Geologists use seismic waves to study the internal structure of the Earth. Explain how the properties of P-waves and S-waves allow scientists to deduce that the Earth has a liquid outer core.

WJEC A-Level Physics (Wales) — 5.2 Gravitational fields (A-Level only) · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

When a large earthquake occurs, seismic monitoring stations around the world detect the waves that travel through the Earth. Scientists have found that P-waves are detected on the opposite side of the Earth from the earthquake, but S-waves are not detected in a large 'shadow zone' on the opposite side.

Model answer (5 marks)

S‑waves are transverse waves, with particle motion perpendicular to the direction of travel. They cannot propagate through fluids or liquids, so they are stopped by any liquid layer. In seismic recordings the S‑waves are absent from the opposite side of the Earth – a large shadow zone – indicating they do not pass through the core. P‑waves, on the other hand, are longitudinal waves, with particle motion parallel to the direction of travel, and they can travel through solids, liquids and gases. P‑waves are still detected on the far side of the Earth, showing that the core does not block them. The fact that S‑waves are blocked while P‑waves are not therefore demonstrates that the outer core is liquid.

Examiner tips

  • Use the terms ‘transverse’ and ‘longitudinal’ to show understanding of wave types.
  • Explain why S‑waves cannot travel through liquids and why P‑waves can.
  • Show the link between the absence of S‑waves and the presence of P‑waves to deduce a liquid core.
  • Keep the answer concise – 5 marks can be earned with 3–4 short points.

Common mistakes

  • Confusing the direction of particle motion for wave type; remember transverse = perpendicular, longitudinal = parallel.
  • Forgetting to mention that P‑waves are detected on the far side, which is the key evidence for a liquid core.
  • Using vague language such as ‘cannot move through the core’ without specifying fluid nature.

Mark scheme (5 marks)

  1. S-waves are transverse waves / S-waves have oscillations perpendicular to the direction of travel
  2. S-waves cannot travel through fluids/liquids
  3. S-waves are not detected on the opposite side of the Earth / S-waves do not pass through the core
  4. P-waves are longitudinal waves / P-waves can travel through all mediums including liquids
  5. P-waves are still detected on the opposite side, so the core must be liquid / the fact that S-waves are blocked but P-waves are not shows the outer core is liquid

Key terms in this question

P-waves · S-waves · seismic waves

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