Seismic Waves and the Earth's Structure

This lesson covers seismic waves and how they provide evidence for the internal structure of the Earth — as required by the Edexcel GCSE Physics specification (1PH0), Topic 4: Waves. You need to understand the properties of P-waves and S-waves, and explain how the behaviour of these waves reveals what is inside the Earth.

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What Are Seismic Waves?

Seismic waves are waves of energy that travel through the Earth, typically produced by earthquakes (but also by explosions and volcanic eruptions). They originate from the focus of an earthquake (the point underground where the rock fractures) and radiate outward in all directions.

Seismic waves are detected by instruments called seismographs (or seismometers). The record they produce is called a seismogram.

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Types of Seismic Waves

There are two main types of seismic waves that travel through the body of the Earth:

P-waves (Primary Waves)

Property	Detail
Type	Longitudinal wave (particles vibrate parallel to the direction of travel)
Speed	Faster than S-waves — they arrive first at seismograph stations
Can travel through	Solids and liquids (and gases)
Typical speed	~5–13 km/s (depending on the material)
Name origin	"P" for primary (arrives first) or pressure

S-waves (Secondary Waves)

Property	Detail
Type	Transverse wave (particles vibrate perpendicular to the direction of travel)
Speed	Slower than P-waves — they arrive second
Can travel through	Solids only (cannot pass through liquids or gases)
Typical speed	~3–7 km/s (depending on the material)
Name origin	"S" for secondary (arrives second) or shear

Key Difference

The critical difference is that S-waves cannot travel through liquids. This is because transverse waves require particles to be bonded in a way that allows shear forces — liquids cannot support shear forces (they flow instead).

Exam Tip: Remember: P-waves travel through everything (solids and liquids). S-waves only travel through Solids. This fact is the key to understanding how we know the outer core is liquid.

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The Earth's Internal Structure

The Earth is made up of several layers with different compositions and physical states:

Layer	Approximate Thickness	State	Key Features
Crust	5–70 km	Solid	Thin outer layer; oceanic crust (~5 km) is thinner than continental crust (~30–70 km)
Mantle	~2,900 km	Solid (but flows very slowly)	Largest layer by volume; the upper mantle is relatively rigid, but deeper mantle rock can flow slowly (convection currents)
Outer core	~2,200 km	Liquid	Made of liquid iron and nickel; generates the Earth's magnetic field
Inner core	~1,200 km radius	Solid	Made of solid iron and nickel; solid despite extreme temperatures because of immense pressure

graph TD
    A["Earth’s Structure"] --> B["Crust<br/>5–70 km<br/>SOLID<br/>Thin rocky outer layer"]
    A --> C["Mantle<br/>~2,900 km<br/>SOLID (flows slowly)<br/>Largest layer"]
    A --> D["Outer Core<br/>~2,200 km<br/>LIQUID<br/>Iron & nickel"]
    A --> E["Inner Core<br/>~1,200 km radius<br/>SOLID<br/>Iron & nickel<br/>(extreme pressure)"]

    style A fill:#2c3e50,color:#fff
    style B fill:#e67e22,color:#fff
    style C fill:#c0392b,color:#fff
    style D fill:#f39c12,color:#fff
    style E fill:#e74c3c,color:#fff

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How Seismic Waves Reveal the Earth's Structure

Scientists cannot drill to the centre of the Earth (the deepest borehole is only about 12 km deep). Instead, they use the behaviour of seismic waves to deduce what the interior is made of. This is a form of indirect evidence.

Evidence 1: The S-wave Shadow Zone (Proves the Outer Core Is Liquid)

When an earthquake occurs, seismograph stations around the world detect the arriving waves. However, there is a large region on the opposite side of the Earth where S-waves are not detected. This region is called the S-wave shadow zone.

Explanation:

• S-waves travel outward from the earthquake focus through the solid mantle.
• When they reach the boundary between the mantle and the outer core, they cannot continue because S-waves cannot travel through liquids.
• Therefore, no S-waves are detected on the far side of the Earth — only the region closer to the earthquake receives S-waves.
• This proves the outer core must be liquid.

Evidence 2: P-wave Refraction (Proves Density Changes)

P-waves can travel through both solids and liquids, so they are detected on the far side of the Earth. However, there is also a P-wave shadow zone — a region where P-waves are not detected.

Explanation:

• As P-waves travel deeper into the Earth, they encounter materials of different densities.
• At boundaries between layers (e.g. mantle/core), the P-waves are refracted (they change speed and direction).
• The refraction at the mantle-core boundary causes P-waves to bend, creating a "shadow zone" where no P-waves arrive.
• The pattern of P-wave refraction tells scientists about the density and state of the materials inside the Earth.

Evidence 3: Changes in Wave Speed

Seismic wave speeds change at boundaries between layers because the properties of the material change:

• Waves speed up in denser, more rigid materials.
• Sudden changes in speed indicate boundaries between different layers.
• By analysing the arrival times of seismic waves at many stations, scientists can map out the internal boundaries.

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Shadow Zones Explained

flowchart TD
    A["Earthquake<br/>Focus"] --> B["P-waves travel<br/>through solid<br/>mantle"]
    A --> C["S-waves travel<br/>through solid<br/>mantle"]
    B --> D["P-waves refract<br/>at core boundary<br/>(enter liquid outer core)"]
    C --> E["S-waves STOP<br/>at core boundary<br/>(cannot enter liquid)"]
    D --> F["P-wave shadow zone<br/>(105°–140° from focus)<br/>No P-waves detected"]
    E --> G["S-wave shadow zone<br/>(105°–180° from focus)<br/>No S-waves detected"]

    style A fill:#e74c3c,color:#fff
    style D fill:#f39c12,color:#fff
    style E fill:#c0392b,color:#fff
    style F fill:#95a5a6,color:#fff
    style G fill:#95a5a6,color:#fff

Summary of Shadow Zones

Wave Type	Shadow Zone	Cause
S-waves	Large shadow zone on far side of Earth (beyond ~105° from focus)	S-waves cannot pass through the liquid outer core
P-waves	Smaller shadow zone (between ~105° and ~140° from focus)	P-waves are refracted by the liquid outer core, bending them away from this region

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How Do We Know the Inner Core Is Solid?

Even though the outer core is liquid, the inner core is solid. Evidence for this:

• P-waves that pass through the inner core are detected at certain stations. The arrival times and speeds of these P-waves indicate they travel through a solid material (they speed up when entering the inner core).
• If the entire core were liquid, the P-wave pattern would be different.
• The inner core is solid because the immense pressure at the centre of the Earth keeps the iron and nickel in a solid state despite the very high temperature.

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Detecting Seismic Waves — Seismographs

A seismograph (seismometer) is an instrument that detects and records seismic waves. It works by:

1. Having a heavy mass suspended on a spring or wire that stays relatively still when the ground shakes (due to inertia).
2. A pen attached to the mass traces a pattern on a rotating drum (or equivalent electronic sensor) that is attached to the ground.
3. When the ground shakes, the drum moves but the pen remains still, producing a seismogram — a record of the ground motion over time.

Reading a Seismogram

• P-waves arrive first (they are faster).
• S-waves arrive second (they are slower).
• The time gap between the arrival of P-waves and S-waves at a station can be used to calculate the distance from the earthquake focus.
• Data from at least three seismograph stations can be used to pinpoint the earthquake's epicentre (the point on the surface directly above the focus).

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Why We Cannot Observe the Earth's Interior Directly