Late Holocene great earthquakes and relative sea-level change at Kenai, southern Alaska

Kenai, located on the west coast of the Kenai Peninsula, Alaska, subsided during the great earthquake Of AD 1964. Regional land subsidence is recorded within the estuarine stratigraphy as peat overlain by tidal silt and clay. Reconstructions using quantitative diatom transfer functions estimate co-seismic subsidence (relative sea-level rise) between 0.28 +/- 0.28 m and 0.70 +/- 0.28 m followed by rapid post-seismic recovery. Stratigraphy records an earlier co-seismic event as a second peat-silt couplet, dated to similar to 1500-1400 cal. yr BP with 1.14 +/- 0.28 m subsidence. Two decimetre-scale relative sea-level rises are more likely the result of glacio-isostatic responses to late Holocene and Little Ice Age glacier expansions rather than to co-seismic subsidence during great earthquakes. Comparison with other sites around Cook Inlet, at Girdwood and Ocean View, helps in constructing regional patterns of land-level change associated with three great earthquakes, AD 1964, similar to 950-850 cal. yr BP and similar to 1500-1400 cal. yr BP. Each earthquake has a different spatial pattern of co-seismic subsidence which indicates that assessment of seismic hazard in southern Alaska requires an understanding of multiple great earthquakes, not only the most recent. All three earthquakes show a pre-seismic phase of gradual land subsidence that marked the end of relative land uplift caused by inter-seismic strain accumulation. Copyright (c) 2005 John Wiley & Sons, Ltd.