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Coseismic and pre-seismic subsidence associated with great earthquakes in Alaska

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Abstract

Alternating beds of peat and mud in sediment sequences on the south-central Alaskan coast record coseismic and inter-seismic relative land and sea-level movements caused by repeated great earthquakes on the Alaska–Aleutian subduction zone. During the AD 1964 Mw=9.2 earthquake, tidal marshes and wetlands around upper Cook Inlet experienced up to 2 m of subsidence, burying peat-forming communities with intertidal mud. Here we use quantitative analyses of fossil diatoms within peat–mud couplets to reconstruct land/sea-level changes for the 1964 and five earlier great earthquakes during the past 3300 years. In contrast to geodetic observations that are limited to the present post-seismic phase, we quantify varying spatial patterns of uplift and subsidence through complete earthquake cycles. Relative land uplift characterises most of the inter-seismic phase of each cycle at our sites, whereas each great earthquake was preceded by a short period of pre-seismic relative land subsidence.

Introduction

Future earthquake forecasting and reduction of loss require knowing the history of large earthquakes, including their frequency and how patterns of coseismic land movement vary during different earthquakes. Coastal wetlands in the Pacific Northwest of Canada and the USA are excellent environments for recording the relative land-level changes (negative of relative sea-level change) that occur during large Holocene earthquakes because different types of sediment accumulate according to their elevation with respect to tide levels at the time of deposition. A great (Mw=9.2), well-documented, plate-boundary earthquake struck south-central Alaska on March 28th, AD 1964 (Plafker, 1969), whereas the last great earthquake on the Cascadia subduction zone occurred 300 years ago (Satake et al., 2003). Observations made before and after the 1964 earthquake (Karlstrom, 1964; Plafker, 1969; Brown et al., 1977) provide analogues that help in estimating pre- and post-seismic relative land-level movements associated with earlier great earthquakes.

Section snippets

Methods and results

Exposed sections and transects of cores establish the continuity of peat–mud couplets at four sites around upper Cook Inlet, in the zone of coseismic subsidence during the 1964 earthquake (Fig. 1). We follow established field stratigraphic procedures (Nelson et al., 1996) and use quantitative analysis of diatom assemblages from peat–mud couplets to confirm whether submergence of a peat–mud couplet records relative land subsidence, the amount of subsidence and its suddenness (Hamilton and

Discussion

Diatom-based reconstructions of coseismic subsidence for the 1964 great earthquake agree closely with observations taken after the earthquake, except for Kasilof (Fig. 1) where we have no sedimentary record of subsidence (Hamilton, 2003). Girdwood, Ocean View and Kenai all record pre-seismic subsidence (Fig. 2, Fig. 4). 137Cs data from all three sites show that pre-seismic subsidence commenced in the early 1950s, which coincides with observations of increased tidal flooding of marshes at

Acknowledgements

This is a contribution to IGCP Project 495. Research supported by the US Geological Survey, award numbers 02HQGR0075 and 03HQGR0101 (the views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the US Government), NERC GT 04/99/ES/57, NERC radiocarbon allocation number 935 0901 and University of Durham Sir James Knott Foundation. We thank Brian Atwater, Stuart Lane,

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