Earthquakes happen when rock beneath the earth suddenly moves. Stresses in the earth make blocks of rock break and move along a fault line, which is a crack in the earth. The rocks may move, slip downward, upward or sideways. Many earthquakes take place on the edge of the seas. As of 2014, earthquakes can't be prevented but can be somewhat predicted.
Scientists believe there may be over a million earthquakes every year, but only a very few of them are destructive.
When an earthquake occurs, seismic waves radiate out from the epicenter. These shock waves are what cause the earth to tremble and crack. The waves might cause sounds like thunder or a runaway freight train. Cities near the epicenter might suffer a great deal of damage and loss of life, especially if the buildings aren't made to withstand earthquakes. Water main lines and gas mains can break and cause flooding and fires.
Earthquakes can also cause landslides, which can bury villages built on the sides of mountains. Earthquakes can change the course of rivers and cause land to shift to form lakes or waterfalls. If an earthquake occurs in the ocean, it can create devastating tsunamis. An earthquake's magnitude is measured with a seismograph.