Properties of Seismic Waves (WJEC GCSE Physics)

• An earthquake is the sudden, violent shaking of the ground
  • When tectonic plates move, they can become locked together causing stress and pressure to build under the surface of the Earth
  • Eventually, the stress becomes so great that the rocks fracture and the pressure is suddenly released
  • This causes intense ground shaking that can last from several seconds to several minutes

• The focus is the point at which the earthquake starts below the Earth's surface
  • The energy released by the earthquake travels out from the focus

• The epicentre is the point on the Earth's surface directly above the focus

The Focus and Epicentre of an Earthquake

The epicentre of an earthquake is on the surface of the Earth directly above the focus

• Earthquakes produce three types of waves:
  • P-waves (primary waves) - these are the first waves to arrive at a point away from the epicentre
  • S-waves (secondary waves) - these are the second waves to arrive
  • Surface waves - these occur on the Earth's surface and are the slowest or last to arrive

The Three Types of Seismic Wave

The three types of seismic wave have different effects on the surface of the Earth

Summary of Seismic Wave Types

	P-waves	S-waves	Surface waves
Type of Wave	Longitudinal (Sound)	Transverse	Longitudinal and Transverse
Speed	Fastest	Slower than P waves but faster than surface waves	Slowest
Material of Travel	Through solids and liquids	Through solids only	Along the surface only

• Seismic waves from earthquakes can be detected using a seismometer (at a recording station) and displayed on a seismogram
  • This provides a visual record of the vibrations of the Earth caused by the earthquake

• Seismometers have a large mass attached to a sensor
  • When a seismic wave is detected the mass and sensor move relative to each other and a small current is produced 
  • The current is recorded by a computer and displayed as a seismogram on a screen

A Seismometer Creating a Seismogram

An earthquake causes the mass and sensor on the seismometer to move relative to each other creating a current which is recorded as the seismogram

Interpreting a Seismogram

• The horizontal axis of the seismogram represents time 
• The vertical axis represents the amplitude (magnitude) of the vibrations
• The faster P-waves arrive first 
  • Then the slower S-waves arrive second
  • This is followed by the surface waves
• Different waves have vibrations of different amplitudes on the seismograph

An Example Seismograph

The seismograph tells us about the time lag between P-waves, S-waves and surface waves

• The most important piece of information that can be obtained from a seismogram is the time delay between the arrival of the P-waves and the S-waves 
  • This is known as the time lag

• The time lag can be used to determine the distance to the earthquake
  • In the example seismograph above the time between the arrival of the P-wave and arrival of the S-wave is 0.95 seconds

• The time lag can be obtained from three different seismometers and the exact position of the epicentre determined
  • This process is known as triangulation

An Example of Triangulation

An earthquake is detected by three seismometers in different locations; Minneapolis, Detroit and Charleston. The time lag between the P and S-waves is calculated to determine the distance from each recording centre and triangulation is carried out. This determines that the epicentre of the earthquake is where all three distances meet