Relative sea-level change inferred from intertidal sediments beneath marshes on Vancouver Island, British Columbia |
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| Authors: | Boyd E Benson John J Clague Kurt A Grimm |
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| Institution: | a University of British Columbia, Earth and Ocean Sciences, 6339 Stores Road, Vancouver, British Columbia, Canada V6T 1Z4;b Geological Survey of Canada, Suite 101, 605 Robson Street, Vancouver, British Columbia, Canada V6B 5J3; Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1A6 |
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| Abstract: | Relative sea-level change at the time of, and since, the most recent great earthquake at the Cascadia subduction zone is estimated from intertidal sediments at three marshes on western Vancouver Island, British Columbia. We compare the elevation of the pre-earthquake surface, which is marked by a tsunami sand sheet, with the modern depositional elevation range of the sediment type upon which the sand was deposited. At a site south of the Nootka fault zone, which is the northern boundary of the subducting Juan de Fuca plate, tidal mud overlies the pre-earthquake marsh surface. The stratigraphy at this site indicates 0.2–1.6 m of coseismic submergence and 1.1 m of subsequent emergence. In contrast, two sites to the north lack obvious stratigraphic evidence for coseismic land-level change and record between 0.1 and 1.7 m of post-earthquake submergence. These results indicate a difference in tectonic environment across the Nootka fault zone and suggest that plate-boundary rupture during the last great Cascadia earthquake probably did not extend north of central Vancouver Island. |
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