An earthquake (or quakes, tremors) is shaking of the surface of earth, caused by sudden movement in the Earth’s crust. They can be extremely violent.

Earthquakes are caused by tectonic movements in the Earth’s crust. The main cause is that when tectonic plates collide, one rides over the other, causing orogeny (mountain building), earthquakes and volcanoes.

The boundaries between moving plates form the largest fault surfaces on Earth. When they stick, relative motion between the plates leads to increasing stress. This continues until the stress rises and breaks, suddenly allowing sliding over the locked portion of the fault, releasing the stored energy.

San Andreas Fault

(caption) Aerial photo of the San Andreas Fault in the Carrizo Plain, northwest of Los Angeles


Earthquakes are usually quite brief, but may repeat. They are the result of a sudden release of energy in the Earth’s crust. This creates seismic waves, which are waves of energy that travel through the Earth. The study of earthquakes is called seismology.Seismology studies the frequency, type and size of earthquakes over a period of time.

A replica of ancient seismometer

(caption) Replica of ancient seismometer with pendulum sensitive to ground tremors. In Luoyang in 133 AD, it detected an earthquake 400 to 500 km (250 to 310 mi) away

There are large earthquakes and small earthquakes. Large earthquakes can take down buildings and cause death and injury. Earthquakes are measured using observations from seismometers. The magnitude of an earthquake, and the intensity of shaking, is usually reported on the Richter scale. On the scale, 3 or less is scarcely noticeable, and magnitude 7 (or more) causes damage over a wide area.

An earthquake under the ocean can cause a tsunami. This can cause just as much death and destruction as the earthquake itself. Landslides can happen, too. Earthquakes are part of the Earth’s rock cycle.


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