Evidence from shear-wave splitting for the restriction of seismic anisotropy to the upper crust

The existence of anistropy in the propagation speed of seismic waves through the Earth's crust has been attributed to the non-random alignment of microcracks in the crust¹. For sparsely distributed, parallel vertical cracks, perturbation theory predicts the splitting of a shear wave into two orthogonally polarized waves^2-4. Here we present extremely clear evidence for shear-wave splitting in three-component seismograms from crustal and sub-crustal microearthquakes recorded in the Shikoku district of Japan. Quantitative analysis of travel-time differences between split shear waves leads to the conclusion that the observed anisotropy is limited to the upper crust, shallower than 10-15 km. This model is consistent with recent results from reflection seismology⁵, and implies that microcrack density or orientation changes from the upper to the lower crust.