Seismic Waves (OCR Gateway GCSE Physics)

• 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

Exploring The Structure of The Earth Using 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. 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

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 returnThe distance the wave travels is twice the depth of the ocean

Sonar is used to determine water depth

Step 1: List the known quantities

• • Wave speed, v = 1500 m/s
  • Time, t = 0.12 s

Step 2: Write out the wave speed, distance and time formula

Step 3: Rearrange the equation to make distance (x) the subject

x = v × t

Step 4: Put known values into the equation

x = 1500 × 0.12 = 180 m

Step 5: Half the distance to obtain the depth

d = 180 ÷ 2

Depth, d = 90 m