Welcome! Nobody has ever drilled more than about 12 km into the Earth – yet we know the centre is a ball of iron with a liquid outer layer. How? In this lesson you’ll use real earthquake evidence to work it out as an earthquake detective. It takes about one hour. (Separate Physics only.)
Before you start
- You need: your exercise book or paper, a pen, a calculator and something round to draw around (like a cup).
- Write the title and date: Seismic waves – Earthquake Detectives.
- The golden rule: write your answer first, then tap Show answer. Correct mistakes in a different colour.
- Look out for the blue 📝 boxes. Copy each one into your book neatly – these are your revision notes.
📝 Copy into your book
By the end of this lesson I will be able to:
1. compare P-waves and S-waves
2. explain how seismic waves show that the Earth has a liquid outer core
3. use wave speeds to calculate arrival times.
Step 1: Starter (5 minutes)
Imagine you’re given a sealed box and told there’s something inside. You can’t open it or X-ray it. Write down three ways you could find out what’s inside.
Show answer
You could shake it and listen, weigh it, tap it and feel the vibrations, or tilt it and feel things move. Scientists study the inside of the Earth the same way – by “listening” to the vibrations from earthquakes as they travel through it.
📝 Copy into your book
Earthquakes produce SEISMIC WAVES that travel through the Earth. They are detected by instruments called SEISMOMETERS.
Step 2: Read and write (8 minutes)
First, copy these three questions into your book. Then read the text below carefully and answer them in full sentences.
- Which type of seismic wave is longitudinal, and which is transverse?
- Which type can travel through liquids?
- What is a shadow zone?
📖 Read: Waves through the Earth
An earthquake sends out two main types of wave through the Earth. P-waves are longitudinal. They travel through both solids and liquids, and they are the faster type, so they arrive first. S-waves are transverse. They can only travel through solids, and they are slower.
When scientists compared seismometer readings from around the world, they found that S-waves are never detected on the far side of the Earth from an earthquake. This large area with no S-waves is called a shadow zone. Because S-waves can’t travel through liquids, this shows that part of the Earth’s core must be liquid. There is also a narrower ring where no P-waves are detected. This happens because P-waves change speed and refract (change direction) when they pass from the mantle into the core. The pattern of these waves also revealed a solid inner core at the very centre.
The study of seismic waves gave scientists evidence for the Earth’s structure – a structure nobody can see directly.
Show answers
(a) P-waves are longitudinal; S-waves are transverse.
(b) P-waves.
(c) An area of the Earth’s surface where a particular type of seismic wave is not detected.
📝 Copy into your book
| P-waves | S-waves | |
|---|---|---|
| Type | Longitudinal | Transverse |
| Travel through | Solids AND liquids | Solids ONLY |
| Speed | Faster – arrive first | Slower |
Step 3: Arrival times (8 minutes)
In this region of the crust, P-waves travel at 6 km/s and S-waves at 4 km/s. A seismometer is 600 km from an earthquake. Use time = distance ÷ speed.
Q1 How long do the P-waves take to arrive?
Show answer
600 ÷ 6 = 100 s
Q2 How long do the S-waves take to arrive?
Show answer
600 ÷ 4 = 150 s
Q3 What is the time gap between the P-waves and S-waves arriving? Would the gap be bigger or smaller for a station further away?
Show answer
150 − 100 = 50 s. The gap is bigger further away, because the faster P-waves pull further ahead. Scientists use this gap to work out how far away an earthquake was.
✅ Score out of 3.
Step 4: Main task – Earthquake Detectives (15 minutes)
A huge earthquake has just happened. Seismometers all around the world recorded it. Their positions are given as an angle around the Earth from the earthquake (0° is the earthquake, 180° is the exact opposite side of the Earth).
| Station | Angle from earthquake | P-waves detected? | S-waves detected? |
|---|---|---|---|
| 1 | 30° | Yes | Yes |
| 2 | 70° | Yes | Yes |
| 3 | 100° | Yes | Yes |
| 4 | 120° | No | No |
| 5 | 150° | Yes | No |
| 6 | 180° | Yes | No |
Your task:
- Draw a circle for the Earth. Mark the earthquake at the top and the six stations around the edge. Shade where S-waves are missing, and where P-waves are missing.
- Clue 1: What do stations 4, 5 and 6 tell you about S-waves? What does this prove about the core?
- Clue 2: Why do P-waves reach stations 5 and 6, but not station 4?
- Verdict: write a short report describing the structure of the Earth that fits all the evidence.
Stuck? Look back at your 📝 table: which wave can’t go through liquids?
Show the detective’s report
Clue 1: no S-waves reach any station beyond about 105°. S-waves can’t travel through liquids, so there must be a large liquid layer in the core blocking them – the liquid outer core.
Clue 2: P-waves can travel through the liquid core, but they change speed and refract at the boundary between the mantle and the core. This bends them away from station 4, leaving a P-wave shadow zone, and they come out again further round at stations 5 and 6.
Verdict: the Earth has a solid crust and mantle that S-waves can pass through, a liquid outer core that stops S-waves and refracts P-waves, and (from more detailed evidence) a solid inner core at the centre.
📝 Copy into your book
S-wave shadow zone → S-waves can’t pass through the core → the OUTER CORE IS LIQUID.
P-wave shadow zone → P-waves REFRACT at the boundary between the mantle and the core.
Step 5: Exam practice (12 minutes)
Look at the marks – they tell you how many points to make. Answer in full sentences, then mark yourself.
Q1 (3 marks) Compare P-waves and S-waves.
Show model answer
P-waves are longitudinal, whereas S-waves are transverse (1). P-waves can travel through solids and liquids, whereas S-waves can only travel through solids (1). P-waves travel faster than S-waves (1).
Q2 (3 marks) Explain how the study of S-waves provides evidence that the Earth’s outer core is liquid.
Show model answer
S-waves are not detected on the opposite side of the Earth from an earthquake / there is an S-wave shadow zone (1). S-waves are transverse and cannot travel through liquids (1). So something in the middle of the Earth is blocking them – the outer core must be liquid (1).
Q3 (2 marks) A P-wave travels 4200 km through the mantle at an average speed of 12 km/s. How long does it take?
Show model answer
t = s ÷ v = 4200 ÷ 12 (1) = 350 s (1)
✅ Add up your marks out of 8 and write the score in your book.
Step 6: Exit ticket (8 minutes)
- We know the outer core is liquid because…
- The P-wave shadow zone is caused by…
- One thing I’m still not sure about is…
🎉 Well done – lesson complete! Your 📝 boxes are your revision notes for this topic. Want more? Try the Sound Investigators lesson, which uses echoes in a similar way.
