Earthquakes produce two types of seismic wave: P-waves and S-waves. Scientists monitoring a distant earthquake detect P-waves at a recording station but do not detect any S-waves. Explain the difference between P-waves and S-waves, and use this to explain why only P-waves are detected at the recording station.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When an earthquake occurs, it releases energy in the form of seismic waves that travel through the Earth. Scientists use networks of recording stations around the world to detect these waves.
Model answer (5 marks)
P‑waves are longitudinal waves, meaning the particle motion is parallel to the direction of wave propagation. S‑waves are transverse waves, with particle motion perpendicular to the direction of propagation.
S‑waves cannot travel through fluids or liquids, whereas P‑waves can move through both solids and liquids.
The Earth’s outer core is liquid. When a distant earthquake occurs, the seismic waves must pass through this liquid layer. The S‑waves are blocked by the liquid outer core and do not reach the recording station, while the P‑waves pass through the liquid outer core and are detected.
S‑waves cannot travel through fluids or liquids, whereas P‑waves can move through both solids and liquids.
The Earth’s outer core is liquid. When a distant earthquake occurs, the seismic waves must pass through this liquid layer. The S‑waves are blocked by the liquid outer core and do not reach the recording station, while the P‑waves pass through the liquid outer core and are detected.
Examiner tips
- Define P‑ and S‑waves clearly with the correct motion description. Explain the liquid outer core and its effect on each wave type. Show the logical chain: liquid core blocks S‑waves, allows P‑waves.
- common_mistakes
- :
- Confusing longitudinal with transverse motion. Forgetting that S‑waves cannot travel through liquids. Not mentioning the liquid outer core as the blocking medium.
Mark scheme (5 marks)
- P-waves are longitudinal waves (oscillations parallel to the direction of travel/energy transfer)
- S-waves are transverse waves (oscillations perpendicular to the direction of travel/energy transfer)
- S-waves cannot travel through fluids / liquids
- The Earth has a liquid outer core (through which the seismic waves must pass)
- P-waves can travel through all mediums (solid and fluid/liquid), so they pass through the liquid outer core and reach the recording station; S-waves cannot, so they are blocked
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Gravitational fields (A-Level only) questions
- Sonar is used on a research vessel to map the seabed. The vessel emits ultrasoun…
- Geologists use seismic waves to study the internal structure of the Earth. Expla…
- The human ear can detect sounds within a certain range of frequencies. Explain h…
- Ultrasound waves are used in sonar and foetal scanning. Explain how ultrasound w…