SKS wave

An SKS wave is a type of seismic wave, with the three letters designating its path through layers of the Earth. The first S indicates that the wave starts as a shear wave or s-wave traveling through the mantle, the K denotes the change to a compressional wave or p-wave while traveling though the outer core, and the final S again indicates an s-wave, returning through the mantle. This occurs because s-waves cannot travel through liquid, such as the liquid iron of the Earth's outer core, and consequently they transform into p-waves.

References

Fowler, C. M. R. (1990) The Solid Earth: An Introduction to Global Geophysics, Cambridge: Cambridge Univ. Press, 472. ISBN 0-521-37025-6