A Large Earthquake Occuring 700 to 800 Years Ago in Aialik Bay, Southern Coastal Area

Mann, Daniel; Crowell, Aron

A conifer forest on the shore of Verdant Cove, an inlet of Aialik Bay on the southeast coast of the Kenai Peninsula, was buried by high-energy beach sediments shortly after 860 ± 50 ‘4C BP. The switch from ocean-distai forest to cobble beach indicates a radical change in depositional environment suggestive of rapid subsidence of 1-3.5 meters. The presence of hemlock, a tree taxon sensitive to salt-water exposure, and the preserved cast of a tree trunk suggest that subsidence and burial occurred rapidly. By 690±60 ‘4C BP, forest peat was accumulating atop the beach sediments burying the forest and rapid spit progradation was underway. Spit progradation implies land emergence or stable sea level, especially in this case where the spit has a limited sediment supply. We infer that subsidence of the spit ca. 860 ± 50 ‘4C BP was followed by slow land emergence up to the time of the 1964 AD great earthquake, when the area subsided coseismically 1.4 m. The sudden drowning of the buried forest in Verdant Cove was probably caused by coseisrnic subsidence and later emergence by gradual, interseismic uplift. Comparison of the Verdant Cove record with previously reported data from south-central Alaska suggests that the penultimate great earthquake in the rupture zone of the 1964 Alaska earthquake occurred between 700 and 800 cal BP.