Microfossil evidence for land movements associated with the AD 1964 Alaska earthquake

Abstract

Microfossil diatom and pollen data from a number of cores at coastal marshes at Girdwood Flats and Kenai Flats in Cook Inlet, Alaska, suggest that there was a phase of gradual land subsidence prior to the main shock of the AD 1964 earthquake. This phase followed a long period of gradual land uplift since the last large earthquake, which occurred about 730-900 years ago. Caesium (137Cs) records indicate that subsidence started approximately 15 years before the 1964 earthquake. This pre-seismic subsidence is evident in the coastal marsh sediment sequences in changes in diatom and pollen assemblages that indicate changes in tidal-marsh environments or a change from raised-bog to marsh conditions. The microfossil evidence indicates ∼0.15 m pre-seismic land subsidence at both Girdwood Flats and Kenai Flats and co-seismic subsidence of ∼1.8 m and ∼0.2 m respectively.