THE APPLICATION OF DIATOM ANALYSIS TO RECONSTRUCT COSEISMIC UPLIFT ON MONTAGUE ISLAND, ALASKA

The 1964 M_w 9.2 Great Alaska Earthquake ruptured >800 km of the eastern segment of the Alaska-Aleutian megathrust, and included rupture of the Patton Bay Splay-Fault System (PBFS). Deformation along the PBFS amplified local tsunami generation (e.g., run-up heights of 6-8 m at Seward, AK) and vertically displaced shorelines on Montague Island, Prince William Sound by as much as 10 m. Pre- and post-1964-earthquake imagery shows significant uplift-induced environmental changes, including the seaward shift of shorelines and the emergence of coastal lagoons along the island’s northwest coast. 1-m/pixel lidar elevation models were used to map coastal geomorphology of a series of abandoned lagoons on the west coast of Montague Island. Sharp and laterally continuous peat over sand/silt contacts were mapped using sediment cores, computed tomography, and radiocarbon dating, that may signal prior instances of abrupt coastal uplift caused by splay-fault ruptures over the past 4.2 ka. Here, we investigate the microfossil signature across the uplift contacts in order to characterize environmental changes (e.g., shift from tidal flat to freshwater peat) and quantify uplift. Microfossils sensitive to changes in salinity, such as diatoms, contained in coastal sediments can indicate sudden changes in relative sea level along the coast and provide an independent test of earthquake-related deformation inferred from coastal stratigraphy. Preliminary diatom results across the penultimate earthquake contact (0.8 ka) reflect an environmental shift from a brackish lagoon to a freshwater peat, consistent with uplift of a coastal lagoon. Our diatom results, in combination with geomorphic and stratigraphic observations, will help assess the rupture history of the PBFS, including whether the PBFS ruptures independently of the megathrust, and improve our understanding of earthquake and tsunami hazards in Prince William Sound.