Late Holocene relative sea-level changes and the earthquake deformation cycle around upper Cook Inlet, Alaska

Multiple peat-silt couplets preserved in tidal marsh sediment sequences suggest that numerous great plate boundary earthquakes caused the coast around Cook Inlet, Alaska, to subside over the past 3500 years. Field and laboratory analyses of the two youngest couplets record the well-documented earthquake of AD 1964 and the penultimate one, approximately 850cal yr BP. Diatom assemblages from a range of modern day estuarine environments from tidal flat through salt marsh to acidic bog produce quantitative diatom transfer function models for elevation reconstructions based on fossil samples. Only nine out of 124 fossil assemblages analysed, including previously published data for the AD 1964 earthquake, have a poor modern analogue. Calibration of fossil samples indicate co-seismic subsidence of 1.50 +/- 0.32m for AD 1964, similar to measurements taken after the earthquake, and 1.45 +/- 0.34m for the similar to 850 cal yr BP earthquake. Elevation standard errors for individual fossil samples range from similar to 0.08m in peat layers to similar to 0.35m in silt units. Lack of a chronology within fossil silt units prevents identification of changes in the rate of recovery and land uplift between the post-seismic and inter-seismic periods. However, preservation of multiple peat-silt couplets indicates no net emergence over multiple earthquake cycles. Glacio-isostatic movements from Little Ice Age glacier advance and retreat explains a similar to 0.15m relative sea-level oscillation recorded within the peat layer subsequently submerged as a result of the AD 1964 earthquake. Before both this and the similar to 850 cal yr BP earthquake, diatom assemblages suggest pre-seismic relative sea-level rise of similar to 0.12 +/- 0.13m, representing possible precursors to great earthquakes. (c) 2004 Elsevier Ltd. All rights reserved.