In an earthquake the ground moves. Earthquakes commonly last for a few seconds.
There are big earthquakes and small earthquakes. Big earthquakes can damage buildings and hurt people.
When the earth moves in an earthquake, it can cause waves in the ocean. A big wave in the ocean caused by an earthquake is called a "tsunami". A tsunami can also hurt people and damage buildings.
The size of an earthquake can be measured. A number is given to an earthquake that shows the size of the earthquake.
Methods for turning the size of an earthquake into a number include the Richter scale and the Modified Mercalli scale.
[change] Cause of earthquakes
The surface of the Earth is divided into pieces called "tectonic plates". These plates move. When the plates rub against each other, they do not move smoothly. When the plates do not move smoothly, earthquakes result.
Some parts of the world get more earthquakes than other parts. The parts of the earth that get most earthquakes are near the edges of these plates.
Earthquakes send sound waves through the earth. Places many kilometers from the edge of the plates can receive these earthquake waves.
Some countries, such as Japan or parts of a country like California in the United States, have a lot of earthquakes. In these places it is a good practice to build houses and other buildings so they will not collapse when there is an earthquake. This is called seismic design or "earthquake-proofing".
For many years earthquakes have occurred. That is why there are earthquake proof buildings. The ability of a building to withstand the stress of an earthquake depends upon its type of construction, shape, mass distribution, and rigidity. Different combinations are used. Different shapes of buildings such as square, rectangular, and shell buildings can withstand earthquakes much better than skyscrapers. To reduce stress, a building's ground floor can be supported...