Evidence of a ULVZ near Vanuatu from Sdiff postcursors

Carl Martin, Lobke Harmsma, James Atkins, Arwen Fedora Deuss, Sanne Cottaar 

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.