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AQA GCSE · Question 04.4 · Waves
Figure 9 shows the layers inside the Earth. An earthquake occurs at the position shown. Which letter shows the position where only P-waves will be detected? Give a reason for your answer.
Figure 9 shows the layers inside the Earth. An earthquake occurs at the position shown. Which letter shows the position where only P-waves will be detected? Give a reason for your answer.
How to approach this question
1. Recall the properties of P-waves and S-waves, specifically which states of matter they can travel through.
2. Look at the diagram. The waves must pass through the Earth's core to reach some of the locations.
3. Identify which type of wave cannot pass through the liquid outer core.
4. The area where this wave cannot be detected is called a "shadow zone". Identify which letter is in this shadow zone.
Full Answer
Position: C
Reason: S-waves are transverse and cannot travel through a liquid. The liquid outer core blocks the S-waves from reaching position C, creating an "S-wave shadow zone". P-waves are longitudinal and can travel through the liquid outer core, so they can be detected at C.
P-waves are longitudinal and can travel through both solids and liquids. S-waves are transverse and can only travel through solids.
The Earth has a liquid outer core. When an earthquake occurs, both P-waves and S-waves travel out.
- To reach positions A, B, and D, the waves can travel through the solid mantle. Both P and S waves will be detected.
- To reach position C, the waves must pass through the core. P-waves can pass through the liquid outer core (though they are refracted), but S-waves are stopped completely.
This creates an "S-wave shadow zone" on the side of the Earth opposite the earthquake. Therefore, at position C, only P-waves will be detected.
Common mistakes
✗ Choosing another position. At A, B, and D, both wave types would be detected (assuming they are not in the P-wave shadow zone caused by refraction).\n✗ Giving an incorrect reason, e.g., saying P-waves cannot travel through liquid.\n✗ Not mentioning the liquid outer core as the reason.
Practice the full AQA GCSE Physics Higher Tier Paper 2
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