CONTINUOUS GPS MONITORING OF ELASTIC STRAIN IN THE NORTHERN CASCADIA SUBDUCTION ZONE

Previous monitoring and modeling of crustal deformation across the nor thern Cascadia margin at Vancouver Is. has provided strong evidence th at the subduction thrust fault is locked and may generate future great earthquakes. The recent establishment of the Western Canada Deformati on Array (WCDA), a network of continuous GPS trackers in south-western British Columbia, provides a new tool for monitoring crustal strain a nd thereby helps to assess the earthquake hazard in this region. 17 mo nths of continuous data from the three longest running WCDA sites (Pen ticton, Victoria, Holberg) indicate: 1.) A 7 mm/yr easterly motion of Victoria relative to Penticton. Victoria is located in the forearc at the southeastern end of Vancouver Is., 230 km from trench axis, wherea s Penticton is located behind the arc, 520 km from the trench axis, an d assumed fixed on the stable North American plate; 2.) A 3 mm/yr nort hwesterly motion and 10 mm/yr uplift (with a large uncertainty) of Hol berg relative to Penticton. Holberg is located on the northern-most pa rt of Vancouver Is., 50 km from the trench and near the plate triple j unction; and 3.) The presence of probably seasonal variations in the a pparent relative positions of the two coastal sites with respect to Pe nticton. The landward motion of Victoria agrees with the deformation p redicted by elastic dislocation models of the interseismic period for a great subduction-thrust earthquake as constrained by previous deform ation data. Although not as well resolved, the motion of Holberg, roug hly parallel to the margin, is significantly different and is inconsis tent with simple subduction models. The non-linear variations in measu red position are dominated by an annual period probably resulting from seasonal biases in the precise orbit estimates or in the modeling of tropospheric delays or tidal effects.