Mapping and Modeling the Seattle Fault Tsunami Inundation in Puget Sound

Session: Understanding Non-Traditional Seismic Tsunami Hazards [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

Directly under the city of Seattle runs the Seattle fault zone, a fault system with a history of generating major earthquakes, including an earthquake (>Mw 7.0) between 900 – 930 A.D., which generated a destructive tsunami that propagated throughout Puget Sound. While this earthquake and tsunami are now common knowledge, how the tsunami behaved as it progressed through Puget Sound, including what effect the next event would have on infrastructure and communities today is less understood. Coastal marshes provide excellent accommodation space for the preservation of high energy event deposits. This study investigates the coastal marsh, Deer Lagoon, with hopes of better defining the inundation associated with the 900-930 A.D. tsunami. A continuous deposit interpreted as a tsunami deposit was observed near the modern day tidal outlet; it was discontinuous at the far reaches of the marsh. Dating of organic material along the event layer correlates with the last Seattle fault rupture. Tsunami modeling on modern topography suggests flooding heights of ~1.5 m would occur in Deer Lagoon. Removing manmade structures (dikes and bridges) using ArcGIS to recreate the paleoenvironment of the study area better simulates flooding of the marsh prior to anthropogenic alteration, indicating the 900-930 A.D. tsunami inundated Deer Lagoon in a pattern matching the observed event deposit. Additional tsunami modeling from other earthquakes (Cascadia and Aleutian sources) shows potential for these far field earthquakes to inundate the Deer Lagoon marsh, with some modeled Cascadia ruptures producing flooding of ~1.5 m as well, although no additional event deposits were confidently identified in Deer Lagoon stratigraphy. By combining modeling and field data, this study enhances the understanding of how tsunamis initiated by crustal fault ruptures behave in the confined water ways of Puget Sound, as well as how large far field events could flood these coastal environments.

Presenting Author: David Bruce

Authors