The Application of Diatom Analysis to Reconstruct Coseismic Uplift on Montague Island, Alaska

Session: Forthcoming Updates of the USGS NSHMs: Hawaii, Conterminous U.S. and Alaska [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

The 1964 M_w 9.2 Great Alaska Earthquake ruptured >800 km of the eastern segment of the Alaska-Aleutian megathrust and remains the second largest earthquake ever recorded. Earthquake-induced faulting along the Patton Bay Splay-Fault System (PBFS), contributed to local tsunami generation and vertically displaced shorelines on Montague Island, Prince William Sound by as much as 10 m. The sudden uplift of the island resulted in significant changes to coastal environments, such as the seaward shift of shorelines and the gradual draining of coastal lagoons. Field mapping, detailed lithologic descriptions and computer tomography (CT) scans of sediments underlying a series of drained lagoons on the western coast of Montague Island reveal distinct and laterally continuous lithologic contacts between sand/silt and peat, potentially signaling prior instances of abrupt coastal uplift over the last 4.2 ka. Here, we investigate the microfossil signature across potential coseismic uplift contacts in order to characterize how environments shifted (e.g., tidal flat to freshwater peat) and quantify uplift. Microfossils, 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 the draining of a coastal lagoon. Our diatom results, in combination with geomorphic and stratigraphic observations, will help assess the rupture history of the PBFS and improve our understanding of earthquake and tsunami hazards in Prince William Sound.

Presenting Author: Jessica DePaolis

Authors