Seismic tomography, converted waves ('receiver' functions), different types of anisotropy, how they are measured, and their relationship with rheology. Seismic definition of LAB.
We first introduced the notion of "birefringence" of shear volume waves in an anisotropic medium, then showed how this property is observed and interpreted in the Earth's upper mantle. We describe how the joint use of SKS volume waves and surface waves improves the depth resolution of azimuthal anisotropy, and reveals several anisotropic layers corresponding to different directions of the fast velocity axis. In particular, the base of the lihosphere can be defined as the transition to a deep layer with a fast axis oriented in the current direction of absolute plate motion, both in ocean basins and beneath continents. This is particularly clear under the Canadian shield, but also in other regions of the globe, such as South Africa. Under the oceans, the "low-velocity" zone is particularly well-developed, but its exact nature is still unknown. Pressure and temperature effects alone predict its existence, but not its amplitude or the speed of the drop in shear velocities at its upper limit. Various models have been proposed: the presence of partial melting, dehydration of the lithosphere at the time of its formation at the ridge, two phenomena that have a major influence not only on shear rates, but also on viscosity. Difficult measurements of electrical conductivity and seismic attenuation should enable us to pinpoint the dominant mechanism in the near future.
