Seismic Waves

What are 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

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

• 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