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Refined Timing and Estimates of Coseismic Subsidence at the Southern Cascadia Subduction Zone: Combining Modern Dendrochronology, Age Modeling, and Relative Sea-level Reconstruction Techniques in the Eel River Valley, CA

Description: 

Geologic evidence from tsunamis and great earthquakes in the Cascadia Subduction Zone is observed at dozens of sites along the coasts of California, Oregon, Washington, and British Columbia. Paleoseismic data at the southernmost of these sites, the Eel River valley, is key to determining the along-strike heterogeneity in rupture characteristics during the most recent, 1700 CE, and older subduction zone earthquakes. Three decades ago, paleoseismic data was first documented at the lower Eel River valley, but conventional dating techniques and microfossil-based paleoenvironmental reconstructions provided coarse estimates on the timing and amount of coseismic subsidence. These coarse estimates place limited confidence on correlations between the Eel River event stratigraphy and more precise chronologies at sites further north. Recent advancements in age-depth modeling, dendrochronology, and paleo-environmental reconstructions provide the opportunity to reexamine the tidal wetland sequences at the Eel River valley.

To improve estimates of earthquake timing and coseismic subsidence, we apply a radiocarbon sampling strategy that includes inter-event stratigraphy, construct multiple Bayesian age-depth models, use a fossil foraminifera Bayesian transfer function, and employ dendrochronological dating on tree roots. We reoccupied previously described sites to collect new sediment cores. We present preliminary results based on 15 radiocarbon ages and changes in fossil foraminifera assemblages across two mud-over-peat contacts. In-growth-position Sitka spruce root wads emerge from cutbanks at two sites along the Eel River. These root wads are just below the shallowest subsidence contact, which is capped by silty sand. We collected 25 root samples to obtain tree ring widths and stable oxygen isotope data to correlate between individual trees at the two sites, crossdate to nearby old-growth redwood records and potentially determine the calendar year-of-death. Refined earthquake ages and more precise coseismic subsidence estimates will advance understanding of past Cascadia earthquake ruptures and their ensuing hazards.

Session: From Faults to Fjords: Earthquake Evidence in Terrestrial and Subaqueous Environments [Poster Session]

Type: Poster

Date: 5/1/2024

Presentation Time: 08:00 AM (local time)

Presenting Author: SeanPaul

Student Presenter: No

Invited Presentation: 

Authors