In geology, an S-wave is the seismic wave that cannot travel through liquids.
S-waves, or secondary waves, are body waves that move through Earth’s interior by shearing material sideways or up and down. Liquids cannot sustain this kind of shear motion, so S-waves stop at liquid layers such as Earth’s outer core.
P-waves, by contrast, are compressional waves and can pass through solids, liquids, and gases. This difference helped scientists infer that Earth has a liquid outer core. S-waves arrive after P-waves because they travel more slowly.
S-waves should not be confused with surface waves, including Love and Rayleigh waves, which travel along Earth’s surface and often cause severe damage during earthquakes.