Explain why S-waves cannot be detected on the opposite side of the Earth from an earthquake, whereas P-waves can.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Seismic waves produced by earthquakes travel through the Earth. Scientists study which types of seismic wave are detected at different points on the Earth's surface to learn about the Earth's internal structure.
Model answer (4 marks)
P‑waves are longitudinal and can travel through both solids and liquids, so they can pass through the liquid outer core and reach the opposite side of the Earth.
S‑waves are transverse and can only travel through solids. The outer core is liquid, so S‑waves are blocked and cannot cross it, meaning they never reach the opposite side of the Earth.
S‑waves are transverse and can only travel through solids. The outer core is liquid, so S‑waves are blocked and cannot cross it, meaning they never reach the opposite side of the Earth.
Examiner tips
- Use the terms ‘longitudinal’ and ‘transverse’ to show you know the wave types. Mention the liquid outer core explicitly. Explain the blocking effect of the liquid core for S‑waves. Show the logical chain: S‑wave → cannot travel in liquid → outer core liquid → S‑wave blocked → no detection on opposite side.
Common mistakes
- Confusing P‑waves with S‑waves or vice versa. Forgetting to state that the outer core is liquid. Saying S‑waves can travel through liquids. Not linking the liquid core to the blocking of S‑waves.
Mark scheme (4 marks)
- P-waves are longitudinal and can travel through both solids and liquids
- S-waves are transverse and can only travel through solids (not liquids)
- The Earth's outer core is liquid
- S-waves are blocked/cannot pass through the liquid outer core, so they do not reach the other side
Key terms in this question
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →