The Richter scale was developed by Charles F. Richter in 1935 during his work at the California Institute of Technology. In California, Charles Richter was measuring the intensity of the earthquakes occurring in the southern section of the state - a fairly common happening. The scale was originally referred to as ML - Magnitude Local.

The Richter scale is designed as a mathematical device for comparing the magnitude of earthquakes. It should be noted that the magnitude does not imply amount of devastation - a relatively small earthquake can cause massive destruction in areas where the local geography amplifies the earthquake while some large ones that happen in deserted areas may cause very little destruction at all. Likewise, it does not mean how noticed it is by people (other than those looking at seismographs) - many earthquakes happen in the depths of the ocean that are not felt at all. The Richter scale simply estimates the energy released by the earthquake.

The magnitude of the earthquake is determined from the waves recorded by a seismograph. The scale used is logarithmic rather than linear - every increase in a point on the Richter scale is 10x more powerful than the previous one and releases about 31x more energy. The value recorded by the seismograph is adjusted for distance to the epicenter to give the reported value. Feel free to work out the values yourself - the difference in energy released between a 3.0 (barely noticeable) and a 6.0 (things move around) is almost 30,000 times more.

Earthquakes less intense than 2.0 are called micro-earthquakes and are typically only noticed on a seismograph. Explosions in quarries often appear in this range and are distinguished by the fact that they are at a known quarry location and less than 0.1km deep. When the earthquake reaches an intensity of 4.5, it is powerful enough to be recorded all across the world.

Within the San Francisco area, there have been about 30 earthquakes since in the past week (Oct 1 - Oct 6); most of these were 2.0 or less with a single one at 3.1. (see for a list and map of earthquakes in the California-Neveda area)

The largest earthquakes are in the 8.0 and above range - for example the 1964 earthquake in Alaska on Good Friday is the second largest recoreded earthquake with a magnitude 9.2. While the largest earthquakes recorded have been about 8.8 to 8.9, there is no upper limit on the scale (likewise, it has no lower limit, but earthquakes of intensity -1 are not as interesting to talk about and far too common).

A selection of major earthquakes:

Date Magnitude  Location
1857 8.0        Fort Tejon
1906 7.8 - 8.25 San Francisco
1933 6.4        Long Beach
1952 7.5        Bakersfield, California
1964 9.2        Alaska
1971 6.6        Sylmar (Los Angeles)
1976 7.8        Tangshan, China
1987 5.9        Whitter Narrows
1988 6.9        Armenia
1989 7.1        Loma Prieta
1994 6.7        Northridge (Los Angeles)
1995 6.7        Kobe, Japan
1999 7.6        Taiwan
2001 7.6        El Salvador
2001 7.7        Gujarat, India
2001 6.8        Puget Sound, Washington
2002 4.8        Dudley, England

(I would welcome links and references to other historical earthquakes)

To compare the magnitude of two earthquakes, take the difference of the numbers, and drop that many '0's after a '1' - The difference between a 3.5 and 4.5 earthquake is 10x. The difference between a 3.5 and 5.5 earthquake is 100x. If the number is not whole or close enough to round, it would be necessary to get out a calculator and plug in 10(abs(M1 - M2)). For a M3.5 and M5.7 earthquake, 102.2 is 158, or the M5.7 earthquake is 158 times more powerful than the M3.5 earthquake.

Mag       Power Energy Released     TNT Yield
--- ----------- --------------- -------------
 1            1               1       30 lbs
 2           10              31        1 ton
 3          100             961       29 tons
 4         1000           29791     1000 tons (small nuke)
 5        10000          923521    32000 tons
 6       100000        28629151        1 Mton
 7      1000000       887503681       32 Mton
 8     10000000     27512614111        1 Bton
 9    100000000    852891037441       32 Bton
10   1000000000  26439622160671        1 Tton
For comparison, a -1.5 Richter magnitude (thats right, -1.5) is about 6 ounces of TNT, about the same as breaking a rock on a table. A tornado unleashes as much energy as a 4.5 Richter magnitude earthquake. A Richter magnitude 9 earthquake releases about twice as much energy as the power consumption of the United States for a year. At Richter 10, the equivalent of one trillion tons of TNT and the power of a San-Andreas type fault circling the earth. Richter 12 would be the equivalent of a fault half way to the center of the earth or about the same amount of energy as the Earth receives from the sun in a day.

See also seismology glossary