Microfossil analysis of sediments representing the 1964 earthquake, exposed at Girdwood flats, Alaska, USA

Abstract

Diatom, pollen, foraminifera and thecamoebian assemblages from an outcrop of peat and silt at Girdwood Flats, in the upper Turnagain Arm of the Cook Inlet, Alaska, record four phases of relative land and sea-level changes. The first phase is the development of freshwater swamp above high marsh sediments during relative land uplift, caused by strain accumulation along the locked portion of the Alaska-Aleutian subduction zone. In second phase, the top 2 cm of the peat, all microfossil groups record pre-seismic relative sea-level rise (relative land subsidence). The third phase is rapid land subsidence, 1.7 m, during the earthquake of March 1964 that initiated intertidal silt accumulation above the peat. The final phase is the colonisation of mudflat by salt marsh communities during post-seismic land uplift. The microfossil data compare favourably with sequences from Washington, Oregon and British Columbia that record late Holocene submergence events caused by earthquakes. The comparable changes in microfossil assemblages record the different phases of relative land and sea-level changes and the magnitude of land subsidence caused by each earthquake (expressed relative to the tidal range at the site). These results raise the question whether preseismic sea-level rise represents any king of warning of large earthquakes.