Horizontal deformation in the Cascadia subduction zone as derived from serendipitous geodetic data

Data from recent Global Positioning System (GPS) surveys are combined with triangulation/trilateration data to estimate horizontal shear-strain rates for two regions in the Cascadia subduction zone. Near Bellingham, Washington, we estimate that the maximum horizontal shear rate (γ) equals 0.116 ± 0.089 μrad/yr and the direction of maximum horizontal contraction (θ) orients N71° E ± 21° for data spanning the 1905–1985 interval. The corresponding estimates for a region near Portland, Oregon, are 0.057 ± 0.027 μrad/yr and N95° E±14° for data spanning the 1881–1988 interval. These estimates are consistent with estimates from independent geodetic data in the area. Moreover, the estimates for θ are consistent with the N68°E direction of ongoing convergence between the Juan de Fuca plate and the North American plate as predicted by the NUVEL-1 plate motion model. This consistency between θ-estimates and the direction of plate convergence supports the argument for the possibility of a great subduction earthquake occurring in the Cascadia subduction zone. The low shear rates, however, imply that the recurrence interval between such earthquakes would be several centuries long.