A large earthquake occurring 700–800 years ago in Aialik Bay, southern coastal Alaska

Abstract

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 high-energy beach sediments shortly after 860 ± 50 I4C years BP. The switch from ocean-distal forest to cobble beach indicates a radical change in depositional environment suggestive of rapid subsidence of 1–3.5 m. 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 14C years 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 14C years 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 coseismic 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 calendric years BP.

Seismic and non-seismic influences on coastal change in Alaska--Fieldtrip guide and conference abstracts, 5th International Conference of IGCP 588

“…Here, building on the original study of Mann and Crowell (1996), we infer that scarps on the spit record the shoreline response to coseismic subsidence during the 1964 earthquake and two earlier events. (Plafker, 1969).…”

“…The coastward-most escarpment forms the seaward side of the active beach ridge and the two landward escarpments separate the composite strand plain into three surfaces. Mann and Crowell (1996) reported the burial of a conifer forest by high-energy beach deposits in Verdant Cove about 800 years ago. Evidence included stumps of mountain hemlock, a salt-intolerant tree, buried below tide level exposed in a tidal pond on Verdant Cove spit.…”

Beach ridges as paleoseismic indicators of abrupt coastal subsidence during subduction zone earthquakes, and implications for Alaska-Aleutian subduction zone paleoseismology, southeast coast of the Kenai Peninsula, Alaska

“…1A and B), but reported no evidence of older Holocene earthquakes. Mann and Crowell (1996) reported ghost forests killed by the 1964 earthquake at several sites on the southeast Kenai coast and evidence for an earlier earthquake about 900 years ago at Verdant Cove. Carver and Plafker (2008) summarize evidence for nine prehistoric earthquakes in the Prince William Sound region inferred from geological evidence for sudden land-level change and, in some cases, tsunami deposits.…”

“…1C), Mann and Crowell (1996) divided the coastal plain into two surfaces, the lower elevation Elymus surface and the higher elevation Tsuga surface. They constructed a levelline topographic profile that depicts the two surfaces as well as a tidal pond situated between the Tsuga surface and the base of the upland.…”

Coastal paleogeography and human occupation of the Western Alaska Peninsula

“…in the Gulf of Alaska and is widely recorded in intertidal stratigraphy from the Copper River delta to Kodiak Island (Combellick, 1991;Plafker et al, 1992;Gilpin, 1995). Mann and Crowell (1996) attribute the destruction of pre-600 yr B.P. sites in coastal fjords of the central Gulf of Alaska to this event, and others have linked it to a 200-year hiatus in occupation that occurs at the Kachemak-Koniag transition on Kodiak Island (Maschner, 1995;Crowell and Mann, 1996;Saltonstall et al, 1996).…”