Seismic Waves (AQA GCSE Physics)

Seismic Waves

Higher Tier Only

• 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

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

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

Discoveries from Seismic Waves

Higher Tier Only

• 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