Rapid communicationCoseismic and pre-seismic subsidence associated with great earthquakes in Alaska
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,
References (24)
- et al.
Late Holocene land and sea-level changes and the earthquake deformation cycle around upper Cook Inlet, Alaska
Quaternary Science Reviews
(2005) - et al.
Evidence of two great earthquakes at Anchorage, Alaska and implications for multiple events through the Holocene
Quaternary Science Reviews
(2005) - et al.
Microfossil analysis of sediments representing the 1964 earthquake, exposed at Girdwood Flats, Alaska, USA
Quaternary International
(1999) Development of the Radiocarbon Program OxCal
Radiocarbon
(2001)- et al.
Postseismic crustal uplift near Anchorage, Alaska
Journal of Geophysical Research
(1977) - et al.
Crustal uplift in the south central Alaska subduction zone: new analysis and interpretation of tide gauge observations
Journal of Geophysical Research
(2001) Investigation of peat stratigraphy in tidal marshes along Cook Inlet, Alaska, to determine the frequency of 1964 style great earthquakes in the Anchorage region
Alaska Division of Geological and Geophysical Surveys Report of Investigations
(1994)Guide to the Little Ice Age landforms and glacial dynamics of Portage Glacier
- et al.
A silent slip event on the deeper Cascadia subduction interface
Science
(2001) - Hamilton, S.L., 2003. Late Holocene relative sea-level changes and earthquakes around the upper Cook Inlet, Alaska,...