Sudden movement of the earth resulting from the abrupt release of accumulated strain, usually a result of faulting or volcanism. An earthquake is classified according to the depth of its focus or hypocenter: shallow <60 km, intermediate <300 km, deep >300 km (sometimes>450 km). The seismicity of an area is its likelihood of having earthquakes. Some of the energy released in an earthquake is radiated as seismic energy. Intensity is a measure of the effects of the ground motion at a specific locality; magnitude is a measure of the energy released in an earthquake; see intensity scale and magnitude of earthquake.