In this topic you will learn what earthquakes are and how they can be detected.

An earthquake is a sudden trembling of the ground. It is caused by something happening in the crust.

When plates move, great forces are exerted on the rocks of the crust. At first the forces can make the rocks bend and stretch. Every material has its limit to how far it will bend before breaking. When rocks in the crust reach their limit, they break and faults form in the crust. A huge crack in the crust, at or below the surface, the sides of which may show evidence of motion, is called a fault.

The point at which the earthquake starts, where the rocks begin to slide past each other, is called the focus. It is usually below the surface. The sudden motion causes a vibration to spread out from the focus when an earthquake happens, and is called a seismic wave. It soon reaches Earth's surface at a point directly above the focus. This point is called the epicenter. It is at the epicenter that people can first feel the ground shaking.

The shaking is what causes most earthquake damage. Damage is usually greatest at the epicenter because it is closest to the focus. As the waves travel away from the focus they get weaker. People far from the epicenter may not feel the ground shaking at all. After the initial shaking of an earthquake, there may be relative quiet, followed by more shaking of the crust called aftershocks. Aftershocks continue the damage of an earthquake.

Seismic waves travel through Earth's crust. They can be detected with a seismograph, which is a sensitive device that detects the shaking of the crust. By carefully studying the waves at several locations, scientists locate the epicenter of an earthquake.

The height of a wave on a seismograph is a measure of the magnitude of an earthquake. Magnitude is the amount of energy released by an earthquake. The more energy an earthquake releases, the more violent the shaking recorded.

Charles Richter devised a scale for comparing the energy of earthquakes. The Richter scale rates earthquakes from 1 to 10 according to magnitude. Each increase of 1 on the scale means an increase of about 30 times the energy released.

Plate tectonics can help us know where earthquakes are more likely to happen. Much earthquake destruction might be prevented if people had warning signs.