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Reconstructing Spatial and Temporal Megathrust Rupture History Using Stratigraphy and Microfossils at Sitkinak Island, Alaska

Description: 

Rupture variability along the Alaska-Aleutian subduction zone complicates seismic hazard assessments at the western extent of the 1964 Mw 9.2 rupture zone. Sitkinak Island, located in the western Kodiak Island archipelago, is well-positioned to study the variability of slip in this region. Previous work identified five episodes of sudden late-Holocene vertical coseismic deformation marked by laterally extensive peat-mud (subsidence) and mud-peat (uplift) contacts, but questions remain about the site-wide continuity of paleoseismic evidence and the amount of land-level change recorded. A multi-core analysis and quantitative approach across diverse depositional settings is needed to corroborate this unique mixed uplift and subsidence record.

Here, we compare stratigraphic and microfossil data from paired sites in different geomorphic environments on Sitkinak Island. In an east-west transect of seven cores, we identified and dated three sharp stratigraphic contacts that indicate one subsidence event (1964 CE) and two uplift events (520–300 and 1050–790 cal yrs BP). Across earthquake contacts we analyzed fossil diatoms, key indicators of salinity changes, and applied a transfer function to statistically relate diatom assemblages to elevations within the tidal frame. Preliminary paleoelevation reconstructions across sharp contacts indicate subsidence of 0.62 ± 0.26 m at 1964 CE, uplift of 0.44 ± 0.25 m at 520-300 cal yrs BP, and uplift of 0.55 ± 0.25 m at 1050-790 cal yrs BP. We find the timing and direction of coseismic land-level change is consistent between two distinctive geomorphic environments in south Sitkinak Lagoon, strengthening our confidence in a mixed uplift and subsidence signal at the site. Ongoing analysis of an additional sediment core will further constrain 1964 CE land-level change estimates. Our quantitative estimates of paleoearthquake rupture behavior will be integrated into rupture models to define slip variability in this region, improving hazard assessments in Alaska.

Session: Linking Subduction Zone Processes and Cascading Hazards in Alaska, Cascadia, Chile and Beyond - II

Type: Oral

Date: 4/16/2026

Presentation Time: 02:15 PM (local time)

Presenting Author: Tabitha Nowak

Student Presenter: Yes

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