Teleseismic travel time tomography ( Travel time tomography )

Continuing in the second lecture, we progressively established the essential elements for building tomographic models of the Earth's mantle : types of data, level of approximation of wave propagation theory, physical and geometric parameterization of the model, inversion method employed to obtain a model. We have presented the techniques employed in the construction of the first tomographic models of the late 1970s, and the first discoveries to which they gave rise. Among these discoveries, confirmed in the decades that followed, was the identification of a very particular morphology at the base of the mantle, which does not resemble that of the first ~ 200 km from the surface (the latter reflecting the distribution of tectonic plates) : two regions with particularly high temperatures located antipodally, one under the Pacific Ocean and the other under Africa, named for want of a better term " LLSVP " (large low shear velocity provinces). We have shown that advances in travel-time tomography ( classic ) over the last few decades have enabled us above all to obtain increasingly precise images of cold regions at great depths. These mainly delineate the tectonic plates that fall back into the mantle in subduction zones (especially around the Pacific). A recent discovery is the realization that these plates do not sink evenly to the bottom of the mantle, but spread out horizontally over thousands of kilometers, to a depth of 660 km or ~ 1 000 km.