Elasticity is described in general by a fourth-order tensor with 21 independent coefficients, which corresponds to the triclinic symmetry class. However seismological observations are usually explained with a higher order of symmetry using fewer parameters. We propose an analytical method to decompose the elastic tensor into a sum of orthogonal tensors belonging to the different symmetry classes. The method relies on a vectorial description of the elastic tensor. Any symmetry class constitutes a subspace of a class of lower symmetry and an orthogonal projection on this subspace removes the lower symmetry part. Orthogonal projectors on each higher symmetry class are given explicitly. In addition, the method provides optimal higher symmetry approximations, which allow us to decrease the number of independent parameters. Consequences of the symmetry approximation of the elastic tensor on shear wave splitting (SWS) are investigated for upper-mantle minerals (olivine and enstatite), natural samples and numerically deformed olivine aggregates. The orthorhombic part of the elastic tensor as well as the presence of enstatite are important second-order effects.
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