Explain why S-waves cannot be detected at seismometer stations on the opposite side of the Earth from an earthquake.

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).

During a large earthquake, seismometer stations positioned more than about 105° away from the earthquake epicentre detect P-waves but record no S-waves.

Model answer (4 marks)

S‑waves are transverse waves that can only propagate through solids.
The outer core of the Earth is liquid.
When S‑waves reach the liquid outer core they are blocked, absorbed or stopped.
This creates an S‑wave shadow zone on the far side of the Earth, so seismometers there record no S‑waves.

Examiner tips

Use the term ‘transverse waves’ and ‘liquid outer core’ – these are key phrases. Explain the blocking effect in one sentence. Show the logical chain: S‑waves → need solid → outer core liquid → blocked → shadow zone.

Common mistakes

Confusing P‑waves with S‑waves. Forgetting to mention the liquid outer core. Using vague terms like ‘cannot travel’ without linking to the core’s state.

Mark scheme (4 marks)

S-waves are transverse waves that can only travel through solids
The outer core of the Earth is liquid
S-waves are blocked / absorbed / stopped when they reach the liquid outer core
This creates an S-wave shadow zone on the far side of the Earth

Key terms in this question

S-waves