link to Home Page

icon Megathrust

Giant Megathrust Earthquakes

A megathrust fault is the boundary between a subducting and an overriding plate. A megathrust earthquake is produced by a sudden slip along this fault. The world's largest earthquakes are all megathrust earthquakes. Megathrust earthquakes have never been observed in the short (~150 year) written history of the west coast of Canada, but there is compelling evidence that they did occur in prehistorical times. Some of this evidence includes:

The last point agrees with some legends of first nation people. One legend describes a severe ground shaking on a winter night accompanied by huge waves that destroyed a coastal village. This legend is likely reporting the effect of the last megathrust earthquake. Geothermal and seismic structure studies are being used to estimate the downdip extent of the potential seismic rupture zone. The landward extent of the rupture is an important factor for the shaking hazard at the inland cities of Victoria and Vancouver. The potential for the next megathrust earthquake for Cascadia is also being studied by monitoring the deformation of the crust using very precise satellite technology (GPS), repeat leveling, changes in gravity, and long term tide gauge measurements.