A regional study of shear wave splitting above the Cascadia Subduction Zone: Margin‐parallel crustal stress

Abstract

Recordings of local earthquakes from 16 three‐component broadband seismic stations in southwestern British Columbia, Washington, and northern Oregon are used to study regional variations of shear wave anisotropy in the North American plate above the subducting Juan de Fuca plate. There is evidence for shear wave splitting at all sites, with good agreement of fast polarization directions and travel time delays at adjacent stations. Most stations exhibit fast directions parallel to the strike of the margin, with anisotropy of 1–2%. These fast polarization directions are consistent with earthquake focal mechanisms and borehole stress studies, indicating that the observed anisotropy is likely due to crustal stresses (i.e., extensive dilatancy anisotropy theory). The margin‐parallel stresses may be due to oblique subduction of the Juan de Fuca plate. However, at the station closest to the coast (OZB), the fast direction shows a more margin‐normal orientation that may be associated with the proximity of the locked portion of the underlying subduction thrust fault.