AQA GCSE Physics

Revision Notes

6.1.14 Seismic Waves

Higher Tier Only

Seismic Waves

  • Earthquakes produce two types of waves:
    • P-waves (primary waves)
    • S-waves (secondary waves)
  • These waves pass through the Earth’s centre and can be detected at various points around the Earth using seismometers
  • By carefully timing the arrival of the waves at each point, the location of the earthquake, along with its magnitude, can be pinpointed

P-Waves

  • P-waves are longitudinal waves
    • These waves can pass through solids and liquids
    • P-waves are faster than S-waves
  • They are very low frequency sound waves known as infrasound
    • Infrasound is any sound below the frequency of human hearing (<20 Hz)
  • The waves refract as they pass through the different layers of the Earth
  • This refraction affects the regions in which waves can be detected, yielding important information about the nature and size of the Earth’s various layers

P-waves, IGCSE & GCSE Physics revision notes

Low frequency sound waves (P-waves) produced by earthquakes, pass through the centre of the Earth, revealing useful information about its structure

S-Waves

  • S-waves are a type of transverse wave
    • Unlike P-waves, S-waves are unable to travel through liquids
    • They pass through solids only
    • S-waves are slower than P-waves
  • This means that they are unable to travel through the Earth’s molten (liquid) outer core – providing important evidence about its state and size

S-Wave, downloadable IGCSE & GCSE Physics revision notes

Transverse S-Waves are unable to pass through the Earth’s liquid outer core

Higher Tier Only

Discoveries from Seismic Waves

  • The interior of the Earth is not directly observable as it is not physically possible to drill that far
    • The furthest humans have managed to drill down is 12.2 km – whereas the radius of the Earth is over 6000 km!
  • Seismic waves provide vital evidence that has led to a greater understanding of the structure of the Earth
  • The two main discoveries are:
    1. On the opposite side of the Earth to an earthquake, only P-waves are detected, not S-waves, this suggests:
      • The mantle is solid – this is because both types of wave can pass through it
      • The outer core of the Earth is liquid – hence no S-waves can penetrate it
    2. Refractions between layers cause two shadow zones, where no P-waves are detected, this suggests:
      • The inner core is solid – this is due to the size and positions of these shadow zones which indicate large refraction taking place

Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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