A geologist uses seismic waves produced by an earthquake to investigate the structure of the Earth's interior. The seismic waves travel from the earthquake's source, pass through different layers of rock, and are detected at monitoring stations on the surface. Explain how the behaviour of seismic waves at boundaries between rock layers provides information about the Earth's interior structure.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Seismic waves are produced by earthquakes and travel through the Earth. When they reach a boundary between two different rock layers, they can be partially reflected back and partially transmitted onward. Monitoring stations on the surface record the time at which the waves arrive.
Model answer (5 marks)
Seismic waves are partially reflected when they encounter a boundary between two rock layers, and the rest is transmitted into the next layer.
The reflected waves travel back to the surface, and because the wave speed is constant within a given medium, the travel time of the reflected wave can be used to calculate the depth of the boundary.
When the waves enter a denser or otherwise different rock layer their speed changes, which causes refraction – the wave changes direction.
By recording the arrival times of both reflected and refracted waves at several monitoring stations, scientists can determine the depth, thickness and type of each layer.
Thus the pattern of reflected and refracted waves allows a geologist to map the internal structure of the Earth.
The reflected waves travel back to the surface, and because the wave speed is constant within a given medium, the travel time of the reflected wave can be used to calculate the depth of the boundary.
When the waves enter a denser or otherwise different rock layer their speed changes, which causes refraction – the wave changes direction.
By recording the arrival times of both reflected and refracted waves at several monitoring stations, scientists can determine the depth, thickness and type of each layer.
Thus the pattern of reflected and refracted waves allows a geologist to map the internal structure of the Earth.
Examiner tips
- Use the word "reflected" and "transmitted" – these are key terms. Show the relationship between travel time, speed and depth. Mention refraction and the change of direction. Explain that multiple stations give a 3‑D picture.
Common mistakes
- Confusing reflected waves with refracted waves. Assuming wave speed is the same in all layers. Not linking travel time to depth calculation.
Mark scheme (5 marks)
- Seismic waves are partially reflected at a boundary between two different rock layers / media
- The remainder of the seismic waves are transmitted / continue through into the next layer
- The speed of seismic waves is constant within a given medium, so the time taken for reflected waves to return to the surface can be used to calculate the depth of the boundary
- When seismic waves pass into a denser / different rock layer their speed changes, causing refraction / change of direction
- By analysing the pattern of reflected and refracted waves detected at multiple monitoring stations, scientists can map the depth, thickness and nature of different rock layers inside the Earth
Key terms in this question
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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