Seismic Waves and the Structure of the Earth

Peering Inside the Earth

We cannot drill to the centre of the Earth, so how do we know what is down there? Every time an earthquake occurs, it sends shockwaves acting like natural "X-rays" directly through the planet.

These vibrations are called seismic waves and are recorded globally by sensitive instruments known as seismometers.
By analysing where these waves can and cannot travel, scientists have mapped the Earth's internal structure without ever seeing it.

Properties of P-waves and S-waves

Earthquakes produce two main types of waves that travel through the Earth, each with distinct behaviours. Comparing their properties is crucial for understanding how they interact with the Earth's internal structure:

Feature	P-waves (Primary)	S-waves (Secondary)
Wave Type	Longitudinal wave	Transverse wave
Relative Speed	Faster (arrive first)	Slower (arrive second)
States Travelled Through	Solids and liquids	Solids only

Because P-waves travel faster than S-waves, they arrive at seismometer stations first. The time interval (or lag) between the arrival of P-waves and S-waves is used to calculate the distance to the earthquake's epicentre. A larger time lag indicates a greater distance from the epicentre.
As seismic waves travel deeper into the mantle, their paths curve. The density of the mantle increases with depth, causing the waves to change speed and undergo continuous refraction.

The S-wave Shadow Zone and the Liquid Outer Core

When an earthquake happens, seismometers located beyond an angle of 105° from the epicentre on the opposite side of the Earth detect absolutely no S-waves.

This creates a massive, wave-free area known as an S-wave shadow zone.
Because S-waves are transverse and cannot travel through liquids, their sudden disappearance proves the existence of a liquid layer deep inside the Earth.
This boundary marks the start of the liquid outer core, which absorbs or reflects the incoming S-waves entirely.

The P-wave Shadow Zone and Core Size

P-waves can travel through liquids, yet they still produce a distinct, ring-shaped shadow zone roughly between 105° and 140° from the earthquake's epicentre.

This happens because P-waves significantly slow down when they hit the liquid outer core.
This sudden drop in speed causes them to refract (bend) sharply inwards towards the normal, and they refract a second time when leaving the core.
This double refraction leaves a clear gap on the surface where no direct P-waves are detected. Measuring the exact angles of these shadow zones allows scientists to mathematically calculate the radius and thickness of the Earth's core.

Evidence for the Solid Inner Core (Higher Tier Only)

For a long time, scientists thought the entire core was liquid, but later observations revealed faint, unexpected signals.

Highly sensitive seismometers eventually detected very weak P-waves arriving inside the P-wave shadow zone.
These faint signals are caused by P-waves reflecting and refracting off a solid inner core located right at the centre of the Earth.
As the waves pass from the liquid outer core into this solid inner core, their speed suddenly increases, proving the very centre of the Earth is solid rock and metal.

Worked Example: Calculating P-wave Wavelength

Seismic waves follow standard wave equations. The wave speed is calculated using:

v = f \lambda

A P-wave generated by an earthquake has a frequency of $0.4\text{ Hz}$ and travels at a speed of $7200\text{ m/s}$ . Calculate its wavelength.

Step 1: Write down the known values.

$v = 7200\text{ m/s}$
$f = 0.4\text{ Hz}$

Step 2: Rearrange the wave equation for wavelength ( $\lambda$ ) and substitute the values.

$\lambda = \frac{v}{f}$
$\lambda = \frac{7200}{0.4}$

Step 3: Calculate the final answer with units.

$\lambda = 18000\text{ m}$

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Exam Tips

Common Mistake
Students often wrongly think S-waves do not enter the inner core because the inner core is liquid; in fact, the inner core is solid, but S-waves never reach it because the surrounding liquid outer core blocks them completely.
2
In 6-mark exam questions comparing seismic waves, always state three distinct differences to secure maximum marks: wave type (longitudinal vs transverse), relative speed, and the states of matter they can travel through.
3
Use the 'S' rule mnemonic in the exam to remember S-wave properties: S-waves are Secondary, Slower, and travel through Solids only.
4
To gain full marks when explaining why P-waves change direction, you must explicitly use the word 'boundary' (e.g., 'refraction occurs at the boundary between the mantle and the core').
5
Higher Tier students must specifically write that 'weak' or 'faint' P-waves are detected in the shadow zone to earn AQA marking points for explaining evidence of the solid inner core.

Key Terms(12)

Seismic waves: Shock waves produced by earthquakes that travel through the Earth and are used to provide evidence of its internal structure.
Seismometers: Instruments used by scientists to detect and record the vibrations produced by seismic waves.
P-waves: Primary seismic waves that are longitudinal, travel the fastest, and can move through both solids and liquids.
S-waves: Secondary seismic waves that are transverse, travel slower than P-waves, and can only move through solids.
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 (at a right angle) to the direction of energy transfer.
Epicentre: The point on the Earth's surface directly above the origin of an earthquake.
Mantle: The layer of the Earth between the crust and the core, which increases in density with depth, causing seismic waves to travel in curved paths.
Refraction: The change in direction of a wave as it passes from one medium to another of different density, resulting in a change in speed.
Shadow zone: A specific region on the Earth's surface where seismometers cannot detect particular seismic waves from an earthquake.
Outer core: The liquid layer of the Earth's core that absorbs S-waves and refracts P-waves, causing shadow zones.
Inner core: The innermost, solid part of the Earth, the existence of which is proven by the refraction of faint P-waves into the shadow zone.

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Key Terms(12)

Seismic waves: Shock waves produced by earthquakes that travel through the Earth and are used to provide evidence of its internal structure.
Seismometers: Instruments used by scientists to detect and record the vibrations produced by seismic waves.
P-waves: Primary seismic waves that are longitudinal, travel the fastest, and can move through both solids and liquids.
S-waves: Secondary seismic waves that are transverse, travel slower than P-waves, and can only move through solids.
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 (at a right angle) to the direction of energy transfer.
Epicentre: The point on the Earth's surface directly above the origin of an earthquake.
Mantle: The layer of the Earth between the crust and the core, which increases in density with depth, causing seismic waves to travel in curved paths.
Refraction: The change in direction of a wave as it passes from one medium to another of different density, resulting in a change in speed.
Shadow zone: A specific region on the Earth's surface where seismometers cannot detect particular seismic waves from an earthquake.
Outer core: The liquid layer of the Earth's core that absorbs S-waves and refracts P-waves, causing shadow zones.
Inner core: The innermost, solid part of the Earth, the existence of which is proven by the refraction of faint P-waves into the shadow zone.

Seismic Waves and the Structure of the Earth

Peering Inside the Earth

We cannot drill to the centre of the Earth, so how do we know what is down there? Every time an earthquake occurs, it sends shockwaves acting like natural "X-rays" directly through the planet.

These vibrations are called seismic waves and are recorded globally by sensitive instruments known as seismometers.
By analysing where these waves can and cannot travel, scientists have mapped the Earth's internal structure without ever seeing it.

Properties of P-waves and S-waves

Feature	P-waves (Primary)	S-waves (Secondary)
Wave Type	Longitudinal wave	Transverse wave
Relative Speed	Faster (arrive first)	Slower (arrive second)
States Travelled Through	Solids and liquids	Solids only

Because P-waves travel faster than S-waves, they arrive at seismometer stations first. The time interval (or lag) between the arrival of P-waves and S-waves is used to calculate the distance to the earthquake's epicentre. A larger time lag indicates a greater distance from the epicentre.
As seismic waves travel deeper into the mantle, their paths curve. The density of the mantle increases with depth, causing the waves to change speed and undergo continuous refraction.

The S-wave Shadow Zone and the Liquid Outer Core

When an earthquake happens, seismometers located beyond an angle of 105° from the epicentre on the opposite side of the Earth detect absolutely no S-waves.

This creates a massive, wave-free area known as an S-wave shadow zone.
Because S-waves are transverse and cannot travel through liquids, their sudden disappearance proves the existence of a liquid layer deep inside the Earth.
This boundary marks the start of the liquid outer core, which absorbs or reflects the incoming S-waves entirely.

The P-wave Shadow Zone and Core Size

P-waves can travel through liquids, yet they still produce a distinct, ring-shaped shadow zone roughly between 105° and 140° from the earthquake's epicentre.

This happens because P-waves significantly slow down when they hit the liquid outer core.
This sudden drop in speed causes them to refract (bend) sharply inwards towards the normal, and they refract a second time when leaving the core.
This double refraction leaves a clear gap on the surface where no direct P-waves are detected. Measuring the exact angles of these shadow zones allows scientists to mathematically calculate the radius and thickness of the Earth's core.

Evidence for the Solid Inner Core (Higher Tier Only)

For a long time, scientists thought the entire core was liquid, but later observations revealed faint, unexpected signals.

Highly sensitive seismometers eventually detected very weak P-waves arriving inside the P-wave shadow zone.
These faint signals are caused by P-waves reflecting and refracting off a solid inner core located right at the centre of the Earth.
As the waves pass from the liquid outer core into this solid inner core, their speed suddenly increases, proving the very centre of the Earth is solid rock and metal.

Worked Example: Calculating P-wave Wavelength

Seismic waves follow standard wave equations. The wave speed is calculated using:

v = f \lambda

A P-wave generated by an earthquake has a frequency of $0.4\text{ Hz}$ and travels at a speed of $7200\text{ m/s}$ . Calculate its wavelength.

Step 1: Write down the known values.

$v = 7200\text{ m/s}$
$f = 0.4\text{ Hz}$

Step 2: Rearrange the wave equation for wavelength ( $\lambda$ ) and substitute the values.

$\lambda = \frac{v}{f}$
$\lambda = \frac{7200}{0.4}$

Step 3: Calculate the final answer with units.

$\lambda = 18000\text{ m}$

Sign up to continue reading

Get full access to revision notes, key terms, and exam tips for every subtopic.

Exam Tips

Common Mistake
Students often wrongly think S-waves do not enter the inner core because the inner core is liquid; in fact, the inner core is solid, but S-waves never reach it because the surrounding liquid outer core blocks them completely.
2
In 6-mark exam questions comparing seismic waves, always state three distinct differences to secure maximum marks: wave type (longitudinal vs transverse), relative speed, and the states of matter they can travel through.
3
Use the 'S' rule mnemonic in the exam to remember S-wave properties: S-waves are Secondary, Slower, and travel through Solids only.
4
To gain full marks when explaining why P-waves change direction, you must explicitly use the word 'boundary' (e.g., 'refraction occurs at the boundary between the mantle and the core').
5
Higher Tier students must specifically write that 'weak' or 'faint' P-waves are detected in the shadow zone to earn AQA marking points for explaining evidence of the solid inner core.

Key Terms(12)

Seismic waves: Shock waves produced by earthquakes that travel through the Earth and are used to provide evidence of its internal structure.
Seismometers: Instruments used by scientists to detect and record the vibrations produced by seismic waves.
P-waves: Primary seismic waves that are longitudinal, travel the fastest, and can move through both solids and liquids.
S-waves: Secondary seismic waves that are transverse, travel slower than P-waves, and can only move through solids.
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 (at a right angle) to the direction of energy transfer.
Epicentre: The point on the Earth's surface directly above the origin of an earthquake.
Mantle: The layer of the Earth between the crust and the core, which increases in density with depth, causing seismic waves to travel in curved paths.
Refraction: The change in direction of a wave as it passes from one medium to another of different density, resulting in a change in speed.
Shadow zone: A specific region on the Earth's surface where seismometers cannot detect particular seismic waves from an earthquake.
Outer core: The liquid layer of the Earth's core that absorbs S-waves and refracts P-waves, causing shadow zones.
Inner core: The innermost, solid part of the Earth, the existence of which is proven by the refraction of faint P-waves into the shadow zone.

Previous NoteUltrasound for Medical and Industrial Imaging Next Topic: Electromagnetic WavesTypes of electromagnetic waves

Back to Waves for detection and exploration

Put your knowledge into practice — try past paper questions for Physics

Key Terms(12)

Seismic waves: Shock waves produced by earthquakes that travel through the Earth and are used to provide evidence of its internal structure.
Seismometers: Instruments used by scientists to detect and record the vibrations produced by seismic waves.
P-waves: Primary seismic waves that are longitudinal, travel the fastest, and can move through both solids and liquids.
S-waves: Secondary seismic waves that are transverse, travel slower than P-waves, and can only move through solids.
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 (at a right angle) to the direction of energy transfer.
Epicentre: The point on the Earth's surface directly above the origin of an earthquake.
Mantle: The layer of the Earth between the crust and the core, which increases in density with depth, causing seismic waves to travel in curved paths.
Refraction: The change in direction of a wave as it passes from one medium to another of different density, resulting in a change in speed.
Shadow zone: A specific region on the Earth's surface where seismometers cannot detect particular seismic waves from an earthquake.
Outer core: The liquid layer of the Earth's core that absorbs S-waves and refracts P-waves, causing shadow zones.
Inner core: The innermost, solid part of the Earth, the existence of which is proven by the refraction of faint P-waves into the shadow zone.