| Summary: | Ultra Low Velocity Zones (ULVZs) are patches on the core-mantle boundary that are highly anomalous in both Vs (10~40%) and Vp (5~20%). Whilst detections of these regions have been going on for several decades, four extremely large ULVZs, dubbed 'mega-ULVZs', have been detected near Hawaii, Samoa, Iceland, and Galapagos in the last decade. It is hypothesised that such mega-ULVZs are co-located with mantle plumes and may be the CMB anchor of hotspots.We performed a global search for ULVZs with Sdiff, of events Mw5.7+ at all depths between 1990-2022. From this, we have significantly augmented the catalogue of events sampling the ULVZs near Hawaii, Iceland, and Galapagos; and discover two previously unknown ULVZs in the south Atlantic near St Helena [Davison et al, in prep] and near Macdonald [Li et al, in prep]. Additionally, we have also found what we believe to be the first direct Sdiff detection of the Samoan ULVZ, which was originally detected and characterised using SPdKS. Furthermore, we have also identified a new mega-ULVZ approximately 15 degrees northeast of Marquesas which does not appear to be related to any hotspot or mantle plume. This ULVZ was detected using higher frequency Sdiff waveforms, 5-12s as opposed to the more typical 10-20s, indicating it is thinner than other mega-ULVZs previously detected. Initial characterisation using a Bayesian inversion with a 2D wavefront tracker forward model [Martin et al, 2023] suggests the anomaly is around 200km in diameter, 8-10km thick, -30 to -40% dVs, and lies on or near the LLSVP boundary.
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