as a potential source. However, tsunami modeling of a Cascadia M2 earthquake source (representing the 1700 earthquake rupture) at Elger Bay preliminarily shows tsunami runup comparable to the Seattle fault runup, perhaps ruling out Cascadia as the source since this event is considered to have occurred at low tide.