Variations of source parameters due to anisotropic focal region

Abstract

Seismic sources in anisotropic medium have more complex moment tensor structures compared with the moment tensors of isotropic medium. Shear sources in an isotropic focal medium generate pure double-couple (DC) moment tensors. However in an anisotropic medium, shear sources can generate moment tensors with DC, compensated linear vector dipole (CLVD) and isotropic (ISO) components. The DC, CLVD and ISO percentages of a moment tensor depend on the magnitude and the orientation of the anisotropy. In this study, we choose five fault types namely, left/right strike slip, normal, reverse and dip-slip faults in a medium of different anisotropy classes; transversely isotropic, orthotropic and monoclinic. We rotated the anisotropic elasticity tensors of the medium for every possible orientation and evaluate the moment tensors of each cases. Then moment tensor decomposition is applied and DC, CLVD and ISO components are found. We plot the DC, CLVD and ISO percentages of the moment tensors generated by different fault types and anisotropy classes. By using the DC components, first we obtained fault plane orientation then we calculate the deviation from the original fault mechanism. Effects of anisotropy of the source region on calculated fault parameters are found. Distance from isotropic space of given anisotropic elasticity tensor and P/S wave velocity anisotropy percentages are measured. These percentages are proportional to the distane from isotropy. There is a correlation between distance to isotropy and P wave anisotropy with variation of fault plane parameters and percentages of non-DC components of earthquake source.