Reconstructing vertical deformation using stratigraphy and microfossils to infer megathrust rupture history on Sitkinak Island, Alaska

Abstract

Characterizing long-term subduction zone behavior is critical to prepare for future seismic hazards along vulnerable coastlines. The Alaska-Aleutian subduction zone (AASZ) has previously produced devastating multi-section, tsunamigenic earthquakes exhibiting variable rupture patterns that complicate hazard assessments. The incomplete historical record (~200 yr) of earthquakes along the AASZ and the fragmentary paleoseismic archive in the western portion of the megathrust require millennial-scale geologic investigations to resolve uncertainties regarding rupture scenarios and timing. Sitkinak Island, located in the southwest Kodiak Archipelago at the western extent of the 1964 rupture zone, is well-positioned to study the persistence of rupture boundaries through multiple earthquake cycles. Previous work in south Sitkinak Lagoon identified a unique mixed coseismic uplift and subsidence record, marked by sharp, laterally extensive peat-over-mud (subsidence) and mud-over-peat (uplift) contacts in coastal marsh stratigraphy. However, questions remain about possible non-seismic sources of stratigraphic change in south Sitkinak Lagoon. A multi-core analysis and quantitative approach across diverse depositional settings are needed to corroborate previous findings, establish site-wide continuity of paleoseismic evidence, and generate estimates of the amount of land-level change recorded. Here, we evaluate evidence for coseismic deformation at a new coring site in south Sitkinak Lagoon. We infer that four sharp contacts, representing two subsidence and two uplift events, record deformation associated with megathrust earthquakes along the AASZ. Age estimates obtained using 137Cs analysis, radiocarbon dating, and age modeling constrain the timing of these events to 1964 CE and 700-554 cal yrs (subsidence), and 524-482 cal yrs BP and 785-735 cal yrs BP (uplift). Across the sharp contacts, we analyzed fossil diatoms, key indicators of salinity changes, and applied a transfer function to statistically relate fossil diatom assemblages to elevations within the tidal frame. Paleoelevation reconstructions indicate 0.29-0.64 m of deformation across the four interpreted earthquake contacts. The timing and direction of land-level change at our site are consistent with those reconstructed from a previously studied core in a different geomorphic setting in south Sitkinak Lagoon, demonstrating site-wide continuity of paleoseismic evidence across the lagoon and strengthening confidence in a mixed uplift and subsidence history. These results support the interpretation that Sitkinak Island occupies a complex deformation setting near the lateral edge of Kodiak section ruptures. Our quantitative estimates of coseismic land-level change provide new model constraints on rupture behavior and slip variability in this region, improving future earthquake and tsunami hazard assessments in Alaska.