A LARGE EARTHQUAKE OCCURRING 700-800 YEARS AGO IN AIALIK-BAY, SOUTHERN COASTAL ALASKA

A conifer forest on the shore of Verdant Cove, an inlet of Aialik Bay on the southeast coast of the Kenai Peninsula, Alaska, was buried by h igh-energy beach sediments shortly after 860 +/- 50 C-14 years BP. The switch from ocean-distal forest to cobble beach indicates a radical c hange in depositional environment suggestive of rapid subsidence of 1- 3.5 m. The presence of hemlock, a tree taxon sensitive to salt-water e xposure, and the preserved cast of a tree trunk suggest that subsidenc e and burial occurred rapidly. By 690 +/- 60 C-14 years BP, forest pea t was accumulating atop the beach sediments burying the forest and rap id spit progradation was underway. Spit progradation implies land emer gence or stable sea level, especially in this case where the spit has a limited sediment supply. We infer that subsidence of the spit ca. 86 0 +/- 50 C-14 years BP was followed by slow land emergence up to the t ime of the 1964 AD great earthquake, when the area subsided coseismica lly 1.4 m. The sudden drowning of the buried forest in Verdant Cove wa s probably caused by coseismic subsidence and later emergence by gradu al, interseismic uplift. Comparison of the Verdant Cove record with pr eviously reported data from south-central Alaska suggests that the pen ultimate great earthquake in the rupture zone of the 1964 Alaska earth quake occurred between 700 and 800 calendric years BP.