Earthquakes are the result of brittle failure within the earth's lithosphere. Earthquakes cause energy to be released in an area of strain that is most often near a tectonic plate boundary.
Earthquakes occur in the solid outer portion of the Earth called the lithosphere. The lithosphere consists of the Earth's crust and the upper mantle. The composition of the lithosphere makes it brittle and vulnerable to failure. As plate tectonics deform the lithosphere over time, strain builds up in certain areas. The release of accumulated strain results in earthquakes.
Plate tectonics is the main cause of strain in the lithosphere. The interaction of plates takes place over thousands of years, and strain builds up slowly over time. In contrast, the release of built-up strain in the form of an earthquake can take just minutes. For this reason, earthquakes are considered high energy releases.
Deformation of the lithosphere occurs most actively at tectonic plate boundaries, which makes these areas the most vulnerable to earthquakes. At these boundaries, fractures occur in the lithosphere. A fault exists when the two sides of a fracture slide past one another. If there is strain buildup in the area of the fracture and the fault slips, an earthquake occurs.