Explain how seismic waves produced by earthquakes provide evidence for the internal structure of the Earth, including the existence of a liquid outer core.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When an earthquake occurs, seismic waves travel through the Earth and are detected at monitoring stations around the world. Scientists have used the patterns of wave detection to draw conclusions about the layers inside the Earth.
Model answer (5 marks)
P‑waves are longitudinal waves that can travel through solids and liquids, so they are detected everywhere around the globe.
S‑waves are transverse waves that can only move through solids; they are absent on the far side of the Earth, creating a shadow zone.
The absence of S‑waves in the shadow zone shows that part of the Earth’s interior is liquid, because S‑waves cannot pass through fluids.
The fact that P‑waves are still recorded on the far side confirms that the inner core is solid, allowing P‑waves to propagate.
Thus the pattern of P‑ and S‑wave arrivals proves the existence of a liquid outer core and a solid inner core.
S‑waves are transverse waves that can only move through solids; they are absent on the far side of the Earth, creating a shadow zone.
The absence of S‑waves in the shadow zone shows that part of the Earth’s interior is liquid, because S‑waves cannot pass through fluids.
The fact that P‑waves are still recorded on the far side confirms that the inner core is solid, allowing P‑waves to propagate.
Thus the pattern of P‑ and S‑wave arrivals proves the existence of a liquid outer core and a solid inner core.
Examiner tips
- Use the terms P‑wave and S‑wave correctly; explain their wave types. Show the link between S‑wave absence and a liquid layer. Mention the P‑wave shadow zone to prove the solid inner core. Keep the answer concise and to the point.
Common mistakes
- Confusing P‑waves with S‑waves or vice versa. Forgetting to mention the shadow zone. Claiming the whole core is liquid or solid without evidence.
Mark scheme (5 marks)
- P-waves are longitudinal seismic waves that can travel through all mediums (solids and fluids)
- S-waves are transverse seismic waves that cannot travel through fluids/liquids
- S-waves are not detected on the opposite side of the Earth from the earthquake, creating a 'shadow zone'
- This shows that part of the Earth's interior must be liquid, since S-waves cannot pass through fluids
- P-waves are still detected on the far side of the Earth, confirming that a solid inner core exists through which they can travel
Key terms in this question
Related
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