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AQA GCSE Physics · Topic 4.6 Waves · Physics only · Higher
Seismic Waves: P-waves, S-waves and the Structure of the Earth
Physics onlyHigher Combined Science students don't need this page.
In this lesson you'll learn to: describe P-waves and S-waves, explain why their paths curve and refract inside the Earth, explain the two shadow zones, and use where the waves are and aren't detected as evidence for a liquid outer core, a place nobody can ever visit.
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Prefer to watch? Scroll to the bottom for the video on this topic
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A video walkthrough is coming soon — it will appear at the bottom of this page
In this lesson you'll learn to: describe P-waves and S-waves, explain why their paths curve and refract inside the Earth, explain the two shadow zones, and use where the waves are and aren't detected as evidence for a liquid outer core, a place nobody can ever visit.
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Start here — make an earthquake
Don't read anything yet. On tab 1, press Earthquake! and watch which seismometers pick up the P-waves. Then do the same with S-waves on tab 2: which ones go quiet? On tab 3, start from an Earth whose inside is a mystery and use the results to work out what's in there, the way scientists did.
The detector results from one earthquake
Angle = how far round the Earth from the earthquake. ✓ detected · ✗ nothing arrived.
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▲ Three tabs: P-waves, S-waves, then work out the structure from the results
Found the liquid core? Now learn to explain it.
The notes below compare P- and S-waves, explain the shadow zones, and give you the four points for "how do seismic waves tell us about the Earth?"
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Revision notes
⭐ What is a seismic wave?
A wave produced by an earthquake that travels through the Earth. There are two types: P-waves and S-waves.
P-waves and S-waves
| P-waves | S-waves | |
|---|---|---|
| The letter stands for | Primary | Secondary |
| Type of wave | Longitudinal | Transverse |
| Speed | fast: arrive first | slow: arrive second |
| Travel through | solids and liquids | solids only |
| When the density changes | refract | refract |
Remember: S-waves are Secondary, transverse, slower and solids only.
Why the paths curve
The density of the mantle changes gradually with depth, so the wave speed changes gradually too. The waves keep refracting a little at a time, which bends their paths into curves. At the edge of the core the density changes suddenly, so the waves change direction sharply there.
The two shadow zones
S
S-wave shadow zone
No S-waves arrive more than about 104° round the Earth from the earthquake. S-waves can't travel through liquids, so the outer core must be liquid. It blocks them, so this zone is huge.
P
P-wave shadow zone
No P-waves arrive between about 104° and 140°. P-waves do pass through the core, but they refract as they go in and out, so they're bent away from this band.
⭐ Why this matters: nobody can go there
The deepest hole ever drilled is about 12 km. The core starts about 2900 km down. We can't observe the inside of the Earth directly, so where seismic waves are, and aren't, detected is our evidence for its structure.
Beno Gutenberg worked out how deep the core starts (1914). Harold Jeffreys showed the outer core is liquid (1926). Inge Lehmann discovered the solid inner core (1936) from tiny P-wave signals arriving inside the shadow zone.
The structure seismic waves revealed
| Layer | State | The evidence |
|---|---|---|
| Crust | solid | thin outer layer |
| Mantle | solid | S-waves travel through it |
| Outer core | liquid | S-waves can't get through it: the S-wave shadow zone |
| Inner core | solid | P-waves speed up and refract again near the centre |
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How to write it in the exam
"How do seismic waves give evidence about the Earth's structure?" (4 marks)
P-waves go through solids and liquids; S-waves only solids →
both refract when the density changes →
no S-waves on the far side, so the outer core is liquid →
where they are and aren't detected is evidence, because we can't observe it directly.
the two wavesrefractionthe conclusion and why it's needed
Exam-style question · page 11 · 9 marks
P-waves from an earthquake at E. No S-waves reach X; no P-waves reach Y.
(a) Give two differences between P-waves and S-waves.
- Any two: P-waves are longitudinal, S-waves transverse · P-waves are faster (arrive first) · P-waves travel through solids and liquids, S-waves through solids only. (2)
(b) Explain why the paths of the P-waves through the mantle are curved.
- The density of the mantle changes with depth (1), so the wave speed changes and the waves refract (change direction) (1).
(c) Explain why no S-waves are detected in region X.
- S-waves are transverse and cannot travel through liquids (1); the outer core is liquid, so they can't pass through it (1).
(d) Explain why no P-waves are detected in region Y.
- P-waves refract as they enter and leave the core, where there's a sudden change in density (1), so they change direction and can't reach region Y (1).
(e) Suggest why seismic waves gave new evidence about the core.
- Nobody can observe or drill to the core: where seismic waves are, and aren't, detected shows its structure (1).
Region X is much bigger than Y: S-waves are stopped by the whole liquid outer core, but P-waves pass through it and are only bent.
📘 Now do it in your workbook
Page 11: define a seismic wave, fill in the P-wave and S-wave table, write the 4-mark explanation of how seismic waves reveal the Earth's structure, and a 9-mark exam question on the shadow zones.
AQA GCSE Physics Workbook · WAVES · page 11The full workbook covers the whole of Topic 4.6 Waves, with exam-style questions and worked answers.
✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this spec point.
Don't forget to hit “Mark complete” at the bottom of the lesson.
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Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
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Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the animation and the notes above cover every mark.
Now put it into practice
Page 11 of the workbook has the P- and S-wave table, the 4-mark explanation and a 9-mark exam question on the shadow zones, with answers at the back.
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