226 Quaternary Research to Characterize Environmental and Geological Hazards (Posters)

Quaternary geosciences research features many valuable tools for understanding environmental and geologic hazards. This multidisciplinary session seeks presentations that encompass a wide range of processes and environments relevant to managing critical infrastructure and resources.

Tuesday, 21 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

Improving Paleotsunami Recurrence and Inundation Estimates at the Southern End of the Cascadia Subduction Zone

Paper number: 226-0
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The Cascadia Subduction Zone (CSZ) stretches along the west coast of North America from British Columbia to Northern California. The CSZ has the potential to produce great earthquakes (magnitude >8), posing a threat to millions of people. Along the coast, these earthquakes produce coupled hazards of ground shaking, co-seismic subsidence, and tsunami inundation. Since the most recent CSZ event occurred in 1700 CE, there is no instrumental record of CSZ earthquakes; we rely on paleoseismic studies to understand the characteristics of this hazard. Certain coastal marshes record key information about recurrence intervals, coseismic subsidence, and tsunami inundation.

Our field site, Lagoon Creek (41.6° N), is a rare location in southern Cascadia with a series of well-preserved tsunami deposits. This impounded coastal estuary forms a freshwater pond that sits 300m from the coast, and supports a freshwater marsh inland. Previous work at this site in 1998 completed initial stratigraphic mapping, dating, and microfossil analysis to identify these deposits as records of subduction zone-generated tsunamis. Our revived coring effort at Lagoon Creek is improving event chronology and inundation estimates. We present initial findings from cores collected in 2024 and 2025.

Tsunami deposits at Lagoon Creek consist of dark grey medium-fine sand, 0.25 to 5 cm thick, containing marine diatoms. At most locations in the marsh, the sand is accompanied by an overlying layer of detrital herbaceous material and silty peat. At this site, five sand layers are present in the top four meters of sediment. We have improved age constraints on these deposits with new radiocarbon samples paired with statistical modelling techniques. The updated recurrence estimates (~340 years) are shorter than previously observed at this site (~590 years). Additionally, we have taken dozens of new cores across the marsh in order to determine the extent of tsunami inundation. The 1700CE sand horizon can be traced 1.1km inland, and upcoming analysis will assess microfossil and geochemical tracers of inundation.

We are grateful to the GSA Quaternary Geology & Geomorphology Division, the NSF Cascadia Coastlines & Peoples Hazards Research Hub, and the University of Washington Quaternary Research Center for financial support; and to the USGS Pacific Coastal and Marine Science Center and the CRESCENT Cascadia Paleoseismology Working Group for in-kind support, assistance and advising.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414074

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