Project results include a great number of newly discovered ULVZs as well as more detailed constraints on their internal structure and morphology. In the process, we developed several new techniques to resolve ULVZs.
We have pushed previous observations of diffracted waves to higher frequencies, finding thinner ULVZs (on the order of ~10 km, Martin et al. 2024) as well as layering within larger ULVZs (~2 km, Li et al. 2022).
We have discovered new observations of postcursors to core-diffracted shear waves that have let us map ULVZs on the CMB near Galapagos (Cottaar et al. 2022), St Helena (Davison et al. 2023), Pitcairn (Li et al. 2023), as well as two in the Pacific that are less easily linked to a hotspot (Martin et al. 2024, and Martin et al. 2025, in revision). We are now in the process publishing our approach to pick a global data set of core-diffracted shear waves, identifying the largest ULVZs around the globe, and also focussing on the extensive areas with no large ULVZs (Atkins et al., in revision). ULVZs in this data set appear to correlate with hotspot volcanism at the surface and can also be related to larger scale structure present in the lowermost mantle.
We have developed a Bayesian approach to mapping the shape and location of ULVZs (Martin et al. 2023a). We have applied this to the Hawaiian ULVZ, showing it is elongated which shows a relationship to the surrounding dynamics (Martin et al. 2023b). The technique is also good to show trade-offs between locations and ULVZ size, particularly when the data available only illuminate a ULVZ from a single direction. To illustrate this trade-off the Bayesian mapping approach has been applied to central Pacific ULVZ (Martin et al. 2024) and a ULVZ near Vanuatu (Martin et al. 2025, in revision).
Additionally, we provided complimentary constraints on the nature of ULVZs. We provided the first ever constraints on the ULVZ using postcursors to Pdiff waves, which are complimentary to Sdiff (Jagt et al. 2024). Our initial interpretation provides potential insights to the compositional nature of the ULVZ. Further complimentary came from the application of vertically scattered shear waves (ScS), which allowed us to map the morphology of the Hawaiian ULVZ (Jenkins et al. 2021). The results could be linked to the surrounding impinging subducted slabs.
We have explored the potential of ULVZs to be caused by present-day partial melting processes (Dannberg et al. 2021). We found that under most-reasonably assumed conditions any melt would sink to the bottom and create a 100% melt layer. Viscous forces are not sufficient to create partial melt zones that are consistent with seismically observed parameters.
We have also zoomed in on the finest-scale structure at the CMB, testing our limits of knowledge here. Russell et al. 2022a showed that the exotic PKKPdf phase would be most sensitive to a layer of melt at the CMB. Global observations show significant scatter, and trade-offs with structures elsewhere means that a layer of several kms could be present without being seen. Russell et al. 2023 shows that modes are okay with the presence of a global slow layer of several km a the CMB, although this layer would have to be solid.
Summaries of our work have been published in short pieces for Astronomy and Geophysics (Cottaar et al. 2024) and Nature Communications (Russell et al. 2024).
