CURRENT DEFORMATION AND THE WIDTH OF THE SEISMOGENIC ZONE OF THE NORTHERN CASCADIA SUBDUCTION THRUST

Evidence has been obtained for the accumulation of elastic strain acro ss the northern Cascadia subduction zone that may be released in a fut ure very large subduction thrust earthquake. Vertical and horizontal s train rates across the southern Vancouver Island region have been dete rmined through (1) long-term trends in tide gauge data, (2) changes in repeated accurate leveling surveys, (3) changes in repeated high-accu racy gravity profiles, and (4) horizontal shortening observed in repea ted precise positioning surveys. The outer coast is uplifting at a rat e 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 th e distribution of strain accumulation predicted from elastic dislocati on and viscoelastic models for a subduction thrust fault. The location of the fault as used in the models is well defined by multichannel se ismic reflection and other geophysical data. Most of the observed curr ent deformation can be explained by interseismic strain accumulation a ssociated 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 c onclusion also results from modeling the coseismic subsidence on the o uter coast of Vancouver Island about 300 years ago deduced from paleos eismicity data. The unusually narrow downdip extent of the subduction thrust seismogenic zone, that extends little if at all beneath the coa st, is a consequence of high temperatures associated with the young ag e of the subducted oceanic lithosphere and the thick blanket of insula ting sediments. The high temperatures limit brittle seismogenic behavi or 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 es timated ground motion at the major centers of Vancouver and Victoria f rom this source. The narrow width may also limit the earthquake size; however, events of magnitude well over 8 are possible.