# 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

> 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.

## 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

- [seismic waves](https://www.gradenine.co.uk/glossary/seismic-waves)
- [P waves](https://www.gradenine.co.uk/glossary/p-waves)
- [S waves](https://www.gradenine.co.uk/glossary/s-waves)

## Related

- [Revision notes for Edexcel A-Level Physics (9PH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-geological-survey-team-detects-seismic-e9dc1186) · Published by Druglandscape Ltd.