Splitting of shear waves, generated by local earthquakes in the Southern California basin, and their polarization changes during propagation through layers with different orientations of anisotropy, are investigated from multicomponent seismograms and their hodograms. Synthetic data for earthquakes in the Southern California basin illustrate the effects on polarization caused by dry vertical microcracks with a crack density of ∼0.04 at the source and receiver locations. Hodograms for observed and synthetic data show that the polarization at the receiver depends only on the orientation of the anisotropy within two wavelengths of the receiver, which qualitatively explains the frequency dependence of anisotropy in previous studies.
Seismograms are simulated for a model with a range of station azimuths and distances. If the anisotropy-producing crack orientations do not change between the source and a receiver, and the source motion is parallel to the cracks, there is no shear-wave splitting. Splitting occurs when the anisotropy orientation at the receiver (or between the source and receiver) is different from the source orientation.