Double‐layer anisotropy resolved from S phases

Abstract

We have developed a technique for the inversion of SKS, SKKS and S waveforms to determine the anisotropic parameters of two homogeneous layers characterized by azimuthal anisotropy with horizontal symmetry axes. We have shown that fast polarization directions (φ1, φ2) and delay times (δt1, δt2) of the two layers can be retrieved by directly inverting high‐quality SKS and S waveforms. The method is based on a systematic grid search over splitting pairs (φ1, δt1) of the lower and (φ2, δt2) of the upper layer, to search for values that most successfully remove the effects of double layers of anisotropy from SKS and S waveforms, measured by minimizing the energy on the component that is orthogonal to the polarization incident on the anisotropic medium. We have tested this method with noise‐free and noise‐contaminated waveforms. Low‐noise waveforms at judiciously chosen incoming polarizations can return both layer parameters, but when waveforms from incoming polarizations that are very close to the fast or slow axis of the lower layer are used alone, the resolution is lost. However, we can always find a unique, well‐resolved minimum near the true model parameters, by stacking transverse energy misfit spaces associated with low‐noise waveforms sampling a variety of polarizations. Thus, the resolution and the uniqueness of the inversion are controlled by the range of incoming polarizations sampled by the data and by the signal‐to‐noise ratio. We have applied this technique to SKS and S waveforms recorded at stations BKS and LAC, located near the San Andreas fault. The best splitting parameters at BKS correspond to a lower layer with (φ1, δt1) = (90 ± 27°, 1.4 ± 0.55s), and an upper layer with (φ2, δt2) = (−45 ± 22°, 1.0 ± 0.5s), and at station LAC correspond to (φ1, δt1) = (50 ± 27°, 0.8 ± 0.55s), and (φ2, δt2) = (‐75 ± 35°, 1.1 ± 0.5s). For both stations, the top layer is parallel to the local strike of the San Andreas fault with a delay time of 1 s. These results are similar to earlier determinations using a different method. Copyright © 1994, Wiley Blackwell. All rights reserved