Section 1 of 6Earthquakes and Seismic Waves
A sudden movement of rock in the Earth's crust releases energy as waves.
Source
- The waves start at the focus of an earthquake and travel through the Earth in all directions.
Detection
- Instruments called seismometers record the arrival of the waves at stations around the world.
Types
- Two kinds of body wave travel through the Earth: primary waves (P-waves) and secondary waves (S-waves).
Information
- The arrival times and paths of the waves allow scientists to work out what the inside of the Earth is like.
Section 2 of 6P-waves and S-waves
P-waves and S-waves
P-waves
- These are longitudinal waves, with particles moving back and forth along the direction of travel.
S-waves
- These are transverse waves, with particles moving at right angles to the direction of travel.
Speed
- P-waves travel faster than S-waves, so they are the first to arrive at a seismometer.
Materials
- P-waves pass through solids and liquids, but S-waves can only pass through solids.
| Feature | P-waves | S-waves |
|---|---|---|
| Wave type | Longitudinal | Transverse |
| Relative speed | Faster | Slower |
| Through solids | Yes | Yes |
| Through liquids | Yes | No |
Section 3 of 6Structure of the Earth
From the surface to the centre the Earth has four main layers.
1
The crust is a thin, solid outer layer of rock on which we live.
2
Beneath it the mantle is a thick layer of hot rock that is solid but flows very slowly over long times.
3
Below the mantle lies the outer core, made mostly of liquid iron and nickel.
4
At the centre is the solid inner core, also mostly iron and nickel, kept solid by the very high pressure.
5
Speeds and paths of seismic waves change at the boundaries between these layers.
Section 4 of 6What the Evidence Shows
S-wave shadow
- S-waves are not detected on the far side of the Earth, so part of the interior must be liquid.
Outer core
- The shadow starts at the edge of the core, showing that the outer core is liquid.
Refraction
- Waves change speed and direction when they pass into a material of different density, so P-waves bend.
Curved paths
- The paths of waves through the mantle are curved because density and speed change gradually with depth.
Section 5 of 6Worked Example
Question
- P-waves travel at 8.0 km/s and S-waves at 4.0 km/s from an earthquake to a station 640 km away.
- How much later do the S-waves arrive?
- Time for P-waves = distance ÷ speed = 640 ÷ 8.0 = 80 s.
- Time for S-waves = 640 ÷ 4.0 = 160 s.
- Difference = 160 − 80 = 80 s.
Answer The S-waves arrive 80 s after the P-waves.
Section 6 of 6Exam Checklist & Tips
8.3.7 – Key Points for the Exam
- Seismic wave A wave of energy that travels through the Earth, produced by an earthquake.
- P-wave A faster longitudinal seismic wave that travels through solids and liquids.
- S-wave A slower transverse seismic wave that travels only through solids.
- Seismometer An instrument that detects and records ground movement.
- Core The central part of the Earth, with a liquid outer part and a solid inner part.
Common Mistakes
- Saying S-waves are longitudinal; S-waves are transverse and P-waves are longitudinal.
- Thinking the whole mantle is liquid because it can flow; it is solid.
- Claiming both wave types pass through the liquid outer core.
- Mixing up which wave arrives first; the P-wave is faster so it arrives first.