Document Type

Thesis

Date of Degree Completion

Spring 2020

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Breanyn MacInnes

Second Committee Member

Lisa Ely

Third Committee Member

Walter Szeliga

Abstract

Coastal communities of Puget Sound are susceptible to tsunami inundation from multiple sources. Two sources of potential tsunami hazard that threaten the region are earthquakes on crustal faults in the Puget Lowlands, or earthquakes originating on the Cascadia subduction zone. This study investigated two coastal marshes in Puget Sound, Deer Lagoon and Doe-Kag-Wats, for evidence of a paleotsunami record to combine with tsunami modeling, in order to predict how future events could inundate Puget Sound. A deposit, interpreted as a paleotsunami, is traceable thought the marsh stratigraphy near the modern day tidal inlet of Deer Lagoon, a site that has been greatly altered in modern times though the construction of dikes and building of infrastructure. I correlated the deposit to the last Seattle fault rupture based on radiocarbon dating of organic material directly below the layer and sedimentation rates of the marsh. Removing manmade structures (dikes and bridges) using ArcGIS allowed me to simulate tsunami inundation of the marsh prior to anthropogenic alteration, which showed that the Seattle fault tsunami could have inundated Deer Lagoon in a pattern matching the observed event deposit. Tsunami modeling on the modified topography shows severity of inundation in the lagoon would be highly dependent on the tidal cycle at the time of rupture: at mean high water (MHW) flow depths across the lagoon are between 1-2 m, whereas at mean low water (MLW) inundation would not occur. Additional tsunami modeling from two Cascadia sources shows potential for tsunamis initiating outside Puget Sound to inundate the Deer Lagoon marsh up to ~0.5 m at high tide for a large Cascadia source (L1), although no additional event deposits were confidently identified in Deer Lagoon stratigraphy. In Doe-Kag-Wats, I did not identify a tsunami deposit in my 10 cores, although modeling suggests a Seattle fault rupture would have inundated the salt marsh with depths of up to 3.5 m above MHW, and up over 1.0 m at MLW. The Cascadia (L1) source inundates Doe-Kag-Wats with 0.7 m wave heights at MHW. The modeling and field data collected in the study is combined to show the variability of tsunami hazard communities in Puget Sound face. This study shows the variability of tsunami hazards in Puget Sound by combining modeling and field data, which resulted in greater comprehension of tsunami inundation styles and extents. This improved understanding can be applied to community hazard planning, resulting in mitigating loss associated with future tsunamis.

Available for download on Saturday, June 25, 2022

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