Lehmann discontinuity

The Lehmann discontinuity is an abrupt increase of P-wave and S-wave velocities at the depth of 220 km (140 mi) in Earth's mantle, discovered by seismologist Inge Lehmann.

[2][3][4] The thickness is 220 km [citation needed].

It appears beneath continents, but not usually beneath oceans,[5] and does not readily appear in globally averaged studies.

Several explanations have been proposed: a lower limit to the pliable asthenosphere, a phase transition,[6] and most plausibly, depth variation in the shear wave anisotropy.

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Velocity of seismic S -waves in the Earth near the surface in three tectonic provinces: TNA = Tectonic North America, SNA = Shield North America and ATL = North Atlantic. [ 1 ]