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.
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