How do scientists measure the size of an earthquake?

The size of the earthquake recording on seismogrammes is related to the size of the event at its source. Scientists, therefore, use various characteristics of the signature of the recording in mathematical equations to calculate the earthquake size or magnitude. As a result, there are different types of magnitude depending on the characteristics used in its determination e.g. body wave, surface wave and duration. The values so obtained may differ, but those using the same type of waves usually agree within a few decimal units.