Seismic observation of the Earth’s small-scale structure

The Earth’s mantle is chemically and thermally heterogeneous varying in 3-dimensions and on many length-scales. Subduction introduces slabs into the mantle while interactions with the core may enrich the mantle in iron. The lower mantle demonstrates the strongest
seismic anomalies outside of the crust. The Large Low Shear Velocity Provinces (LLSVPs), two volumes 1000s km across with seismic velocity reductions of 1-3 %, are likely thermally and chemically distinct from the surrounding mantle. Smaller velocity anomalies are detected close to the Core-Mantle Boundary (CMB) such as velocity increases ∼100 km thick often related to subducted slabs, and strong velocity decreases 10-100 km thick called
Ultra Low Velocity Zones.

Array analysis of signals arriving before PKP demonstrates that they are waves scattered from volumes of anomalous material with 10 km scale-lengths in the lowermost
mantle under South Africa. The data image a heterogeneous 80 km tall ridge at the CMB likely related to the edge of the African LLSVP. Scattering is likely caused by heterogeneities with strongly reduced velocities and increased densities probably elevated above
the CMB by entrainment into the LLSVP.

Scattered PKKP waves (PK•KP) reveal heterogeneities irregularly distributed in the lowermost 300 km of the mantle. Scattering is also seen under South Africa, co-located with PKP observations. Anomalies are preferentially located towards the edges of the LLSVPs and regions of subducted material. The predisposition of small-scale anomalies towards the edge of the large-scale structure suggests control by dynamic processes.

P(Pdiff )-travel-times are used to resolve the boundary of the Pacific LLSVP. The east of the LLSVP displays a sharp (60 km wide) transition and is traced steeply upwards sloping at 70◦. The transition at the northern edge is broader (120 km wide) and shallower (30◦slope). The proximity to active subduction may sharpen and steepen the boundary of the LLSVP, providing insight into the dynamics of the lowermost mantle.