A regional study of shear wave splitting above the Cascadia subduction zone: Margin-parallel crustal stress

Recordings of local earthquakes from 16 three-component broadband seismic s tations in southwestern British Columbia, Washington, and northern Oregon a re used to study regional variations of shear wave anisotropy in the North American plate above the subducting Juan de Fuca plate. There is evidence f or shear wave splitting at all sites, with good agreement of fast polarizat ion directions and travel time delays at adjacent stations. Most stations e xhibit fast directions parallel to the strike of the margin, with anisotrop y of 1-2%. These fast polarization directions are consistent with earthquak e focal mechanisms and borehole stress studies, indicating that the observe d anisotropy is likely due to crustal stresses (i.e., extensive dilatancy a nisotropy theory). The margin-parallel stresses may be due to oblique subdu ction of the Juan de Fuca plate. However, at the station closest to the coa st (OZB), the fast direction shows a more margin-normal orientation that ma y be associated with the proximity of the locked portion of the underlying subduction thrust fault.