Seismic Observation of a New ULVZ Beneath the Southern Pacific

We present new observations of core-diffracted shear waves which contain anomalous waveforms sampling the lowermost mantle beneath the southern Pacific region. Data in two distinct geometries, one from New Zealand to North America and the other from the Fiji and Solomon Islands to South America, show evidence of postcursor phases. The postcursor delays and move-outs imply that they are caused by an ultra-low velocity zone (ULVZ). Beamforming analyses of the observed diffracted postcursors show a strong backazimuthal deviation, suggesting this new ULVZ is likely to have a cylindrical shape similar to broad ULVZs sampled by shear diffracted waves elsewhere. Full-waveform modeling suggests that the postcursors seen in North America might be due to the previously modeled ULVZ located to the west of the Galapagos, while those seen in South America are due to a previously unknown ULVZ beneath the Southern Pacific. We cannot fit observations in both geometries by a single ULVZ. For the new location, we propose one cylindrical ULVZ model with a radius of 400 km and a shear wave velocity decrease of 20% centered at geographical coordinates (-33.6, 130) close to the Pitcairn hotspot. Despite some uncertainty in the west-east direction, this new ULVZ observation likely provides another example to support the hypothesis that ULVZs exist at the base of mantle plumes where primordial signatures are observed in the ocean island basalts. We analyze shear waves that diffract along the core-mantle boundary, called Sdiff, beneath the southern part of the Pacific Ocean. Sdiff waves are very sensitive to the seismic velocity structure of the lowermost mantle just above the boundary. For diffracted phases from several earthquakes and across a range of seismic stations, we observe anomalous waveform signals, which we refer to as Sdiff postcursors, and which arrive up to tens of seconds after the main Sdiff phase. We use the seismic data from many seismic stations to find from what direction the postcursor energy is coming from. We infer the cause of the anomalous postcursors are ultra-low velocity zones (ULVZs) on the core-mantle boundary, which are the strongest heterogeneities observed in the lower mantle. Additionally, we compute synthetic data for a range of potential ULVZ models. We find that some of the data can be explained by a previously modeled ULVZ near the Galapagos, but that other data point to a previously unknown ULVZ, beneath the southern Pacific region, although its exact location is somewhat uncertain, particularly in the west-east direction. ULVZs, like this one, might represent the bases of upwelling mantle plumes that feed the hotspots at the surface. This particular one is in a location roughly beneath several southern Pacific hotspots, where geochemists find primordial isotope anomalies. Detection of anomalous Sdiff postcursors sampling the core-mantle boundary beneath the Southern Pacific Beamforming and forward waveform modeling analyses find a new ultra-low velocity zone (ULVZ) with an uncertain location beneath the southern Pacific This provides a potential new seismic example to support the correlation between surface ocean island basalts and ULVZs