Radially anisotropic upper mantle structure beneath the Australian plate using full-waveform inversion

Abstract:

We construct the radially anisotropic upper mantle structure beneath the Australian plate by applying full-waveform inversion technique. 248 regional earthquakes from 1993 to 2019, and 1,102 seismographic stations are used in this study. Twenty-one preconditioned conjugate gradient iterations are utilized to minimize phase discrepancies between observed and synthetic seismograms, generating our current model AU21 using 32,655 body-wave (15-40 s) and 35,897 surface-wave (25-100 s) measurements. The point spread function tests demonstrate good resolution and weak inter-parameter trade-off in most of the study region. AU21 confirms the sharp contrast in shear wavespeed perturbations between the Phanerozoic eastern continental margin with the Precambrian western and central Australia, and this boundary is offset to the east of the Tasman Line. The lithosphere thickness is about 250-300 km beneath central and western Australia, and relatively large faster SH radial anisotropy is observed at shallow depths (80-150 km) beneath the Australian continent. However, a unique weak radial anisotropy layer shows up beneath the western Australian craton, which may arise from changes in tilt angles of seismic azimuthal anisotropy, by dipping layers alignments or tilted symmetry axes of anisotropic minerals. Slow anomalies extending down to the uppermost lower mantle beneath east of New Guinea, the Tasman Island, and the Tasman Sea may suggest deep thermal activities beneath these regions, possibly contributing to the formation of the low wavespeed band along the eastern Australian margin. Furthermore, AU21 confirms that the Tonga slab is stagnant within the mantle transition zone, while the Kermadec slab penetrates through the 660-km discontinuity down to the lower mantle.