Explain why seismologists can use seismic waves to determine the internal structure of the Earth.

Seismologists can determine the internal structure of the Earth because seismic waves behave differently in different materials. P‑waves are longitudinal and can travel through solids, liquids and gases, whereas S‑waves are transverse and can only move through solids. When an earthquake occurs, the P‑waves and S‑waves generated travel outward. If a wave passes from one layer to another, its speed changes and it refracts, producing a predictable change in arrival time and direction. By recording the arrival times of P‑waves and S‑waves at many seismic stations, seismologists can map the zones where S‑waves are absent (the shadow zones) and where P‑waves are delayed. These patterns show that the outer core is liquid (no S‑waves) and that there are distinct solid layers above and below it, allowing the depth and nature of each layer to be inferred.