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12.7 Seismic Waves

GCSE Physics · AQA · Chapter 12: Wave Properties

A free GCSE Physics lesson for AQA (8463) from OBA School: physics only (Higher tier). P-waves and S-waves, the structure of the Earth, shadow zones and the evidence they give. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe how earthquakes produce seismic waves and how they are detected
  • Compare P-waves and S-waves: type of wave, speed and the materials they can travel through
  • Explain why seismic waves follow curved paths through the Earth
  • Explain how the shadow zones give evidence for a liquid outer core and a solid inner core
  • Use speed = distance ÷ time with seismic data, including the time lag between P- and S-waves

Key words

  • Seismic waves: Waves that travel through the Earth (and along its surface) carrying energy from an earthquake.
  • Focus: The point inside the Earth’s crust where the earthquake starts.
  • Epicentre: The point on the Earth’s surface directly above the focus.
  • Seismometer: An instrument that detects and records the ground movement caused by seismic waves.
  • P-waves: Primary waves: longitudinal, the fastest, travel through solids and liquids.
  • S-waves: Secondary waves: transverse, slower than P-waves, travel through solids only.
  • Shadow zone: A region of the Earth’s surface where P-waves and/or S-waves from a particular earthquake are not detected.

Earthquakes and seismic waves

Forces build up in the rocks of the Earth’s crust until the rock suddenly breaks and slips. The energy released spreads out as seismic waves. The earthquake starts at the focus, below ground; the point on the surface above it is the epicentre. Seismometers around the world record the waves when they arrive.

  • type; P-waves (primary) longitudinal (push and pull); S-waves (secondary) transverse (shake side to side); L-waves (long / surface) surface waves
  • speed; P-waves (primary) fastest (≈ 5–13 km/s); S-waves (secondary) slower (≈ 3–7 km/s); L-waves (long / surface) slowest
  • travel through; P-waves (primary) solids and liquids; S-waves (secondary) solids only; L-waves (long / surface) the crust, along the surface
  • arrive; P-waves (primary) first; S-waves (secondary) second; L-waves (long / surface) last – cause most damage

Key fact: P-waves: Primary, Push–pull (longitudinal), Pass through everything. S-waves: Secondary, Sideways (transverse), Solids only.

The structure of the Earth

Nobody can drill more than about 12 km into the Earth, so we cannot see its inside. Our knowledge of the deep Earth comes mainly from studying how seismic waves travel through it.

Curved paths and shadow zones (Higher tier)

Records from many seismometers after one earthquake show three key facts:

  • The paths curve. The density and stiffness of the mantle change gradually with depth, so the wave speed changes gradually. The waves therefore refract a little all the time and follow curved paths (bending back towards the surface).
  • No S-waves are detected more than about 105° from the epicentre. S-waves are transverse and cannot travel through a liquid, so the core must have a liquid outer part. This large S-wave shadow zone shows the size of the core.
  • No P-waves are detected between about 105° and 142°. P-waves change speed suddenly at the mantle–core boundary, so they are refracted sharply as they enter and leave the core and miss this band of the surface.
  • Weak P-waves are detected inside the P-wave shadow zone. They have been refracted again by a boundary deeper down – evidence for a solid inner core.

A sudden change in wave speed about 5–70 km down also showed seismologists where the crust ends and the mantle begins.

Seismic calculations

Worked example: speed of P-waves

P-waves reach a seismometer 2400 km from the epicentre 5.0 minutes after the earthquake.
time = 5.0 × 60 = 300 s; speed = distance ÷ time = 2400 ÷ 300

Answer: 8.0 km/s

Worked example: distance from the time lag

P-waves travel at 8.0 km/s and S-waves at 4.5 km/s. At one station the S-waves arrive 70 s after the P-waves.
For a distance d (km): time lag = d ÷ 4.5 − d ÷ 8.0 = 70
d × (0.2222 − 0.125) = 70, so d = 70 ÷ 0.09722

Answer: d ≈ 720 km

The bigger the time lag between the P- and S-waves, the further away the earthquake. Using the distances from three or more seismometers, scientists can locate the epicentre.

Common mistakes

  • Mixing up the waves: P-waves are longitudinal and travel through liquids; S-waves are transverse and cannot.
  • Saying the inner core is liquid. The outer core is liquid; the inner core is solid.
  • Explaining the curved paths with “the waves are reflected”. They are gradually refracted because their speed changes with depth.
  • Forgetting to change minutes to seconds before using speed = distance ÷ time.

Revision questions

Try these, then check your answers below each one.

Question 1. Which seismic waves can travel through the Earth’s liquid outer core? (Choose from: P-waves only, S-waves only, both P-waves and S-waves, neither.)

Answer: P-waves only

Working: Only P-waves (longitudinal) pass through liquids; S-waves cannot.

Question 2. P-waves travel 3600 km in 450 s. Calculate their speed in km/s.

Answer: 8 km/s

Working: speed = 3600 ÷ 450 = 8.0 km/s.

Question 3. (Higher tier) True or false? P-waves and S-waves follow curved paths through the mantle because their speed changes gradually with depth.

Answer: True

Working: True. The speed changes gradually with depth, so the waves keep refracting and follow curved paths.

Related lessons

Continue in the full course

This lesson is section 12.7 (AGCPH1207) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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