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 one to ten, though there is no upper limit. It is logarithmic, which means that an earthquake measuring magnitude five is ten times more powerful than an earthquake measuring four.
Earthquakes measuring one to two on the scale happen regularly, and they are so small people cannot feel them. Earthquakes measuring upwards of seven are less frequent but very powerful and can cause a lot of destruction. The largest earthquake ever recorded was in Chile in 1960, it measured 9.5 on the Richter scale.