Azimuthal Anisotropy and Lateral Velocity Gradient Beneath Australia From Teleseismic P-wave Polarization Anomalies

Kotaro Tarumi , Kazunori Yoshizawa 

Teleseismic P-wave polarization anomalies provide useful constraints on lateral heterogeneity and anisotropic structure beneath seismic stations, but their application to continental-scale structures has remained limited. Here, we analyze long-period teleseismic P-wave polarization anomalies recorded at permanent and temporary seismic stations across Australia. We apply a harmonic analysis to the back-azimuth-dependent polarization anomalies to separate the contributions of the ϕ-term (lateral heterogeneity) and the 2ϕ-term (azimuthal anisotropy) using Bayesian inference. We then map the positive direction of the P-wave velocity gradient and the fast-axis direction, together with their uncertainties. The inferred directions of positive P-wave velocity gradients commonly correlate with crustal thinning along the continental margins, whereas several stations may reflect localized variations in the depth of the lithosphere-asthenosphere boundary (LAB). The spatial patterns of inferred fast-axis directions show clear tectonic-province-dependent variations. In eastern Phanerozoic Australia and the southern North Australian Craton, the anisotropy inferred from P-wave polarization anomalies is likely influenced by present-day asthenospheric mantle flow. In contrast, across much of cratonic Australia, the inferred fast-axis patterns are more plausibly attributed to frozen anisotropy within the lithosphere inherited from ancient tectonic processes. Our results demonstrate the potential of teleseismic P-wave polarization anomalies as a complementary dataset for investigating upper-mantle anisotropy beneath continents. Integrating these constraints with other seismological observations, such as receiver functions and surface-wave tomography, will help resolve the laterally variable and depth-dependent anisotropic architecture beneath the Australian continent and improve our understanding of its tectonic evolution.