Seismic scale

A seismic scale is used to calculate and compare the severity of earthquakes.

Two fundamentally different but equally important types of scales are commonly used by seismologists to describe earthquakes. The original force or energy of an earthquake is measured on a magnitude scale, while the intensity of shaking occurring at any given point on the Earth's surface is measured on an intensity scale.

Magnitude and intensity

The severity of an earthquake is described by both magnitude and intensity. These two frequently confused terms refer to different, but related, expressions. Magnitude, usually expressed as an Arabic numeral characterizes the size of an earthquake by measuring indirectly the energy released. By contrast, intensity indicates the local effects and potential for damage produced by an earthquake on the Earth's surface as it affects humans, animals, structures, and natural objects such as bodies of water. Intensities are usually expressed in Roman numerals, and represent the severity of the shaking resulting from an earthquake. Ideally, any given earthquake can be described by only one magnitude, but many intensities since the earthquake effects vary with circumstances such as distance from the epicenter and local soil conditions. In practice, the same earthquake might have magnitude estimates typically differing by few tenths of a unit, depending on which magnitude scale is used and which data are included in the analysis.