The Hole in the Pacific LLVP and Multipathed SKS

In contrast to a relatively simple whole structure of the African Large Low Velocity Province (LLVP), the Mid-Pacific LLVP appears to be much more complex and likely interacts more with the down-going slab debris from the circum-Pacific subduction zones. Tomographic models show an apparent hole in the Mid-Pacific LLVP, coinciding with observed anomalous SPdKS arrivals. Previous studies have linked these anomalies to a large-scale mega ultra-low velocity zone (ULVZ) exhibiting up to a 45% S-wave velocity reduction. To further investigate this anomaly, we analyzed SKS waveforms from Fiji–Tonga earthquakes recorded by the USArray. Many events display pronounced travel time jumps and waveform distortions near epicentral distances of 100°, consistent with strong multipathing effects. Notably, such complexities are absent in S and SKKS phases, indicating that only the down-going SKS leg is affected. Using waveform modeling, we find that a northeast-dipping high-velocity anomaly approximately 300 km wide, 800 km long, and with a shear velocity increase of ~2% provides a good fit to the observed SKS data. This apparent LLVP hole may represent a localized downwelling within the LLVP or a remnant slab fragment interacting with the deep mantle.