Tsunamis and tectonic deformation at the northern cascadia margin: a 3000-year record from Deserted Lake, Vancouver Island, British Columbia, Canada

Three layers of sand occur within a sequence of muddy gyttja in a 3 m core recovered from Deserted Lake, which lies at the head of a Fjord on the cent ral west coast of Vancouver Island, British Columbia, Canada. The sedimento logy and microfossil assemblages of these layers are consistent with deposi tion by tsunamis. Radiocarbon ages suggest that the tsunamis were generated by plate-boundary earthquakes at the Cascadia subduction zone about 2600, 1600 and 300 years ago. Diatom and protozoan contents of the gyttja layers indicate that relative sea level fell in the periods between tsunamis. The sea-level changes are probably a product of interseismic uplift of the Nort h America plate margin. Post-tsunami diatom and protozoan assemblages are g enerally more marine than their pre-tsunami counterparts, suggesting that t he study site subsided during earthquakes. Estimates of interseismic and co seismic deformation during one earthquake cycle indicate that less than hal f of the interseismic uplift was recovered during the earthquake, leading t o cumulative, or permanent. uplift of the area.