The Richter Scale
The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology. The mathematical device is used to compare the size earthquakes.
On the Richter Scale, the magnitude of quake is expressed in whole numbers and decimal fractions.
Each whole number increase on the scale increases the magnitude tenfold and corresponds to a release of about 31 times more energy than the preceeding whole number.
There is no upper limit on the scale.
The Richter Scale is not used to express damage.
Earthquakes with a magnitude of about 2.0 or less are usually referred to as microearthquakes. These quakes are usually not felt by people and are generally only recorded on local seismographs.
Quakes with magnitudes of 4.5 or greater are recordable by seismographs all over the world. Thousands of these quakes occur each year.
Events that record magnitudes of 8.0 or higher, on average, occur only once a year.
The largest quake known took place in Chile on May 22, 1960. It measured 9.5 on the scale.
U.S. Geological Survey: http://www.usgs.gov
Earthquake Information Center: http://neic.usgs.gov