The Richter Scale for measuring the magnitude of earthquakes was developed by Dr. Charles F. Richter in 1934. Dr Richter was working at the California Institute of Technology studying the waves produced by earthquakes using an instrument called a seismograph. He found that the amplitude of the shear waves detected could be related to the strength of the earthquake, which produced them.
The Scale he used to measure the amplitudes is a logarithmic scale in which seismic waves from a Richter Scale 2 earthquake are 10 times the amplitude of those produced by a Scale 1 earthquake. While the amplitude of the waves increases by a factor of ten for each whole unit on the Richter Scale, the amount of energy released by an earthquake goes up by a factor of 32. An earthquake measuring 6.0 will release the equivalent energy at I million tons of TNT and one measuring 7.0 releases the equivalent of 32 million tons of TNT this is the same amount of energy as the worlds’ largest thermonuclear device.
Very minor earthquakes releasing energy equivalent to 6 ounces of TNT have a negative value on the Richter Scale of minus 1.5. There is no upper limit on the Richter Scale but the strongest earthquake measured was the Chile earthquake of 1960 at 9.5.
If we were ever to experience a magnitude 12 earthquake, it would be equivalent to 160 trillion tons of TNT.
Earthquakes can be grouped within the Richter Scale according to their magnitude class. Minor earthquakes measure between 3.0 and 3.9. Light earthquakes measure between 4.0 and 4.9. Moderate earthquakes measure between 5.0 and 5.9. Strong earthquakes measure between 6.0 and 6.9. Major earthquakes measure between 7.0 and 7.9. Any earthquake measuring above 8.0 is classed as a Great earthquake.
Earthquakes with a measurement below 3.0 cannot usually be felt and are detectable only by seismographs. There are about 900,000 of these very small earthquakes every year. There are about 20 major earthquakes every year, fortunately not all of these strike populated areas. Great earthquakes are far less common with one occurring about every five to ten tears. In 2008, there was a Great earthquake measuring 8.0 in Sichuan, China, which killed an estimated 650,000 people. The earthquake responsible for the Indian Ocean Tsunami in 2003 was a Great earthquake with a magnitude of 9.3.
Richter’s original work related only to earthquakes in California detected by the Wood-Anderson seismograph. By standardizing and calibrating seismographs made by different manufacturers earthquakes around the world can be assigned a value on the Richter Scale.
There are other scales use to measure earthquakes. The Mercalli Scale, invented by Giuseppe Mercalli in 1902, is a subjective scale based on people’s obsevations and the damage caused by the earthquake. It is not an accurate comparative scale as the damage an earthquake can cause will depend on the geology of the earthquake area and on the building materials used in the damaged structures, as well as the strength of the earthquake. Recently another scale for use when measuring Great earthquakes has been introduced. This is the Moment Magnitude Scale and it involves a complicated calculation that relates magnitude to the seismic moment or torque of the earthquake.