We employ full finite-difference modeling to study effects of teleseismic shear-wave splitting in 2D upper-mantle models with lateral variations of anisotropy. In our models the anisotropy is confined to a layer at variable depth within the top 550 km of the upper mantle. We consider propagation of plane shear wavefronts and investigate the effects of lateral variations in symmetry-axis orientation of orthorhombic olivine. Splitting parameters obtained from the synthetic waveforms exhibit characteristic variations as functions of frequency and polarization of the incident shear wave. We estimate the effective Fresnel zones (sensitivity range) from the lateral variations of splitting parameters. This leads to constraints for the extent of homogeneous anisotropic domains and for the depth of inhomogeneous regions. ¿ 2000 American Geophysical Union