Evidence of a ULVZ near Vanuatu from Sdiff postcursors

Thin anomalous structures known as ultra-low velocity zones (ULVZs) have been found on the core-mantle boundary (CMB) and have extreme velocity reductions. These features are detected due to their effect on seismic waves that travel through them, typically producing precursors or postcursors. In this study we use postcursors to shear core-diffracted waves (Sdiff+) that sample the CMB near Vanuatu to detect and characterise the properties of a ULVZ. We identified a total of 19 earthquakes originating from the South Pacific Rise region detected by stations across East Asia — particularly Japan — showing Sdiff+ signals. Of these events, six with the highest quality Sdiff+ signals are included in a Bayesian inversion of travel times using the 2D Wavefront Tracker we previously developed. A subset of events was selected for further analysis by modelling using 3D full waveform synthetics for a range of parameters. The comparison of the real data with the synthetic waveforms suggests that a ULVZ is located to the southeast of Vanuatu at 172.2±0.9 °E and 22.9±1.1 °S and its broad-scale structure can be approximated as a cylinder with a height of 20±5 km, radius 240±50 km, and shear wave velocity reduction of 30±5%. These parameters are comparable to other ULVZs previously detected and modelled with Sdiff and Sdiff+. There are appreciable uncertainties in the location along the NW-SE direction due to the distribution of earthquakes and seismic arrays, as well as trade-offs between the height, size and velocity reduction of the ULVZ. Other studies using SPdKS, ScP and PcP have reported detections of ULVZs in the proximate region, some of which are consistent with the well-fitting parameter space of the ULVZ in this study. The Vanuatu ULVZ lies within the southwest edge of the Pacific large low velocity province. There is potentially a mantle plume rooted by this ULVZ that has diverted towards the hotspots on the eastern Australian plate around the Tonga slab, although most tomographic models do not show a continuous plume here.