Earthquake is the sudden shaking of the ground caused when masses of rock change position below our planet’s surface. This shifting causes shock waves that may alter the surface, thrust up cliffs or open great cracks in the ground.
The vast majority of Earth’s earthquakes occur within fault zones. Faults are areas where plates move in different directions – sometimes grinding against each other, or slipping past each other as they slide along a continental plate boundary. The friction that causes these movements generates energy, which accumulates until a threshold value is reached and suddenly releases the built-up stress, producing an earthquake.
When an earthquake occurs, the friction produces two kinds of energy waves. The first, called P waves (for compressional) travel fastest and shake the ground near you first. The second wave, known as S waves (for shear) follow P waves and shake the ground a little bit later. The time difference between the two P and S waves tells scientists how far away the earthquake occurred.
Scientists can also determine the size of an earthquake from the pattern of wiggles on the seismogram recorded at a seismic station. A short wiggly line means the quake was small, and a long wiggly line indicates a large earthquake.
While the science of predicting when and where an earthquake will happen is still very much in its early stages, there are some signs that indicate a quake may be coming. For example, some earthquakes have been preceded by low frequency electromagnetic signals that geophysicists detect with satellites in orbit. But these signals don’t provide a lot of information about the direction or location of the impending quake, so it’s not possible to issue a public warning. Likewise, some people have reported feeling unusual vibrations in the ground or seeing cracks in the road and buildings just before an earthquake happens. There are even reports of animals, from toads to chickens, acting strangely just before a quake.