The geological signature of great earthquakes off Canada's West Coast

Geological and geophysical evidence, gathered in the last 15 years by a num ber of scientists working in Canada and the United States, leaves little do ubt that some of the largest earthquakes on Earth occur at the Cascadia sub duction zone on Canada's western doorstep. No such earthquake has occurred since European settlement of the region in the early 1800s, but the entire 900 km length of the thrust fault separating the Juan de Fuca and North Ame rica plates apparently ruptured during a magnitude-g event on 26 January 17 00. Evidence for this and older subduction earthquakes has been found at co astal wetlands from Vancouver Island to northern California. The geological evidence includes buried wetland soils produced by sudden, seismically ind uced subsidence, sheets of sand and gravel deposited by tsunamis, and sand dykes and blows generated by liquefaction during strong ground shaking. Dat ing of the buried soils and tsunami deposits in Washington and Oregon has s hown that great Cascadia earthquakes have an average recurrence of 500 year s; however, intervals between the seven most recent events range from less than 200 years to 700-1300 years. The hypothesis that subduction earthquake s occur in Cascadia is independently supported by geodetic measurements and the results of geophysical modelling, which collectively indicate that par t of the plate boundary is locked and accumulating elastic strain that will be released during a future earthquake.