Crustal Anisotropy in a Subduction Zone Forearc: Northern Cascadia

Abstract:

S-wave splitting analyses using high SNR low frequency earthquake (LFE) templates at 3-component stations across northern Cascadia indicate the presence of a heterogeneous distribution of crustal anisotropy in the North American plate. On southern Vancouver Island (SVI), we investigate the contribution to anisotropy from the Leech River Complex (LRC), an allochthonous terrane comprising strongly foliated greenschist-facies phyllites and amphibolite-facies schists with steeply dipping foliations striking E-W. Estimates of initial S-wave polarization direction are consistent with radiation patterns predicted from LFE focal mechanisms, providing corroboration for thrust along the plate boundary. Fast directions across mainland SVI are subparallel to the dominant foliation direction in the LRC. An eastward increase in depth normalized delay times combined with small-scale azimuthal variations in fast directions suggest a heterogeneous distribution of anisotropy. We test azimuthally anisotropic LRC models constrained by surface geology and seismic reflection studies using 3D spectral element method simulations. The preferred model of a NNE shallowly dipping wedge of LRC material with varying orientation of anisotropy terminating at mid crustal levels is able to recreate mean and azimuthal variations in fast directions along with variations in delay times, supporting the hypothesis of the LRC as a primary contributor to crustal anisotropy beneath SVI. For select stations where anisotropic LRC models do not recreate observations, fast directions are subparallel to local estimates of maximal compressive horizontal stress, suggesting fluid-filled cracks could be a source of anisotropy. We refute the idea that anisotropy along mainland SVI is primarily due to stress related cracks as has been suggested by prior studies. Fast directions at northern Washington stations exhibit variations with azimuth and incidence angle suggesting complex anisotropy interpreted as due to a combination of cracks and preferred mineral orientation of metamorphosed slates of the Olympic core rocks. These slates may also underlie stations on SVI and represent another source of anisotropy.