Richter Scale

Richter Scale, method of ranking the strength or size of an earthquake. The Richter scale, also known as the local magnitude scale, was devised in 1935 by the American seismologist Charles F. Richter to rank earthquakes occurring in California. Richter and his associates later modified it to apply to earthquakes anywhere in the world.

The Richter scale ranks earthquakes based on how much the ground shakes 100 km (60 mi) from the earthquake’s epicenter, the site on Earth’s surface directly above the earthquake’s origin. The amount of ground movement is measured by an instrument called a seismograph. Seismographs can detect movements as small as about 0.0001 mm (about 0.000004 in) to movements as large as about 1 m (about 40 in). In order to deal with numbers in such a broad range, the Richter scale is a logarithmic scale—each increase of 1 on the Richter scale represents a tenfold increase in movement. Thus, an earthquake registering 7 on the scale is 10 times as strong as an earthquake registering 6, and the earth moves 10 times as far.

Earthquakes of magnitude 5 are considered moderate, while quakes of magnitude 6 are considered large, quakes of magnitude 7 are considered major, and quakes of magnitude 8 or larger are considered great. For example, the Los Angeles earthquake of 1994 was a magnitude 6.7 earthquake and the San Francisco earthquake of 1906 was a magnitude 7.9 earthquake. Although there is no upper limit to the Richter scale, earthquakes of magnitude 8 or greater are rare. Worldwide, they occur only about once a year. Scientists believe that the crust cannot store enough energy to release a magnitude 10 earthquake. However, the energy from the impact of an asteroid or a comet from space striking the crust could exceed this natural upper limit. There is also no lower limit on the Richter scale. An earthquake one-tenth the size of a magnitude 1 earthquake would be a magnitude 0 earthquake, and an earthquake one tenth that size would be a magnitude -1 earthquake. Earthquakes with negative Richter scale magnitudes occur every day, but are so small that they are difficult to detect.

The amount of energy released by an earthquake is related to how much the earth moves. The energy released by an earthquake increases 32 fold for each increase of 1 on the Richter scale. Thus, an earthquake registering 7 on the Richter scale releases 32 times as much energy as an earthquake registering 6, even while the earth moves only 10 times as far. The amount of energy released by a magnitude 4.3 earthquake is equivalent to the energy released by the atomic bomb that destroyed Hiroshima, Japan, which is equivalent to about 20 kilotons of TNT. The largest earthquakes recorded to date measured about 9.5 and released as much energy as 66,000,000 Hiroshima-sized atomic bombs. It is estimated that a magnitude 12 earthquake would release enough energy to split Earth in half.

The Richter scale is only one of several scales used to measure earthquakes. Currently, the scale most commonly used by seismologists to rank the effects of earthquakes is the Modified Mercalli Intensity Scale, or MM scale. The MM scale measures the effects of an earthquake at different sites and thus the same earthquake has different MM scale values at different sites. The MM scale is marked from I (for barely detectable) to XII (for almost complete destruction).