Stratigraphic and Microfossil Evidence of Repeated Late Holocene Tsunami Inundation at Sitkalidak Island, AK

Session: Forthcoming Updates of the USGS NSHMs: Hawaii, Conterminous U.S. and Alaska [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

Data on the spatial and temporal variability of prehistoric earthquakes along the Alaska-Aleutian subduction zone remain scarce. Recent studies west of Kodiak Island and the western edge of the 1964 M_w9.2 rupture have used tidal marsh stratigraphy and seismic reflection data to establish the potential of this region to generate giant tsunamis directed at far-field sites such as the west coast of the United States and Hawaii and to question the persistence of proposed subduction rupture segment boundaries. Here, 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, Alaska. Peat cores collected from an estuary in southern Sitkalidak Island reveal three anomalous, laterally continuous sand beds with sharp upper and lower contacts. The sediment properties and diatom assemblages of the sand beds—for example, the upward fining sand sequences, the presence of anomalous marine planktonic diatoms and the high fragmentation of diatoms within the beds—point to a marine sediment source and high-energy deposition consistent with tsunami inundation. Radiocarbon dating constrains the deposition of the sand beds to 1964 CE, 1788 CE and ~400 BP, consistent with records preserved at sites ~90 km to the west (Sitkinak Island) and ~80 km to the east (Middle Bay, Kodiak Island). Despite the lack of significant lithologic changes between the peat under- and overlying the sand beds, preliminary foraminifera and diatom results suggest decimeter-scale subsidence concurrent with the deposition of the 1964 CE sand bed. Deformation concurrent with the 1964 CE rupture as well as sand beds associated with the 1788 CE and ~400 BP ruptures at Sitkalidak help better define the western (1964 CE and ~400 BP) and eastern (1788 CE) limits and thus permissible maximum magnitudes of past Alaska-Aleutian subduction zone ruptures.

Presenting Author: Alexa Prater

Authors