What is earthquake magnitude?

Earthquake magnitude is a measure of the "size," or amplitude, of the seismic waves generated by an earthquake source and recorded by seismographs

Earthquakes, until recently, have been measured on the Richter scale. The Richter scale measures the magnitude of an earthquake (how powerful it is). It is measured using a machine called a seismometer which produces a seismograph. A Richter scale is normally numbered 1-10, though there is no upper limit.