LIMITS OF PRIOR MEGATHRUST RUPTURE FROM STRATIGRAPHIC AND MICROFOSSIL OBSERVATIONS AT SITKALIDAK ISLAND, AK

Seismic hazard models for Alaska require estimates of the size and frequency of prehistoric megathrust earthquakes. However, observations that place limits on the size of subduction paleoearthquakes along the Alaska-Aleutian subduction zone are scarce. To help place bounds on the along-strike extent of prehistoric Alaska-Aleutian subduction ruptures, we present stratigraphic and microfossil evidence of repeated tsunami inundation over the last ~500 years at Sitkalidak Island, located 0.5 km off the coast of south-central Kodiak Island. Peat cores collected from an estuary in southern Sitkalidak Island reveal three anomalous, laterally continuous sand beds with sharp upper and lower contacts preserved within a coastal peat sequence. The biostratigraphic and lithostratigraphic characteristics of the sand beds, including the presence of anomalous marine planktonic diatoms, high fragmentation of diatoms, and upward fining sand sequences, indicate high-energy marine incursions consistent with tsunami inundation. Radiocarbon dating constrains the deposition of the sand beds to 1964 CE, 1788 CE, and ~400 cal yr B.P. The peat core stratigraphy and dates are consistent with tidal wetland stratigraphic records observed at sites ~90 km to the west at Sitkinak Island, and ~80 km to the east at Middle Bay, Kodiak Island. Preliminary foraminifera and diatom results from Sitkalidak Island suggest decimeter-scale subsidence during the deposition of the 1964 CE sand bed. Deformation concurrent with the 1964 CE rupture along with the presence of sand beds associated with the 1788 CE and ~400 cal yr BP ruptures at Sitkalidak help better define the western (1964 CE and ~400 cal yr BP) and eastern (1788 CE) rupture limits, and thus the permissible maximum magnitudes, of past Alaska-Aleutian subduction zone ruptures.