Earthquakes produce two types of seismic wave: P-waves and S-waves. Explain the differences between P-waves and S-waves, including how the behaviour of these waves as they travel through the Earth's interior can be used to determine that the Earth has a liquid outer core.

P‑waves are longitudinal waves, with particle motion parallel to the direction of travel. S‑waves are transverse waves, with particle motion perpendicular to the direction of travel.

P‑waves travel faster than S‑waves; consequently they arrive first at seismic stations.

S‑waves cannot propagate through fluids, because a fluid offers no shear resistance. P‑waves, being compressional, can travel through both solids and liquids.

On the opposite side of the Earth, no S‑waves are recorded – a shadow zone – because they are blocked by the liquid outer core. The absence of S‑waves in this zone shows that the outer core must be liquid.