Investigating Elastic Anisotropy of the Leech River Complex, Vancouver Island using finite-frequency sensitivity kernels

The Leech River Complex (LRC) of southern Vancouver Island is part of a once continuous belt of Cretaceous sandstone, mudstone and volcanics that formed an accretionary wedge along the northwestern margin of North America. Metamorphism at 50 Ma to prehnite-pumpellyite, greenschist, amphibolite and blueschist facies produced pervasive foliations with strong phyllosilicate lattice preferred orientations. Laboratory measurements and in-situ S-wave splitting analysis of tectonic tremor wavetrains indicate that this fabric produces substantial S-wave anisotropy of up to 30%. In this study we seek to gain further understanding on the nature of anisotropy within the LRC using high signal to noise ratio low frequency earthquake (LFE) templates and 3-D simulations from the spectral element method (SEM). The LFEs are characterized by impulsive, double couple, point sources and lie along a surface between 27 and 37 km depth that is inferred to be the plate boundary, immediately underlying the LRC. The SEM modelling employs a regional mesh that incorporates realistic topography, bathymetry and a 3-D tomographic P-wave velocity model of southern Vancouver Island. It allows us to readily simulate wave propagation in general anisotropic media with up to 21 independent elastic constants. We will investigate the orientation and distribution of anisotropy within the LRC by employing sensitivity kernels determined using adjoint methods in conjunction with SEM.