Document Type

Thesis

Date of Degree Completion

Spring 2026

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Breanyn MacInnes

Second Committee Member

Lisa Ely

Third Committee Member

Isabel Hong

Abstract

Puget Sound is a densely populated, tectonically active area that can be affected by earthquakes from local crustal faults and the Cascadia subduction zone capable of producing abrupt coseismic subsidence. Tidal wetlands are particularly sensitive to abrupt changes because they are depositional environments that reside at sea-level and their lithology and ecological communities are strongly controlled by elevation within the tidal frame. As a result, coseismic subsidence can disrupt established environmental gradients and leave distinguishable signatures preserved within the stratigraphy, like tidal mud directly over peat deposits. These preserved records provide an opportunity to reconstruct past earthquakes and associated relative sea-level changes. Paleo-environmental reconstruction using diatom microfossils within wetland stratigraphy can be an especially valuable proxy for assessing past sea-level and earthquake histories due to their distinct ecological niches driven by salinity preferences that correspond to different positions within the wetland where a gradient of conditions, from freshwater high marsh to marine tidal flat conditions, are observed.

This study focuses on Deer Lagoon, Whidbey Island, WA, which lies along the trace of the Southern Whidbey Island Fault (SWIF), located in Puget Sound. The goal of this study is to determine if an observed peat-to-mud transition within the wetland stratigraphy resulted from abrupt coseismic subsidence associated with the SWIF or from gradual relative sea-level rise. The stratigraphy was correlated among ~34 cores across a distance of ~2.5 km. Diatom analysis from three cores shows the transition from a more freshwater (peat) to a more brackish (mud) environment occurs over a few centimeters into the mud, not abruptly at the contact. Radiocarbon dating of samples vary with a wide range between calibrated ages of 1174 and 673 BP, providing a ~500-year age range that show the lithologic transition is not coeval throughout the marsh, suggesting gradual marine transgression is more likely. This research works to improve our capability to parse gradual sea-level changes and coseismic land-level change in Puget Sound and to increase our understanding of the seismic history of Puget Sound in order to make better-informed decisions regarding public safety against geologic hazards.

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