Focal mechanisms in anisotropic media

Focal mechanisms of seismic sources in anisotropic media are more complicated than in isotropic media. Planar shear faulting produces pure double-couple (DC) mechanism in isotropy, but generally non-double-couple (non-DC) mechanism in anisotropy. The non-DC mechanism can comprise both the isotropic (ISO) and compensated linear vector dipole (CLVD) components. The amount of the ISO and CLVD components depends on strength and symmetry of anisotropy and on the orientation of faulting. Shear faulting in anisotropy generates a pure DC mechanism, provided that faulting is situated in symmetry planes of orthorhombic or of higher anisotropy symmetries. The fault plane solution, i.e. the orientation of the fault normal and the slip direction, can be retrieved from the seismic moment tensor and from elastic parameters describing anisotropy at the source. Numerical modelling shows that shear faulting in anisotropic rocks present in the Earth crust, the Earth mantle or in the subduction zones can produce mostly mechanisms with the CLVD up to 30 per cent and with the ISO up to 15 per cent. The fault plane solutions calculated under the assumption of isotropy typically deviate from the true solutions by an angle of 5° to 10°.