A major earthquake occurs and seismometers are placed at different distances from the epicentre. Explain why P-waves are always detected before S-waves at any given seismometer station.

WJEC GCSE Physics (Wales) — 1.7 Seismic waves · Explain · 4 marks · View as Markdown

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

Seismometers around the world record vibrations caused by earthquakes. Both P-waves and S-waves are produced at the same moment during an earthquake.

Model answer (4 marks)

P‑waves are longitudinal waves and S‑waves are transverse waves.
P‑waves travel faster than S‑waves through the same material.
Both types of wave are produced at the same time at the epicentre.
Because P‑waves travel faster over the same distance, they arrive at the seismometer first.

Examiner tips

  • Use the word ‘because’ to link speed and arrival time. Show the causal chain: same source → different speeds → different arrival. Keep each point short and use the exact terminology (longitudinal, transverse).

Common mistakes

  • Confusing the order of arrival with the order of generation. Using vague terms like ‘fast’ without linking to speed. Omitting the fact that both waves are produced simultaneously.

Mark scheme (4 marks)

  1. P-waves are longitudinal waves and S-waves are transverse waves
  2. P-waves travel faster than S-waves through the same material
  3. Both types of wave are produced at the same time / same source (epicentre)
  4. Because P-waves travel faster over the same distance, they arrive at the seismometer first

Key terms in this question

P-waves · S-waves · seismometer · epicentre

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