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Spec links: 4.6.1.1 Transverse and longitudinal waves · 4.6.1.2 Properties of waves · 4.6.1.5 Waves for detection and exploration (Physics only) (HT) · Reading time: about 4 minutes

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Looking where no one can go

The deepest hole ever drilled goes down about 12 km. That sounds deep, but the centre of the Earth is about 6400 km below our feet. Nobody has ever been there. Nobody ever will. So how do we know that the Earth has a core, and that part of it is liquid?

The answer came from earthquakes. Every large earthquake sends waves right through the planet. By listening to them, scientists worked out what is inside.

Two kinds of wave

An earthquake produces seismic waves. Two types travel through the inside of the Earth.

P-waves are longitudinal. The rock vibrates backwards and forwards, parallel to the direction the wave travels. P-waves can travel through solids and liquids. They are the faster type, so they arrive first.

S-waves are transverse. The rock vibrates at right angles to the direction the wave travels. S-waves can only travel through solids. They cannot pass through liquids.

Instruments called seismometers record these waves at stations all over the world.

Timing the waves

Seismic waves travel at different speeds in different rock. Near the surface, a P-wave moves at roughly 6 km/s. That is about 6000 m/s – over seventeen times faster than sound in air.

We can use distance = speed × time (s = v t). If a P-wave travels at 6000 m/s for 50 s, how far does it go?

s = v t = 6000 m/s × 50 s = 300 000 m, which is 300 km.

Seismic waves also obey the wave equation, v = f λ. A P-wave with a speed of 6000 m/s and a frequency of 2 Hz has a wavelength of 6000 ÷ 2 = 3000 m.

Because S-waves are slower, they arrive after the P-waves. The longer the gap between them, the further away the earthquake was. Comparing the gaps at three or more stations lets scientists pinpoint where the earthquake started.

The shadow zone

In the early 1900s, scientists noticed something strange. After a big earthquake, stations on the far side of the Earth detected P-waves, but no S-waves at all. There was a huge area where S-waves never arrived. This is called the S-wave shadow zone.

Something deep inside the Earth was blocking them. S-waves cannot travel through liquids. So the outer part of the core must be liquid. In 1926 the British scientist Harold Jeffreys argued that the core was liquid. Today we know the outer core is mostly molten iron and nickel, starting about 2900 km down.

P-waves gave more clues. As they cross into the core, their speed changes. This makes them refract (change direction), which leaves a smaller P-wave shadow zone too. In 1936 the Danish scientist Inge Lehmann studied faint P-waves that appeared inside this zone. She realised they had been bent by a boundary deep inside the core. Later evidence showed this was a solid inner core.

Still learning

Seismic waves are still our best way to explore the deep Earth. Every new earthquake adds data. It is a great example of how scientists use evidence to build models of things they can never see directly.

Key words

• Seismic waves: waves produced by earthquakes that travel through the Earth.
• P-wave: a longitudinal seismic wave that can travel at different speeds through solids and liquids.
• S-wave: a transverse seismic wave that cannot travel through a liquid.
• Longitudinal wave: a wave in which the oscillations are parallel to the direction of energy transfer.
• Transverse wave: a wave in which the oscillations are perpendicular to the direction of energy transfer.
• Shadow zone: an area of the Earth’s surface where a particular type of seismic wave is not detected after an earthquake.
• Refraction: the change in direction of a wave when its speed changes as it passes into a different medium.

Questions

1. Which type of seismic wave arrives first at a seismometer? [1 mark]
2. Name the scientist who found evidence for an inner core, and give the year. [2 marks]
3. What is the outer core mostly made of? [1 mark]
4. Explain how the S-wave shadow zone provides evidence that the outer core is liquid. [3 marks]
5. Explain why P-waves change direction as they pass from the mantle into the core. [2 marks]
6. An S-wave travels at 3500 m/s. It takes 120 s to reach a seismometer. Calculate the distance from the earthquake to the seismometer. Give your answer in kilometres. [3 marks]
7. A P-wave has a frequency of 4 Hz and a wavelength of 2000 m. Calculate the speed of the wave. Give the unit. [3 marks]
8. Describe how scientists used seismic waves to find out about the structure of the inside of the Earth. [6 marks]

Show answers

1. P-wave (1)
2. Inge Lehmann (1); 1936 (1)
3. (Molten) iron and nickel (1)
4. S-waves are not detected in a large area on the opposite side of the Earth from the earthquake (1); S-waves are transverse and cannot travel through liquids (1); so something in the way, the outer core, must be liquid (1)
5. The speed of the P-wave changes when it enters the core / different material (1); this causes refraction / a change in direction (1)
6. s = v t = 3500 × 120 (1); = 420 000 m (1); = 420 km (1)
7. v = f λ = 4 × 2000 (1); = 8000 (1); m/s (1)
8. Indicative content:
   • earthquakes produce P-waves and S-waves that travel through the Earth
   • seismometers around the world detect the waves and record arrival times
   • P-waves are longitudinal and travel through solids and liquids
   • S-waves are transverse and cannot travel through liquids
   • an S-wave shadow zone exists on the far side of the Earth
   • this shows the outer core is liquid
   • wave speeds change with depth / material, so the waves refract and follow curved paths
   • P-waves refract at the core boundary, giving a P-wave shadow zone
   • faint P-waves detected in the P-wave shadow zone showed a solid inner core (Lehmann)
   Level 3 (5–6 marks): a detailed, coherent description linking the properties of P- and S-waves to the shadow zones and to correct conclusions about a liquid outer core and solid inner core; Level 2 (3–4 marks): describes the S-wave shadow zone and links it to a liquid outer core, with some detail about wave types; Level 1 (1–2 marks): simple statements, e.g. earthquakes make waves that are detected, or S-waves cannot pass through liquid

More reading: See all 30 GCSE Physics comprehension articles