Abstract
Thin sheets of sand occur within Holocene mud and peat deposits beneath tidal marshes at Tofino, Ucluelet, and Port Alberni on Vancouver Island, British Columbia. The sand sheets are extensive and have sharp upper and lower contacts. In most cases they consist of moderately sorted, massive sand and silty sand with abundant wood and other plant detritus. At Port Alberni, the thickest sheet has gravel and is graded. The upper two sand sheets in the Tofino-Ucluelet area, and possibly the third, are also present at Port Alberni.
Eyewitness accounts and 137Cs analysis suggest that the uppermost, thinnest sand was deposited by the tsunami triggered by the great Alaska earthquake in 1964. The next oldest sand sheet has been radiocarbon dated at <500 yr old. At Tofino and Ucluelet, it sharply overlies a former marsh surface that probably subsided suddenly during a large earthquake; the sand was deposited by a tsunami generated by this earthquake. The third sand sheet at Tofino is 500-800 yr old. The tsunami that deposited it may have been caused by the penultimate, great subduction earthquake in southern Alaska, a previously unrecognized earthquake on the Cascadia subduction zone, a great earthquake elsewhere in the North Pacific, or a large submarine landslide.
Our data suggest that large tsunamis have struck the southern British Columbia coast several times during the late Holocene and that some were much larger than the 1964 tsunami, which caused about $10 million damage (1964 Canadian dollars) to communities on Vancouver Island. Because such tsunamis can be expected in the future, they pose a hazard to people and property in some coastal areas.
