Inversion of body-wave waveforms to infer high-resolution 3-D S-velocity structure in the D″ region at the western margin of the Pacific LLSVP: Evidence for small-scale plumes and paleoslabs

Abstract:

Although previous global tomographic studies found a large low S-velocity province (LLSVP) in the lowermost mantle beneath the Pacific, due to a lack of resolution it was unclear whether the LLSVP consists of clusters of small-scale low-velocity anomalies or large-scale anomalies. We recently deployed a seismic array in Thailand (TSAR; Tanaka et al., BERI, 2019) which provides a dataset with wide azimuthal coverage at the western edge of the Pacific LLSVP. We then analyzed the new dataset using localized waveform inversion (Kawai et al., GJI, 2014), which has better resolution, especially for low-velocity anomalies, than traveltime tomography. We find high-velocity anomalies extending vertically to a height of ~400 km above the core-mantle boundary (CMB) beneath the Philippine Sea and small-scale low-velocity patches with a diameter of ~300 km at the CMB beneath New Guinea. The locations of the high-velocity anomalies are consistent with the past Izanagi-plate subduction boundary, and the low-velocity anomalies can be interpreted as a small-scale plume cluster. Hence, we concluded that vertical flow (upwelling plumes and downwelling of slabs) is dominant in the lowermost mantle beneath the western Pacific region (Suzuki et al., PEPI, 2020). We also discuss the various validation tests used to confirm the reasonableness of the inversion results.