Seismic Observation of a New ULVZ Beneath the Southern Pacific

Abstract

AbstractWe 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 Galápagos, 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.