A geological survey team detects seismic waves after an earthquake. Seismometers on the opposite side of the Earth from the earthquake detect P waves but do not detect S waves. Explain what this tells scientists about the structure of the Earth's core, and why the two types of seismic wave behave differently when passing through the Earth.

Edexcel A-Level Physics (9PH0) — 4.1 Stress, strain and the Young modulus · Explain · 5 marks · View as Markdown

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

Seismic waves produced by earthquakes travel through the Earth. There are two types: P waves, which are longitudinal, and S waves, which are transverse. Seismometers placed at various points around the Earth record which waves arrive and when.

Model answer (5 marks)

S waves are transverse waves, with particle motion perpendicular to the direction of travel. Transverse waves can only propagate through solids, they cannot travel through liquids. P waves are longitudinal waves, with particle motion parallel to the direction of travel. Longitudinal waves can propagate through both solids and liquids. Because only P waves reach the opposite side of the Earth, the core must be liquid, which blocks S waves but allows P waves.

Examiner tips

  • Use the exact terms ‘transverse’ and ‘longitudinal’ and state the direction of particle motion.
  • Explain the medium restriction for S waves and the ability of P waves to pass through liquids.
  • Show the logical link: S waves absent → core liquid.
  • Keep the answer concise and to the point.

Common mistakes

  • Confusing the direction of particle motion for S and P waves.
  • Claiming S waves can travel through liquids.
  • Failing to state that the core is liquid as the conclusion.

Mark scheme (5 marks)

  1. S waves are transverse waves / S waves have oscillations perpendicular to the direction of travel
  2. Transverse (S) waves can only travel through solids / S waves cannot travel through liquids
  3. P waves are longitudinal waves / P waves have oscillations parallel to the direction of travel
  4. Longitudinal (P) waves can travel through both solids and liquids
  5. Because only P waves reach the opposite side, the Earth's core must be liquid

Key terms in this question

seismic waves · P waves · S waves

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