Abstract

Tectonic processes at Earth’s surface, are driven by convection deep in Earth’s mantle. Seismic tomography using earthquakes is the main tool to directly image the lower mantle. Such images show two large continental size regions just above the core mantle boundary, one located under the Pacific and the other one under Africa. These two regions have low shear wave velocity, but their role in mantle convection as either a thermal plume or a stable compositional pile is still heavily debated. Here, I will show that the key to unravelling the nature of these two regions are whole Earth oscillations. These normal modes do not only provide shear wave velocity, but also additional information such as their density and attenuation. We find that the low velocity regions are partially dense at their base and have weak attenuation, requiring puzzling new interpretations in terms of mantle dynamics.