REVISED CHRONOLOGY OF EVENT DEPOSITS BEFORE THE 1700 CASCADIA TSUNAMI LAYERS FROM THE SOUTHWEST COAST OF VANCOUVER ISLAND, CANADA

To improve the earthquake history of the southwest coast of Canada, coastal deposits at Tofino, Ucluelet, and Port Alberni in Vancouver Island facing the Cascadia subduction zone were re-examined. We found sand sheets interbedded within peat and mud, suggesting that they have been deposited by strong flows under the low-energy environment. The ages of sand sheets below the tsunami deposits associated with the most recent Cascadia earthquake in 1700 CE were presumed to be 1420–1450 CE, based on limiting max. and min. ages derived from plant macrofossils. The newly constrained ages range between those of coseismic subsidence events Y and W in southern Washington, which have been recognized as the 1700 CE and the penultimate Cascadia earthquakes, respectively. Some of the onshore paleoseismic evidence in Vancouver Island and Oregon could be correlative, but there is no correlative evidence in Washington. In contrast, the new ages overlap with the averaged age of turbidite T2 interpreted to have been originated from the penultimate Cascadia earthquake. Our revised chronology of event deposits prior to the 1700 CE Cascadia tsunami layers from Vancouver Island may be a clue to understand the timing of the penultimate Cascadia earthquake.