Moment tensors of kinematically double-couple microseismic events triggered in anisotropic formations are known to exhibit non-double-couple focal mechanisms. The weak anisotropy approximation of these mechanisms reveals the combinations of anisotropy coefficients of vertically transversely isotropic and orthorhombic focal regions responsible for the deviations of moment tensors from double couples. Numerical examples for models of typical unconventional shales indicate the non-double-couple components of moment tensors to be sufficiently large to cause misinterpretation of the nature of ruptures associated with hydraulic fracturing.

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