An earthquake is one of the worst natural disasters on our earth. We think that the ground we stand on is very stable, but it isn't. It moves quite a lot. In the last few decades scientists have been able to find out why earthquakes happen.

Earthquakes happen when there is a sudden vibration in the earth's crust. It's like a large lorry that travels down your street. When it passes by, you feel your house shake.

Earthquakes can be caused by a lot of things:

• Volcanoes that suddenly erupt
• Meteorites that hit the earth
• Undergrounds explosions
• Buildings that fall apart

But most earthquakes happen because the earth's plates move.

In the middle of the 20th century scientists found out that continents do not always stay in the same place. They have been moving on plates for millions of years. The earth's surface is made up of many such plates. Where two plates meet magma comes out of the inner part of the earth. These areas are called faults—breaks in the earth's crust.

How plates move

When two plates move away from each other lava or magma comes out of the earth. Most of this happens at the bottom of oceans, where the earth's crust is very thin. Lava cools down when it reaches the water and underwater mountains are formed.

When plates push towards each other—one of them slides under the other. Rocks are pushed up and new mountains are formed.

Some plates slide past each other— for example, one moves north and the other moves south. When these plates move along faults a lot of energy is released and the biggest earthquakes happen.

We only hear about earthquakes once in a while, but they really happen every day. There are more than 3 million earthquakes every year—about 8,000 every day or one every 11 seconds.

But most of them are very weak or they happen in places where nobody lives. Some of them take place on the sea floor.

Where do earthquakes happen?

Earthquakes occur all over the world but there are places where they happen more often. Big earthquakes can be found where plates meet.

80% of the world's earthquakes happen around the Pacific Ocean—near the east coast of Asia and the west coast of America. Japan has over 2,000 earthquakes every year and California and South America are also very active earthquake zones. The edge of the Pacific Ocean is also called the "Ring of Fire" because there are also many active volcanoes in this region.

Earthquake waves

When there is a sudden movement in the earth's crust, energy moves in the form of waves. It's like dropping something into water.

Body waves move through the inner part of the earth and surface waves travel over the earth's surface.

Body waves can travel very fast—up to 8km a second. They travel through rock, water and gas. When they reach other places on the earth's surface they can be registered there. They are usually the first waves to get to the surface.

Surface waves cause the most damage, but they move very slowly. These waves come at the end of an earthquake.

Man-made earthquakes

Sometimes people can make earthquakes happen. They can fill man-made lakes with water after building a dam—or they test atomic bombs underground. Some of these tests can help scientists find out how quakes happen.

How earthquakes are measured

With a machine called a seismograph scientists can tell where an earthquake happened and how strong it was.

The place in the earth where the movement takes place is called the focus or hypocentre. From here, waves start to spread out in all directions. This focus can be very near to the surface or it can be hundreds of km below it. The area on the surface exactly above the focus is called the epicentre. This is the place where the waves hit first and where the most damage is done.

Whenever an earthquake hits us you hear how powerful it is. The Richter Scale is used to rate the magnitude of earthquakes. Small quakes have a rating of under 4. You won't see a lot of damage here. Medium-sized earthquakes reach between 5 and 7 on the scale, and the really big ones are above 7. The largest earthquake that has ever been registered was at 9.5 on the Richter scale.

There are more than 100 seismograph stations all over the world. When the earth shakes seismologists compare the information they get and then they can tell where the earthquake really happened.

Effects of earthquakes

Earthquakes make the ground move. Buildings shake and many of them collapse. Landslides also happen when rocks get loose.

Another danger is fire. In 1906 San Francisco was hit by a big earthquake and many houses burned down because they were made of wood.

When an earthquake occurs on the sea floor, big waves - called tsunamis - hit the coast. They often come without any warning and they kill many people and destroy buildings and streets near the coast.

Earthquakes also can lead to diseases, especially in developing countries. When water supplies are destroyed people don't have safe water to drink. Sometimes earthquakes also hit hospitals where injured people are treated.

Dealing with earthquakes

We understand earthquakes a lot better today than we did 50 years ago, but we still can't do very much about them. They are so powerful, that we cannot control them.

Scientists can tell us in which regions earthquakes will probably happen, but they can't tell us exactly where.

So what can we do about earthquakes? We can make our houses we live in and buildings we work in safer. Today architects use materials that won't collapse when an earthquake hits—like steel and concrete.

The biggest earthquakes in history

856

200,000 people are killed in one of the deadliest earthquakes in history in Damghan, Iran.

1138

An earthquake in Aleppo, Syria kills 230,000

1556

The deadliest earthquake in history hits Shensi province in China. Almost 1 million people are killed.

1755

The most serious earthquake in Europe hits Lisbon, Portugal. The whole city is destroyed. About 70,000 people get killed.

1906

The city of San Francisco is hit by a large earthquake. A big fire destroys most of the city. About 500 people die.

1908

Messina, Sicily is totally destroyed by a large earthquake. About 70,000 people are killed.

1923

Tokyo, Japan is destroyed by an earthquake with a magnitude of 8.3. 150,000 people are killed.

1960

The strongest earthquake ever recorded hits Chile. It reaches 9.5 on the Richter scale. 20,000 people die.

1964

The strongest earthquake in North America hits Anchorage Alaska. Ocean waves reach a height of 20 metres. 200 people are killed.

1976

The biggest earthquake ever recorded in China leaves 242,000 people dead.