The Lehmann discontinuity is an abrupt increase of P-wave and S-wave velocities at the depth of , discovered by seismologist Inge Lehmann. The thickness is 220 km . It appears beneath continents, but not usually beneath oceans, and does not readily appear in globally averaged studies. Several explanations have been proposed: a lower limit to the pliable asthenosphere, a phase transition, and most plausibly, depth variation in the shear wave anisotropy. Further discussion of the Lehmann discontinuity can be found in the book Deformation of Earth Materials by Shun-ichirō Karato. Notes General references – some historic background. External links Inge Lehmann, UCLA Career highlights of Inge Lehmann from UCLA Structure of the Earth