A Lithostratigraphic and Microfossil Investigation into Late Holocene Coseismic Land Level Change and Tsunami Inundation on Nagai Island, Shumagin Islands, Alaska.

Abstract

Cycles of earthquakes and tsunamis are complex, occurring over timescales greater than the instrumental data, therefore, palaeoseismic reconstructions best constrain seismic and tsunami hazards. This research investigates late Holocene great earthquakes (Mw >8.0) and high tsunami inundation (>present-day intertidal zone) on Nagai Island in the Shumagin section of the Alaska-Aleutian subduction zone (AASZ) through palaeoseismic reconstruction of sediment
cores from four sites across Nagai Island. DV.19.03 and 19.DV.17 are selected for detailed lithostratigraphic, diatom, grain size, and chronological analysis. Nagai Island does not exhibit identifiable land level change or sand beds conclusively demonstrating high tsunamis over the last ~2900 years, supporting the geological evidence from nearby Simeonof Island where no land level change >0.3 m or tsunami inundation exceeding the present-day intertidal zone are
identified. Thus, it appears that the Shumagin section of the AASZ has accommodated plate convergence throughout the late Holocene through long term persistent creep, neither
generating a great earthquake nor coseismically weakening from the propagation of a great earthquake from adjacent Semidi or Unimak sections. Further, absence of geological evidence for high tsunami inundation on Nagai Island alongside Simeonof Island suggests that the Shumagin section of AASZ has neither generated a high tsunami nor exhibited deposition of high tsunamis sourced from teletsunamis generated elsewhere along the AASZ or other subduction zones throughout the late Holocene. The geological evidence from Nagai Island combined with Simeonof Island implies low hazard for great earthquakes and high tsunamis generating or propagating into the Shumagin section. This suggests a low hazard for Shumagin sourced tsunamis to far field communities in the Pacific. However, great earthquakes and high tsunamis cannot be ruled out because the geological evidence may not extend to the Shumagin section recurrence intervals of great earthquakes, potential future changes in the locking of the plate interface, and possible missing geological evidence.