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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 .

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.

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.

Use the 'S' rule mnemonic in the exam to remember S-wave properties: S-waves are Secondary, Slower, and travel through Solids only.

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').

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.

Shock waves produced by earthquakes that travel through the Earth and are used to provide evidence of its internal structure.

Instruments used by scientists to detect and record the vibrations produced by seismic waves.

Primary seismic waves that are longitudinal, travel the fastest, and can move through both solids and liquids.

Secondary seismic waves that are transverse, travel slower than P-waves, and can only move through solids.

A wave in which the oscillations are parallel to the direction of energy transfer.

A wave in which the oscillations are perpendicular (at a right angle) to the direction of energy transfer.

The point on the Earth's surface directly above the origin of an earthquake.

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.

The change in direction of a wave as it passes from one medium to another of different density, resulting in a change in speed.

A specific region on the Earth's surface where seismometers cannot detect particular seismic waves from an earthquake.

The liquid layer of the Earth's core that absorbs S-waves and refracts P-waves, causing shadow zones.

The innermost, solid part of the Earth, the existence of which is proven by the refraction of faint P-waves into the shadow zone.