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Reconstructing Vertical Deformation Using Stratigraphy and Microfossils to Infer Megathrust Rupture History on Sitkinak Island, Alaska

Description: 

Historical records of great (Mw>8.0) earthquakes and tsunamis along the Alaska-Aleutian Subduction Zone (AASZ) are too short to forecast future events. Does the recent tectonic history of the subduction zone reflect long-term rupture patterns? Coastal marsh stratigraphy can provide centennial to millennial-scale archives of abrupt coseismic vertical deformation from past earthquakes and give clues about long-term rupture patterns. Quantitative estimates of coseismic vertical deformation can be obtained using diatom-based transfer functions (TFs). Diatoms are siliceous, unicellular algae that are sensitive to salinity, restricting species to vertical elevation zones. TFs statistically relate modern diatom assemblages to tidal elevation and can be applied to fossil diatom assemblages within the sedimentary record to reconstruct paleo-elevations. Recent work on Sitkinak Island, located at the western limit of the 1964 CE rupture boundary in southwest Kodiak Island, utilized qualitative diatom analyses and indicated a mixed record of coseismic uplift and subsidence suggesting a non-persistent rupture boundary. Here, we present additional stratigraphic and microfossil evidence of a series of great earthquakes on Sitkinak Island. We described stratigraphy from a low-energy coastal marsh to the east of the previous Sitkinak lagoon study site. We collected a 130 cm-long sediment core that was representative of the site stratigraphy. We identified two peat-mud contacts indicating subsidence, and three mud-peat contacts indicating uplift, that radiocarbon and Cs137 analyses suggest are consistent with previously identified earthquakes at Sitkinak dated to 1964 CE and 640-510 cal yrs BP (subsidence), and 290-0, 520-300, and 1050-790 cal yrs BP (uplift). Ongoing work will apply the diatom-based TF across each of the earthquake contacts to generate quantitative estimates of vertical deformation. Our data will be used to help constrain past rupture models and improve our knowledge of slip variability along the AASZ over time.

Session: Temporally Variable Records of Earthquake Behavior and Considerations for Seismic Hazard Analyse [Poster]

Type: Poster

Date: 4/17/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: Tabitha

Student Presenter: Yes

Invited Presentation: 

Poster Number: 34

Authors