Magnitude 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. 

