The magnitude of an earthquake is a measure of its strength, or the energy released by it, as determined by observations from seismographs. Magnitude is usually expressed in terms of the Richter scale, developed by Charles Richter in 1935. Because the Richter scale is logarithmic, an increase in magnitude of 1.0 (e.g., from 4.0 to 5.0) represents a tenfold increase in the the strength of an earthquake.
People generally can’t feel quakes of magnitude 2.0 or lower. Earthquakes of magnitude 6.0 or higher are considered major. The Richter scale has no upper limit.