Earthquakes are tremors caused by movement of parts of the earth. They happen millions of times a year, but they are usually so small that people don’t feel them. When they do occur, they can cause buildings and their contents to collapse and kill or injure people. They can also cause landslides, tsunamis and fires.
The shaking from an earthquake is caused by the sudden slip of rocks on a fault. This releases elastic energy in waves that move through the earth’s outer layer. The point on the surface directly above the fault is called the epicenter.
Seismic waves travel differently through different types of rock. When they pass from hard rock to soft soil, they slow down and get bigger (they amplify). They can continue shaking for weeks, months or years after the main shock of an earthquake.
The size of an earthquake is measured by its magnitude, a number based on how much the rocks on a fault moved. There are four different kinds of faults: normal, reverse, strike-slip and oblique-slip. The larger the fault, the greater its magnitude.
It takes many smaller earthquakes to release the same amount of energy as a large one. For example, it would take 32 earthquakes of magnitude 5 to equal the energy of one earthquake of magnitude 6. Earthquakes occur in a series of linked belts around the globe that are related to plate tectonics. They can also be triggered by volcanoes and landslides.
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