Continuous GPS monitoring of elastic strain in the Northern Cascadia Subduction Zone

Publisher Website
Google Scholar
Abstract
Previous monitoring and modeling of crustal deformation across the northern Cascadia margin at Vancouver Is. has provided strong evidence that the subduction thrust fault is locked and may generate future great earthquakes. The recent establishment of the Western Canada Deformation Array (WCDA), a network of continuous GPS trackers in southwestern British Columbia, provides a new tool for monitoring crustal strain and thereby helps to assess the earthquake hazard in this region. 17 months of continuous data from the three longest running WCDA sites (Penticton, 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, whereas Penticton is located behind the arc, 520 km from the trench axis, and assumed fixed on the stable North American plate; 2.) A 3 mm/yr northwesterly motion and 10 mm/yr uplift (with a large uncertainty) of Holberg relative to Penticton. Holberg is located on the northern‐most part of Vancouver Is., 50 km from the trench and near the plate triple junction; and 3.) The presence of probably seasonal variations in the apparent relative positions of the two coastal sites with respect to Penticton. The landward motion of Victoria agrees with the deformation predicted by elastic dislocation models of the interseismic period for a great subduction‐thrust earthquake as constrained by previous deformation data. Although not as well resolved, the motion of Holberg, roughly parallel to the margin, is significantly different and is inconsistent with simple subduction models. The non‐linear variations in measured 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.