Observations of D" anisotropy from P and S-wave reflectivity

Abstract:

Anisotropy in the deep Earth is often recognised through splitting of shear waves. Especially in the D" region, differential travel times of the two polarised S-waves provide information on the polarisation direction and strength of the anisotropy. Another possibility to test for the presence of anisotropy in the deep Earth is given by using the reflectivity of seismic waves: the reflection coefficient of P and S-waves is distance dependent but in the case of anisotropic material also azimuth dependent. Using P- and S-waves simultaneously allows for testing and ruling out several possible candidate mechanisms of anisotropy. The region in D" beneath Eurasia is characterised by a number of crossing ray paths which opens the possibility for the detection of reflected P- and S-waves from three different directions and the results show a clear variation in the polarity of the reflected seismic waves. Although a number of models produce the observed P-wave reflectivity, they fail to give the correct result for the S-wave reflectivity and vice versa. The model containing a phase transition from isotropic perovskite to anisotropic post-perovskite can explain the P- and S-wave observations. We also extend our observation and analysis to other regions in the Earth where we have a sufficient number of crossing paths, and compare the anisotropic signature of these regions.