Seismic anisotropy in the mantle transition zone beneath Fiji‐Tonga

We demonstrate a rare, unequivocal case of anisotropy in the transition zone (TZ), evident from many horizontal‐polarized shear‐waves (SH) that arrive up to 3 s earlier than vertical‐polarized shear‐waves (SV). The anisotropic region correlates with deep earthquakes away from the active subduction zone and known boundaries of lateral heterogeneities in compression‐ and shear‐wave speeds. We interpret the anisotropy as strong fabric associated with a large petrologic anomaly in the core of remnant lithosphere which is buoyant and stagnates in the TZ. Fast subduction of cold slab actually favors such a configuration, so slab penetration into the lower mantle does not uniformly occur.