What causes an earthquake?

• There are more than one million earthquakes a year
• Tectonic plates float on soft rock and are always moving
• The plates rub against each other which causes friction and pressure
• Plates slip and cause a jolt that we feel it as a sudden earthquake

Tectonic plates are floating on a bed of soft magma and rock below the earth's crust. Sometimes they move away from each other and sometimes they go towards another plate. When the plates touch they may rub against each other which causes friction.

The pressure builds up until one of the plates may no longer be able to withstand the force. The weaker plate slips out of position. It may even crack in places. Humans on the earth's surface feel this sudden movement and it is known as an earthquake.

The movement of the earth's crust is called seismic waves. The size of these seismic waves is measured on the Richter Scale.

Earthquakes usually only last for a few seconds. However, earthquakes are often followed by a series of aftershocks. These are caused by the continued moving of the plates pushing against each other as they settle into their new positions. Aftershocks are never usually as powerful as the initial earthquake.

Sometimes where earthquakes happen below the sea they cause a tsunami.

Earthquakes happen regularly and are fairly common in areas that lie on fault lines - the places where tectonic plates meet. One such area is Japan. The country may feel several earthquakes a year. However, the buildings have been designed to withstand minor tremors.

Earthquakes cause problems when they are very powerful. The force of the seismic waves causes buildings to shake. Sometimes the buildings will collapse. Over the years, many thousands of people have been killed when earthquakes have caused buildings to fall down.

An earthquake hits Haiti in the Caribbean. It wrecked the nation, and it's thought to have killed between 100,000 and 200,000 people. Countless buildings were devastated and 1.5 million individuals were made homeless. Troops and support had to be sent in to help the recovery

Around 150 individuals lost their lives in Italy when a quake struck in a metropolis called L'Aquila. More than 50,000 individuals were made homeless by the tremor.

Around 87,000 people are thought to have died in China by when a quake struck the Sichuan region of the nation. Some five million homes were obliterated in the shudder which measured 7.5 on the Richter scale, and was the largest to strike the nation for 30 years.

More than 450 individuals recorded dead in Peru by a tremor measuring 7.9 on the Richter scale.

More than 3,000 people were left dead by a seismic tremor that struck the Indonesian island of Java. Something greater than 200,000 people were made homeless by the quake, which measured 6.2 on the Richter scale.

The death count rose to 300,000 when a quake in the Indian Ocean measuring 9.0 on the Richter scale caused a colossal tsunami to hit some Asian nations. The worst affected nations were Indonesia, Sri Lanka, India and Thailand.

An estimated 50,000 people were killed after a tremor in Iran, which measured 6.3 on the Richter scale.

Over 1,000 folks are killed and practically 7,000 harmed in a shake in Algeria, which measured 6.7 on the Richter scale.

Thousands lost their lives in a remote part of Afghanistan when a tremor measuring 6.0 on the Richter scale struck.

Thirty-thousand individuals died and around 50,000 were seriously injured by an earthquake measuring 7.9 on the Richter scale in Gujarat, India.

More than 1,000 residents died in a 7.2 quake in El Salvador.