Body wave magnitude is a way of determining the size of an earthquake

Earthquake

An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...

, using the amplitude of the initial P-wave

P-wave

P-waves are a type of elastic wave, also called seismic waves, that can travel through gases , solids and liquids, including the Earth. P-waves are produced by earthquakes and recorded by seismographs...

 to calculate the magnitude. The P-wave is a type of body wave that is capable of traveling through the earth at a velocity of around 5 to 8 km/s, and is the first wave

Seismic wave

Seismic waves are waves of energy that travel through the earth, and are a result of an earthquake, explosion, or a volcano that imparts low-frequency acoustic energy. Many other natural and anthropogenic sources create low amplitude waves commonly referred to as ambient vibrations. Seismic waves...

 from an earthquake to reach a seismometer

Seismometer

Seismometers are instruments that measure motions of the ground, including those of seismic waves generated by earthquakes, volcanic eruptions, and other seismic sources...

. Because of this, calculating the body wave magnitude can be the quickest method of determining the size of an earthquake that is of a large distance from the seismometer.

Limitations in the calculation method mean that body wave magnitude saturates at around 6-6.5 , with the figure staying the same even when the moment magnitude

Moment magnitude scale

The moment magnitude scale is used by seismologists to measure the size of earthquakes in terms of the energy released. The magnitude is based on the seismic moment of the earthquake, which is equal to the rigidity of the Earth multiplied by the average amount of slip on the fault and the size of...

may be higher.