Seismic Waves

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CognitoSeismic waves and Earth's structure

Key facts & equations

Types of seismic wave

P-waves (primary waves): longitudinal compression waves. Travel through solids AND liquids.
S-waves (secondary waves): transverse waves. Travel through solids ONLY. Cannot travel through liquids.
P-waves travel faster than S-waves. P-waves arrive at seismograph first.
Both generated by earthquakes at the focus (point of origin beneath ground).
Surface waves (L-waves): travel along Earth's surface. Cause most earthquake damage.

What seismic waves reveal about Earth

S-waves do not arrive at seismograph stations on the opposite side of Earth from the earthquake.
This means there is a liquid zone inside Earth — the outer core is liquid (S-waves cannot pass through it).
P-waves refract (change direction) as they pass through layers with different densities.
The pattern of P-wave arrival times reveals the layered structure of Earth.
Shadow zone: region where neither P nor S-waves arrive — evidence for liquid outer core.

Earth's internal structure

Crust: thin outer layer. Continental (30–70 km) and oceanic (5–10 km).
Mantle: mostly solid rock, but flows very slowly. S-waves pass through.
Outer core: liquid iron and nickel. S-waves CANNOT pass through — key evidence.
Inner core: solid iron and nickel (high pressure compresses it despite high temperature).
Seismic waves travel faster through denser, more rigid materials.

Exam questions — 4 questions · 10 marks · AQA 8463 style

1 markS-wave propertyAQA 8463 style

Why do S-waves not reach seismograph stations on the opposite side of Earth from an earthquake?

A S-waves are absorbed by the mantle and run out of energy

B S-waves are transverse and cannot travel through the liquid outer core

C S-waves travel slower than P-waves and take too long to reach the far side

D S-waves are reflected back at the mantle-core boundary before reaching the outer core

3 marksEvidence for liquid outer coreAQA 8463 P2 style

Explain how the study of seismic waves provides evidence that the outer core of the Earth is liquid. (3 marks)

Hint: Chain: S-waves are transverse → can only travel through solids → S-waves have a shadow zone on far side → something is blocking them → that something must be liquid → outer core is liquid.

+30 XP

3 marksP-wave refractionAQA 8463 Higher style

P-waves travel through the Earth's interior and are detected at seismograph stations. Explain why P-waves change direction (refract) as they pass through the Earth, and what this tells us about the Earth's interior. (3 marks)

+30 XP

1 markP-wave vs S-wave speedAQA 8463 style

At a seismograph station, P-waves arrive before S-waves from the same earthquake. What does this tell us?

A S-waves were produced later in the earthquake

B P-waves travel faster through the Earth than S-waves

C The seismograph is closer to the S-wave source

D P-waves take a shorter path through the Earth

Types of seismic wave

What seismic waves reveal about Earth

Earth's internal structure

Module complete! 🎉