Kotaro Tarumi , Kazunori Yoshizawa

Lateral structural variations in the upper mantle generate azimuthal dependence in receiver functions (RFs) based on incoming directions of body waves. Although these azimuthal variations in RFs have not been considered in earlier studies of RF inversions, they provide a means to image localized changes in upper mantle interfaces. In this study, we incorporate azimuth-dependent RFs into a joint Bayesian inversion with multimode surface waves, applying this approach to major permanent broadband stations in Australia. The resulting models reveal dependence on event directions, and by identifying P-to-S conversion depths, we constructed a localized map of conversion points, reflecting local lateral variations of upper mantle discontinuities beneath each station. At the CTAO station in northeastern Australia, the lithosphere thickens rapidly northwestward, from 70 km to 120-130 km depth, corresponding to the tectonic boundary with the western cratonic region. At stations in western and central Australia, lithospheric thickness also varies laterally within the stable cratons, though these changes are more gradual than those in northeastern Australia. In addition to the lithosphere-asthenosphere boundary (LAB), both mid-lithospheric discontinuities (MLDs) and X-discontinuities (X-Ds) are observed in the local 1-D profiles. The X-Ds, characterized by seismic velocity jumps below the LAB, are found at multiple depths around 170, 220, 260, and 310 km, depending on location, accompanying the weakened radial anisotropy across these depths. The multiple MLDs are also identified in the cratonic regions, showing substantial variations in their seismological properties, including both positive or negative S-velocity jumps, which vary with location and depth. Our approach, incorporating azimuth-dependent RFs, enables the detection of localized changes in the upper mantle discontinuities and associated elastic properties, providing new insights into the complex layering of the upper mantle.