1.7.2Electricity, Energy and Waves
Discoveries from Seismic Waves
5 min read 5 parts Lesson 40 of 54 Unit quiz
Section 1 of 5Layers of the Earth
The deepest borehole reaches only about 12 km, whereas the centre is over 6000 km down, so indirect evidence is needed.
Crust
- The thin, solid outer layer on which we live.
Mantle
- A thick layer of hot rock beneath the crust that is solid but can flow very slowly.
Outer core
- A layer of molten metal beneath the mantle.
Inner core
- The central region, which is solid and mostly iron and nickel.
Section 2 of 5Evidence from P-waves
Evidence from P-waves
Refraction
- P-waves change speed and direction when they cross boundaries between layers of different density.
Curved paths
- This refraction makes the waves follow curved paths through the Earth.
Layers
- Analysis of these paths shows the Earth consists of layers with different densities.
Shadow zones
- Refraction at the core leaves regions on the far side where no direct P-waves are detected.
Section 3 of 5Evidence from S-waves
Evidence from S-waves
Solids only
- S-waves are transverse and cannot travel through liquids.
Large shadow zone
- On the side of the Earth opposite an earthquake, no S-waves are detected.
Conclusion
- This shows that the outer core is liquid.
Inner core
- Further evidence from seismic waves, reflected from the inner core boundary, shows it is solid.
| Observation | Meaning |
|---|---|
| P-waves refract at boundaries | Layers have different densities |
| S-waves absent on far side | Outer core is liquid |
| P-waves still detected on far side | Waves pass through the core |
Section 4 of 5Interpreting Seismograms from Different Stations
1
A station near the epicentre records P- and S-waves with a short time lag.
2
A station further away, but on the same side, shows the same waves with a longer lag and smaller amplitude.
3
A station on the opposite side of the Earth records P-waves only.
4
The absence of S-waves at that station shows they have been blocked by the liquid outer core.
5
The P-waves arrive later and are smaller, because they travel a longer path and lose energy.
Section 5 of 5Exam Checklist & Tips
1.7.2 – Key Points for the Exam
- Mantle The thick layer of hot rock between the crust and the core.
- Outer core The liquid metal layer around the inner core.
- Shadow zone A region where particular seismic waves are not detected.
- Refraction Change in direction of a wave when its speed changes.
Common Mistakes
- Saying S-waves travel through the outer core; they are stopped by it.
- Describing the mantle as liquid, when it is solid but flows slowly.
- Assuming the inner core must be liquid because it is hot; the enormous pressure keeps it solid.
- Forgetting that evidence comes from waves detected at the surface, not drilling.