Earthquakes - Major Causes of Earthquakes. Focus, Epicentre and seismic waves

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Introduction and Causes of Earthquakes

  • Earthquakes are sudden vibrations of the lithosphere.
  • Over 10 million earthquakes occur each year, but most of them are too mild to be felt.
  • They cause loss of lives and property.

 Major Causes of Earthquakes

  • Most earthquakes are the result of plate movements.
  • When plates move, friction causes pressure to build up slowly in the plate and energy is stored.
  • When the pressure exceeds the strength of rock, the rock breaks.
  • This releases the stored energy and produces seismic waves.
  • These waves move out in all directions and cause the ground to shake, resulting in earthquakes.
  • Some earthquakes are caused by volcanic eruptions.
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Focus, Epicentre and seismic waves

  • Some earthquakes are mild, some others are rather destructive.
  • The severity of an earthquake depends on the amount of energy that is released.
  • The point of origin of shock waves is usually at a certain depth in the earth’s crust, known as focus.
  • The epicentre is the point on the Earth’s surface vertically above the focus.
  • Shock waves that travel outwards from the epicentre at different speeds in all directions are called seismic waves.
  • These waves cause the ground to shake, resulting in earthquakes.


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