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. It is logarithmic which means, for example, that an earthquake measuring magnitude 5 is ten times more powerful than an earthquake measuring 4. Earthquakes measuring 1-2 on the scale happen regularly, and they are so small that people cannot feel them. Earthquakes measuring upwards of 7 are less frequent but very powerful, and can cause a lot of destruction.
The largest earthquake ever recorded was in Chile in 1960, which measured 9.5 on the Richter scale. The Richter scale is not very accurate in measuring these larger earthquakes and today scientists use the Moment Magnitude Scale which uses the same logarithmic scale but which more accurately measures the strength of larger earthquakes.