As early as the 1920s, Gutenberg (1926) equated the low-viscosity asthenosphere with a seismic low-velocity zone. Regions of high and low velocity in the mantle are today usually determined with tomographic methods. These methods are, however, not very sensitive to sharp boundaries. Converted waves have been used for many years to study boundaries in the mantle. A velocity reduction with depth (possibly the lithosphere-asthenosphere boundary, LAB) was found with P-to-S converted waves (P-receiver functions) beneath North America at ∼100 km depth with a sharpness of about 10 km (Rychert and Shearer 2009)....

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