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The stratigraphy and neotectonic significance of tsunami deposits beneath tidal marshes on Vancouver Island, British Columbia Benson, Boyd E.


Two tsunami deposits in intertidal marshes on Vancouver Island, British Columbia, were studied to understand their stratigraphy, sedimentology and neotectonic significance. Tsunami sand sheets on Vancouver Island thin and fine landward, drape the marsh surface, contain marine microfossils, may be internally graded or stratified, and are similar in age and stratigraphic position to deposits of known earthquakes. These characteristics provide criteria for recognizing tsunami deposits in other areas. Historical accounts and 137rjs analysis indicate that the upper tsunami deposit is the product of the great Alaska earthquake in 1964. Radiocarbon ages of organic material associated with the lower deposit show that it was deposited after AD 1660. This deposit is attributed the most recent earthquake on the Cascadia subduction zone 300 yr ago. This study extends the known effects of Cascadia seismicity north of the Nootka fault zone, the northern boundary of the subducting Juan de Fuca plate. Two methods were used to decipher the neotectonic history at the northern end of the Cascadia subduction zone: (1) comparison of the two tsunami deposits; (2) alithofacies method, permitting comparison of land level change experienced by the older sand sheet on opposite sides of the Nootka fault zone since the earthquake about 300 yr ago. North of the Nootka fault zone, the older tsunami deposit becomes thinner and less continuous, whereas the 1964 tsunami deposit shows little change at the different sites. This contrast suggests that the 300-yr-old tsunami approached the study area from the south, and that the earthquake source that triggered it may have been south of the study area. A lithofacies method was developed to estimate coseismic and interseismic land level changes since that earthquake. This method involves comparison of the current elevation of the pre-earthquake surface with the elevation range of the lithofacies in which it was deposited. Between 0.2 and 1.6 m of coseismic subsidence and 1.1 m of post-seismic emergence occurred at a site south of the Nootka fault zone. Such coseismic subsidence and interseismic emergence is indicative of coseismic and interseismic deformation on a locked subduction zone. At two sites north of the Nootka fault zone there is no obvious stratigraphic evidence for coseismic land level change. Post-seismic submergence of 0.1 to 1.7 m is attributed mainly to eustatic sea level rise. The lack of coseismic subsidence and interseismic emergence at the northern sites suggests that subduction and earthquake rupture on the Cascadia subduction zone may not extend north of the Nootka fault zone.

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