P-waves are longitudinal seismic waves that travel fastest and can pass through both solids and liquids. Free GCSE Physics notes and 15 FAQs.
Updated 20 Mar 2026 · Revise Right Team
Seismic Waves and Earth's Structure is part of Waves in air, fluids and solids in GCSE AQA Higher Physics.
5 min
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19
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GCSE AQA Higher
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What this lesson covers
P-waves are longitudinal seismic waves that travel fastest and can pass through both solids and liquids. Free GCSE Physics notes and 15 FAQs.
Explain how P-waves and S-waves are produced by earthquakes and describe their properties
Compare the behaviour of P-waves and S-waves when travelling through different materials
Analyse how seismic wave data provides evidence for Earth's internal structure
Evaluate the significance of seismic wave studies in discovering non-observable parts of Earth
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Use this lesson to stay inside Waves in air, fluids and solids while connecting the detail back to the wider Waves topic.
Waves
Waves in air, fluids and solids
GCSE AQA Higher Physics
Revision notes
Read the core explanations from Seismic Waves and Earth's Structure before testing yourself from memory.
1. When Earth Shakes - The Birth of Seismic Waves
Picture the ground beneath your feet suddenly cracking and shifting. The energy released doesn't just disappear - it travels outward as waves through the entire planet.
Earthquakes occur when tectonic plates suddenly move past each other, releasing enormous amounts of stored energy. This energy travels as seismic waves - vibrations that can journey right through Earth's centre to the other side of the planet.
These waves carry vital information about what lies deep inside our planet. Before seismic studies, we could only guess what Earth's interior looked like. Now we can 'see' inside without digging.
2. The Two Types - P-waves and S-waves
Not all seismic waves behave the same way. There are two main types, each with distinct properties that help us understand Earth's structure.
P-waves (Primary waves) arrive first at seismic stations because they travel fastest. These are longitudinal waves - the particles vibrate back and forth in the same direction as the wave travels, like a spring being pushed and pulled.
S-waves (Secondary waves) arrive later because they travel more slowly. These are transverse waves - particles vibrate at right angles to the wave direction, like a rope being shaken up and down.
The key difference lies in how they interact with different materials. P-waves can travel through anything - solid rock, liquid magma, even the liquid outer core. S-waves are more selective - they stop completely when they hit liquid.
3. The Journey Through Earth - Wave Behaviour in Different Materials
When seismic waves encounter different materials inside Earth, they behave predictably. This behaviour is what allows scientists to map the planet's interior.
P-waves slow down when they enter denser materials and speed up in less dense ones. They also change direction (refract) at boundaries between different layers. Most importantly, they can pass through both the solid mantle and the liquid outer core.
S-waves follow similar refraction rules in solids but have one crucial limitation - they cannot travel through liquids at all. When an S-wave hits liquid, it stops completely. This creates 'shadow zones' on Earth's surface where S-waves never arrive.
The speed changes are measurable and consistent. P-waves travel at about 6-8 km/s through the mantle but slow to about 8 km/s in the outer core. S-waves travel at 3-4 km/s through solid rock but disappear entirely in liquid regions.
4. Earth's Hidden Structure Revealed
Before seismic studies, Earth's interior was a complete mystery. Scientists could only speculate about what lay beneath the deepest mines and boreholes.
Seismic waves changed everything. By studying how P-waves and S-waves travel through Earth, scientists discovered that our planet has distinct layers with different properties. The evidence comes from wave arrival times and shadow zones.
The most significant discovery was Earth's liquid outer core. S-waves create a shadow zone between 103° and 143° from an earthquake's epicentre - no S-waves arrive in this region. This can only happen if S-waves are blocked by liquid.
Direct P-waves exhibit a shadow zone roughly between 103° and 143° due to strong refraction at the core-mantle boundary; however, refracted P-waves continue to traverse Earth's interior and reach most regions of the surface. In contrast, S-waves are completely blocked by the liquid outer core, resulting in distinct shadow zones where they do not appear.
The combination of both shadow zones provided the first solid evidence that Earth has a liquid outer core surrounded by a solid mantle. Later studies revealed an inner solid core within the liquid outer core.
5. The Scientific Revolution - Discovering the Unobservable
The study of seismic waves represents one of science's greatest detective stories. Scientists used indirect evidence to discover parts of Earth that no human will ever see directly.
This work provided new evidence that completely changed our understanding of Earth's structure. Before seismic studies, many scientists thought Earth was solid throughout or had a different internal arrangement.
The discoveries went beyond just mapping layers. Seismic evidence revealed that the outer core is liquid iron and nickel, explaining Earth's magnetic field. It showed the mantle behaves like a very thick liquid over geological time, driving plate tectonics.
Modern seismology continues this tradition. Scientists now use seismic waves to locate oil deposits, monitor nuclear tests, and even study other planets. The same principles that revealed Earth's core help us understand Mars and the Moon.
Seismic wave questions often test your understanding of wave properties and their relationship to Earth's structure. The key is linking wave behaviour to what it tells us about materials.
When describing P-waves and S-waves, always mention their wave type (longitudinal/transverse) and their key property (can/cannot travel through liquids). For explain questions, focus on the cause-and-effect relationship between wave properties and Earth's structure.
Shadow zone questions are common. Remember that both P-waves and S-waves create shadow zones, but for different reasons. S-waves are blocked by liquid; P-waves are refracted away from certain angles.
Worked example: Worked Method: Seismic Waves and Earth's Structure
Use the relevant formula, substitute values with units, calculate, then state the final answer clearly.
Write down the correct physics formula. — Write down the correct physics formula.
Substitute known values with units. — Substitute known values with units.
Rearrange if needed and calculate carefully. — Rearrange if needed and calculate carefully.
Give the final answer with correct unit and sensible precision. — Give the final answer with correct unit and sensible precision.
Answer: Final answer should include unit and appropriate significant figures.
Sources and content standards
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
What is the difference between P-waves and S-waves?
P-waves are longitudinal seismic waves that travel fastest and can pass through both solids and liquids. S-waves are transverse seismic waves that travel more slowly and cannot travel through liquids, so they stop completely when they reach liquid.
What is the outer core made of?
Seismic evidence revealed that Earth's outer core is liquid iron and nickel, which explains Earth's magnetic field. Later studies revealed a solid inner core within the liquid outer core.
What are seismic waves?
Seismic waves are energy waves produced by earthquakes that travel through Earth's layers, carrying information about the planet's internal structure. Earthquakes happen when tectonic plates suddenly move past each other, releasing stored energy.
Are S-waves longitudinal or transverse?
S-waves are transverse: particles vibrate at right angles to the wave direction, like a rope being shaken up and down. They shake material from side to side and cannot travel through liquids.
What is a seismic shadow zone?
A shadow zone is a region of Earth's surface where certain seismic waves do not arrive. Both P-waves and S-waves create shadow zones, but for different reasons: S-waves are blocked by the liquid outer core, while P-waves are refracted away from certain angles.
How fast do P-waves and S-waves travel?
P-waves travel at about 6–8 km/s through the mantle, while S-waves travel at 3–4 km/s through solid rock. S-waves disappear entirely in liquid regions, which is why they cannot cross the outer core.
What else are seismic waves used for?
Seismic waves are used to locate oil deposits, monitor nuclear tests and study other planets such as Mars and the Moon. The same principles that revealed Earth's core apply.
Why can't S-waves travel through liquids?
S-waves cannot travel through liquids because they are transverse waves that shake material from side to side, and liquids cannot support shear forces. When an S-wave hits liquid, it stops completely.
How do seismic waves show the Earth has a liquid outer core?
S-waves create a shadow zone on Earth's surface where no S-waves arrive, which can only happen if they are blocked by liquid. Together with the P-wave shadow zone caused by refraction at the core-mantle boundary, this gave the first solid evidence that Earth has a liquid outer core.
Which arrive first, P-waves or S-waves?
P-waves (primary waves) arrive first at seismic stations because they travel fastest. S-waves (secondary waves) arrive later because they travel more slowly. The gap in arrival times carries information about Earth's internal structure.
How do P-waves move particles?
P-waves are longitudinal: the particles vibrate back and forth in the same direction as the wave travels, like a spring being pushed and pulled. They compress and stretch material and can pass through both solids and liquids.
Why is there a P-wave shadow zone?
There is a P-wave shadow zone, roughly between 103° and 143° from the epicentre, because direct P-waves are strongly refracted at the core-mantle boundary. Refracted P-waves still reach most of the surface, unlike S-waves, which are blocked by the liquid outer core.
What causes earthquakes?
Earthquakes occur when tectonic plates suddenly move past each other, releasing enormous amounts of stored energy. This energy travels outward as seismic waves, which can journey right through Earth's centre.
Do seismic waves refract inside the Earth?
Seismic waves refract, changing direction, at boundaries between different layers inside Earth. P-waves slow down in denser materials and speed up in less dense ones.
How do you describe P-waves and S-waves in an answer?
P-waves and S-waves should be described by their wave type and key property: P-waves are longitudinal and can travel through liquids, while S-waves are transverse and cannot travel through liquids.