Measurements ofSKSsplitting parameters have been made at stations of the Canadian National Seismograph Network for events over the period 1993–mid1994. Here we report on results obtained from western stations. In the northern Cordillera the alignment of polarization directions with major transcurrent fault systems suggests that the crust and upper mantle are being deformed coherently. Splitting parameters in the southern Cordillera, however, are oriented approximately perpendicular to the strike of major tectonic features, notably the Cascadia subduction zone. This contrast in the character of fast polarization directions between the southern and northern Cordillera is thought to reflect the change in the tectonic style of the adjacent plate margin from convergent along the southern B.C. coast to transform north of the triple junction. One plausible explanation is that upper mantle anisotropy in the southern region is due primarily to mantle flow perpendicular to the margin involving upwelling at the Juan de Fuca ridge, subduction along the coast and associated back‐arc asthenospheric flow beneath the B.C. inter