The strength of an earthquake is generally expressed in two ways: magnitude and intensity. The magnitude is a measure that depends on the seismic energy radiated by the earthquake as recorded on seismographs. An earthquake's magnitude is expressed in whole numbers and decimals (e.g, 6.8). The intensity at a specific location is a measure that depends on the effects of the earthquake on people or buildings. Intensity is expressed in Roman numerals or whole numbers (e.g, VI or 6). Although there is only one magnitude for a specific earthquake, there may be many values of intensity (damage) for that earthquake at different sites.

Magnitude Scales
Several magnitude scales have been developed by seismologists. The original is the Richter scale of magnitude, developed in 1932 by the late Dr. Charles F. Richter who was a professor at the California Institute of Technology (CalTech).
The most commonly used scale today is the Moment magnitude (Mw) scale, jointly developed in 1978 by Dr. Thomas C. Hanks of the U.S. Geological Survey and Dr. Hiroo Kanamori, a professor at CalTech.
Moment magnitude is related to the physical size of fault rupture and the movement (displacement) across the fault, and as such is a more uniform measure of the strength of an earthquake.

Several magnitude scales have been developed that measure the amplitude of different portions of a seismogram. The most widely known scale, the Richter scale, was originally designed to use the maximum trace amplitude registered on a seismogram from a standard instrument, called a Wood-Anderson torsion seismograph, as a measure of earthquake size.
When an earthquake is recorded on the standard instrument, the greatest excursion of the wiggly trace is measured and compared with that of a reference magnitude 3.0 earthquake at the same epicenter-to-station distance. The result is a number that directly corresponds to the size of the earthquake relative to the reference earthquake.
The reference magnitude 3.0 earthquake was defined by Richter to have a maximum trace amplitude of 1 millimeter on a standard Wood-Anderson seismograph at a distance of 100 kilometers from the epicenter. With appropriate distance corrections for the recorded amplitude, the magnitude value is constant and is an effective means of earthquake size classification.