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Crustal anisotropy in a subduction zone forearc : Northern Cascadia Matharu, Gian


S-wave splitting analyses using high signal-to-noise ratio low frequency earthquake (LFE) templates at 3-component stations across southern Vancouver Island (SVI) and northern Washington indicate the presence of a heterogeneous distribution of crustal anisotropy in the North American plate. For SVI, we investigate the contribution to anisotropy from the Leech River Complex (LRC), an allochthonous terrane comprised of strongly foliated greenschist facies phyllites and amphibolite facies schists with steeply dip- ping foliations striking E-W. On SVI, estimates of initial S-wave polarization direction are consistent with predictions from radiation patterns generated by LFE focal mechanisms, providing corroboration for thrust mechanisms at the plate boundary. Fast directions across mainland SVI are subparallel to the dominant foliation direction in the LRC. Increases in depth normalized delay times from east to west, combined with small-scale azimuthal varia- tions in fast directions suggest a heterogeneous distribution of anisotropy. We test azimuthally anisotropic LRC models based upon analyses of geological fabric and geometrically constrained by reflection studies, through forward modeling using 3D spectral element method (SEM) simulations. The preferred model of a north/northeast shallowly dipping wedge of LRC material with varying orientations of anisotropy terminating at mid crustal levels is able to recreate mean and azimuthal variations in fast directions along with variations in delay times, thereby 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 stations on northern Washington 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 underlay stations on SVI and represent another source of anisotropy.

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