Seismic Full-Waveform Inversion Reveals Radially Anisotropic Upper Mantle Structures Beneath the Australian Plate

To explore seismic structures beneath the Australian continents and subduction zone geometry around the Australian plate, we introduce a new radially-anisotropic shear-wavespeed model, AU21. By employing full-waveform inversion on data from 248 regional earthquakes and 1,102 seismographic stations, we iteratively refine AU21, resulting in 32,655 body-wave and 35,897 surface wave measurements. AU21 reveals distinct shear-wavespeed contrasts between the Phanerozoic eastern continental margin and the Precambrian western and central Australia, with the lithosphere-asthenosphere boundary estimated at 250–300 km beneath central and western Australia. Notably, a unique weak radial anisotropy layer at 80–150 km is identified beneath the western Australian craton, possibly due to alignments of dipping layers or tilted symmetry axes of anisotropic minerals. Furthermore, slow anomalies extending to the uppermost lower mantle beneath the east of New Guinea, Tasmania, and the Tasman Sea indicate deep thermal activities, likely contributing to the formation of a low wavespeed band along the eastern Australian margin. In addition, our findings demonstrate the stagnant Tonga slab within the mantle transition zone and the Kermadec slab's penetration through the 660-km discontinuity into the lower mantle.