An S wave cannot travel through liquids such as Earth’s outer core.
S waves, or secondary waves, are body waves that move through solid material by producing shear motion perpendicular to the direction of travel. Liquids cannot sustain the sideways rigidity required for this motion, so S waves stop at the liquid outer core.
This behavior provided important evidence about Earth’s interior. Seismologists observed that earthquakes create an S-wave shadow zone on the far side of Earth, beginning at about 103 degrees from the earthquake’s source. The missing S waves indicated that a liquid layer lies deep inside the planet.
P waves, or primary waves, can pass through both solids and liquids, although they change speed and direction at boundaries. Surface waves such as Love and Rayleigh waves travel along Earth’s exterior rather than passing through the whole planet. S-wave strength is also important in earthquake engineering because these waves can cause strong side-to-side shaking.