Seismological methods : long-period surface waves

We introduced the main seismological tools for studying the lithosphere and asthenosphere, and recalled the properties of surface waves, in particular their frequency dispersion and the way in which depth variations in shear velocity are deduced. We then described the main results obtained on a global scale concerning variations in lithospheric thickness, the progression of the oceanic low-velocity zone with plate age, and the characteristics of the "cratons", the oldest (Archean) regions of the continents. We then showed that the variation in depth of radial anisotropy (polarization anisotropy) can also be used to distinguish lithosphere from asthenosphere, but with little precision. We concluded by introducing azimuthal anisotropy and showing its variations on a global scale in the upper mantle, obtained from surface waves.