Global full-waveform inversion reveals previously undetected positive wave speed anomalies beneath the Pacific Ocean

Seismic tomography, a critical tool for studying Earth's interior structure and dynamics, has revealed positive seismic wave speed anomalies in the mantle that are commonly interpreted as slabs, the remnants of subducted lithosphere. However, classical travel-time tomography relies on the inversion of travel times of a few easily identifiable body wave phases along ray paths or volumetric sensitivity kernels, which is strongly dependent on the geometry of seismic sources and receivers. Since both of these are primarily clustered on modern convergent plate boundaries, the resulting tomographic resolution is highly variable across the mantle. Full-waveform inversion (FWI) attempts to reduce this dependence by fitting whole seismograms, thereby including many reflected and refracted body wave phases to enhance the volumetric sensitivity of the inversion.Here, we analyse a new global tomographic model constructed using FWI. The mantle structure imaged in this model reveals significantly more positive seismic wave speed anomalies in the mantle when compared to travel-time tomography, particularly in regions with low seismic activity and limited station coverage. Notably, FWI detects positive wave speed anomalies with slab-like morphologies at ~1000 km depth beneath the Pacific Ocean that fall outside the coverage of classical travel-time tomography. We demonstrate the sensitivity of FWI to wave speed anomalies below the western Pacific using forward wavefield modelling. Importantly, we find that these newly imaged positive wave speed anomalies do not correspond to reconstructed subduction zones in existing global plate reconstructions.Our work challenges the widespread assumption that positive wave speed anomalies (exclusively) represent subducted slabs, highlighting potential gaps in either global plate reconstructions or the current understanding of the nature of seismic anomalies in the mantle.