Low-velocity zone

[2] A second low-velocity zone (not generally referred to as the LVZ, but as ULVZ) has been detected in a thin ≈50 km layer at the core-mantle boundary.

[2][3][4] The LVZ has been interpreted to indicate the presence of a significant degree of partial melting, and alternatively as a natural consequence of a thermal boundary layer and the effects of pressure and temperature on the elastic wave velocity of mantle components in the solid state.

[4][5] The existence of the low-velocity zone was first proposed from the observation of slower than expected seismic wave arrivals from earthquakes in 1959 by Beno Gutenberg.

For this to occur at the depths where the LVZ is observed, small amounts of water and/or carbon dioxide must be present to depress the melting point of the silicate minerals.

The lack of LVZ beneath continental shields is explained by the much lower geothermal gradient, preventing any degree of partial melting.

Velocity of seismic waves in the Earth versus depth. [ 1 ] S -waves (seismic shear waves) cannot propagate in liquids, leading to negligible velocity in the liquid outer core. The seismic velocities very near the surface ( ≲ 220±30 km ) are markedly lower than at greater depths, demarking the LVZ.
Velocity of seismic S -waves in the Earth near the surface in three tectonic provinces: TNA= Tectonic North America SNA= Shield North America & ATL = North Atlantic. [ 8 ]