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 ~400 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 microfossil 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 AD 1964, AD 1788, 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. Diatom results from Sitkalidak Island suggest decimeter-scale subsidence during the deposition of the 1964 CE and 1788 CE sand bed. Deformation concurrent with the 1964 and 1788 ruptures along with the presence of a sand bed associated with the ~400 cal yr BP rupture at Sitkalidak help better define the western and eastern rupture limits, and thus the permissible maximum magnitudes, of past Alaska-Aleutian subduction zone ruptures. / Master of Science / The Alaska-Aleutian subduction zone experiences frequent great earthquakes over magnitude 8 which often produce far-reaching tsunamis. Seismic hazard models that help coastal communities predict and prepare for future hazards require estimates of the size and frequency of prehistoric earthquakes. Data for prehistoric earthquake events in the western region of the subduction zone is scarce. To help address the lack of prehistoric data and understand the rupture path and magnitude of past Alaska-Aleutian earthquakes, we present stratigraphic and microfossil evidence of repeated tsunami inundation over the last ~400 years at Sitkalidak Island, located 0.5 km off the coast of south-central Kodiak Island, Alaska. Sediment cores collected from a marsh in southern Sitkalidak Island record three laterally continuous sand beds with sharp stratigraphic contacts preserved within a coastal peat. Diatom microfossil and grain-size characteristics of the sand beds indicate high-energy marine incursions consistent with chaotic tsunami inundation. Radiocarbon dating places the age of sand bed deposition to AD 1964, AD 1788, and ~400 cal yr B.P. The sediment core stratigraphy and dating correlates well with sites investigated ~90 km to the west and ~80 km to the east. Diatom results from Sitkalidak Island point to small, decimeter scale coseismic subsidence during the deposition of the AD 1964 and AD 1788 sand beds. Land-level change concurrent with the 1964 and 1788 earthquakes along with the presence of a sand bed associated with the ~ 400 cal yr B.P. earthquake found at Sitkalidak Island help better define the earthquake rupture limits and maximum magnitudes of past Alaska-Aleutian subduction zone earthquakes.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/105104 |
| Date | 28 September 2021 |
| Creators | Prater, Alexa Brianne |
| Contributors | Geosciences, Dura, Tina, Spotila, James A., Romans, Brian W. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Coverage | Alaska |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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