Document Type

Thesis

Date of Degree Completion

Summer 2021

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Breanyn MacInnes

Second Committee Member

Lisa Ely

Third Committee Member

Megan Walsh

Abstract

Puget Sound has a history of earthquakes and tsunamis with an ever-expanding knowledge of these events. The focus of this study is the Seattle fault earthquake and resulting tsunami 1100 years ago. This study aimed at refining the extent of tsunami inundation north of the fault using a two-phased approach: a field study at Elger Bay and tsunami modeling. Tsunami deposits dating to this event have been observed in six sites total sites in Puget Sound, and four of them are in northern Puget Sound. At Elger Bay I found one tsunami deposit in cores from the southwest corner of the marsh, near the inlet, however, 14C dates of charcoal near the deposit range from 1478-1664 cal AD – too young to be from the Seattle fault tsunami. Other possible sources within the last 500 years could be the 1820’s Camano Head landslide, 1700 Cascadia event, or nearby local fault earthquakes, or undiscovered landslides. Environmental analysis of Elger Bay sediments suggests that at the time of the Seattle fault rupture, the site was a tidal lagoon and therefore would have been unlikely to preserve a tsunami deposit.

Tsunami modeling consisted of three earthquake scenarios run at three tide levels for the 4 published northern Puget Sound Seattle fault tsunami deposit sites (West Point, Cultus Bay, Deer Lagoon, the Snohomish delta) and Elger Bay, resulting in 45 total models. All models agreed that the tide needed to be at or near mean higher high water (MHHW) to get inundation at all locations – no significant inundation occurred at NAVD88 (0 m) or mean lower low water (MLLW) datums. At Elger Bay, tsunami wave heights ranged from ~0.6 m to 0.75 m, which is at the threshold of whether it could leave a deposit or not. However, as the field study concluded, even if the tsunami left a deposit in Elger Bay, it would not have been preserved because it was a lagoonal environment. The data and conclusions from this study can be used by hazard mitigation programs such as the Island County Department of Emergency Management to further their knowledge and improve their plans for future tsunami events.

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