Evidence has been obtained for the accumulation of elastic strain across the northern Cascadia subduction zone that may be released in a future very large subduction thrust earthquake. Vertical and horizontal strain rates across the southern Vancouver Island region have been determined through (1) long-term trends in tide gauge data, (2) changes in repeated accurate leveling surveys, (3) changes in repeated high-accuracy gravity profiles, and (4) horizontal shortening observed in repeated precise positioning surveys. The outer coast is uplifting at a rate of a few millimeters per year decreasing landward, and shortening is occurring across the 100-km-wide coastal region at a rate of about 0.1 microstrain per year (mm km-1yr-1). The results are compared with the distribution of strain accumulation predicted from elastic disclocation and viscoelastic models for a subduction thrust fail. The location of the fault as used in the models is well defined by multichannel seismic reflection and other geophysical data.

Most of the observed current deformation can be explained by interseismic strain accumulation associated with the subduction thrust of southern Vancouver Island and northern Washington, provided the locked portion is restricted to a 60-km-wide band offshore beneath the continental shelf and slope. This conclusion also results from modeling the coseismic subsidence on the outer coast of Vancouver Island about 300 years ago deduced from paleoseismicity data. The unusually narrow downdip extent of the subduction thrust seismogenic zone, that extends little if at all beneath the coast, is a consequence of high temperatures associated with the young age of the subducted oceanic lithosphere and the thick blanket of insulating sediments. The high temperatures limit brittle seismogenic behavior downdip to where the thrust fault is at a depth of less than 15 km. The distance from the seismic portion of the megathrust limits the estimated ground motion at the major centers of Vancouver and Victoria from this source. The narrow width may also limit the earthquake size; however, events of magnitude well over 8 are possible.