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
• Earth and waves:

  A) Mantle

  B) P-waves

  C) Liquid

  D) Inner

  E) Shadow

  
• 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-waves and earth

  A) S-waves

  B) Liquid

  
• Exploring The Structure of The Earth Using Waves

  • The interior of the Earth is not observable as it is not possible to drill that far
  • 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
  • The outer core of the Earth is liquid – hence no S-waves

  1. Refractions between layers cause shadow zones for P- waves this suggests:

  • The inner core is solid
• Exploring Objects in Deep Water

  • Waves can also be used to image objects in deep water
  • This is a process known as sonar
  • Sonar uses ultrasound to detect objects underwater
  • The sound wave is reflected off the ocean bottom
  • The time it takes for the sound wave to return is used to calculate the depth of the water
  • This is the distance to the ocean floor plus the distance for the wave to return the distance the wave travels is twice the depth of the ocean