APPLICATION OF SALT-MARSH AND TIDAL-FLAT FORAMINIFERA TO IDENTIFY LATE HOLOCENE LAND-LEVEL CHANGES CAUSED BY MEGATHRUST EARTHQUAKES AT SITKINAK ISLAND, ALASKA

The Alaska-Aleutian subduction zone is the source of frequent great earthquakes and accompanying destructive tsunamis, as demonstrated by the remarkable series of M8-9 ruptures during the 20^th century. Almost nothing, however, is known about great earthquake and tsunami prehistory west of central Kodiak Island (154.3° W). This uncertainty limits assessments of southern Alaska’s earthquake hazard and forecasts of potentially damaging tsunamis along North America’s west coast. Sitkinak Island lies at the western end of the rupture zone of the 1964 earthquake. A rancher on the north coast observed ~0.6 m of shoreline uplift immediately following the 1964 Mw9.2 earthquake, which was reduced to ~0.3 m when it was measured on a later survey (Plafker, 1969; USGS Professional Paper 543-I). Here we describe salt-marsh and tidal-flat foraminifera from modern transects at two sites west of Kodiak Island (Sitkinak Island, and Simeonof Island, 370 km southwest). We use these modern foraminiferal assemblages to infer the direction of coseismic land-level changes from a stratigraphic record deposited during prehistoric earthquakes west of central Kodiak Island.

The diversity of foraminifera on Sitkinak and Simeonof islands is low, but distinctive assemblages with different elevational ranges constitute viable sea-level indicators. Three assemblages are recognized along five transects. The high marsh (> -0.2 m mean high water (MHW)) is dominated by Balticammina pseudomacrescens, Miliammina fusca characterizes the low-marsh (< 0 m MHW) environment, and a tidal-flat assemblage (< -0.5 m MHW) is recognized by high abundance of Cibicides lobatulus. Multiple radiocarbon-dated stratigraphic contacts of the past 1000 years in tidal sections fringing a lagoon on Sitkinak Island may indicate coseismic uplift (peat overlies mud) or subsidence (mud overlies peat). For example, foraminiferal assemblages at a pre-16^th-century contact indicate rapid uplift, with a facies consisting of >95% B. pseudomacrescens replaced by a freshwater marsh facies barren of foraminifera. The record of land-level changes identified by foraminiferal assemblages will improve the understanding of recurrence intervals of great Alaska-Aleutian earthquakes and associated tsunamis.