A scale used to rate the strength or total energy of earthquakes. The scale has no upper limit but usually ranges from 1 to 9. Each increase in whole number represents a tenfold increase in magnitude, so that an earthquake rated as 5 is ten times as powerful as one rated as 4. An earthquake with a magnitude of 1 is detectable only by instruments (seismographs); one with a magnitude of 7 is a major earthquake. See Note at earthquake.