Richter Develops a Scale for Measuring Earthquake Strength

Date January, 1935

Charles Francis Richter devised a scale for measuring the strength of earthquakes based on their seismograph recordings.

Locale Pasadena, California

Key Figures

• Charles Francis Richter (1900-1985), American seismologist
• Beno Gutenberg (1889-1960), German American seismologist
• Kiyoo Wadati (1902-1995), Japanese seismologist
• Giuseppe Mercalli (1850-1914), Italian physicist, volcanologist, and meteorologist
• Michele Stefano de Rossi (1834-1898), Italian geologist and archaeologist
• François-Alphonse Forel (1841-1912), Swiss geologist and geographer

Summary of Event

Earthquakes range in strength from barely detectable tremors to catastrophes that devastate large regions and cause the loss of hundreds of thousands of lives. The human impact of earthquakes is not an accurate measure of their power; minor earthquakes in heavily populated regions may cause great destruction, whereas powerful earthquakes in remote areas may go unnoticed. To study earthquakes, it is essential to have an accurate means of measuring their power.

The first attempts to measure the power of earthquakes involved the development of intensity scales that relied on damage effects and the reports of witnesses as measures of the force of vibration. The first such scale was devised by Michele Stefano de Rossi and François-Alphonse Forel in 1883. It ranked earthquakes on a scale of 1 to 10. The de Rossi-Forel scale proved to have two serious limitations: Its level 10 encompassed a great range of effects, and its description of effects on human-made and natural objects was so specifically European that the scale was difficult to apply elsewhere. To remedy these problems, Giuseppe Mercalli published a revised intensity scale in 1902. The Mercalli scale, as it came to be called, added two levels to the high end of the de Rossi-Forel scale, making its highest level 12. It also was rewritten to make it more globally applicable. With later modifications by Charles Francis Richter, the Mercalli scale is still in use.

Intensity measurements, even though they are somewhat subjective, are very useful in mapping the extent of earthquake effects. Nevertheless, intensity measurements are still not ideal measuring techniques. Intensity varies from place to place and is strongly influenced by local geologic factors, and different observers frequently report different intensities. There is a need for an objective method of describing the strength of earthquakes with a single measurement.

Richter devised an objective technique for determining the power of earthquakes in the early 1930’s at the California Institute of Technology in Pasadena, California. The eventual usefulness of what became known as the Richter scale was completely unforeseen at first. In 1931, the California Institute of Technology was preparing to issue a catalog of all earthquakes detected by its seismographs in the preceding three years. Several hundred earthquakes were listed, most of which had not been felt by humans, only detected by instruments. Richter was concerned about possible misinterpretations of the listings. With no indication of the strength of the earthquakes, the public might overestimate the risk of earthquakes in areas where seismographs were numerous and underestimate the risk in areas where seismographs were few. To remedy the lack of a measuring method, Richter devised the scale that now bears his name.

Richter defined the magnitude of an earthquake as the logarithm of the height of its seismograph trace in microns (thousandths of a millimeter), as recorded on a standard instrument. Thus an earthquake that produces a trace one millimeter (1,000 microns) high would be magnitude 3, one that produces a trace 1 centimeter high (10,000 microns) would be magnitude 4, and so on. These measurements were defined for a standard seismograph magnifying ground motion twenty-eight hundred times and located 100 kilometers (about 62 miles) from the earthquake. The magnification of the instrument means that the actual ground motion caused by a magnitude 3 earthquake 100 kilometers away is not 1 millimeter but only 1/2,800 millimeter, or only about three thousand times the diameter of an atom. By comparing records for earthquakes recorded on different devices at different distances, Richter was able to create conversion tables for measuring magnitudes for any instrument at any distance. He also set up the scale so that any event likely to be felt by humans would have a positive magnitude, because scales with zero and negative numbers tend to be confusing to most people.

Richter had hoped to create a rough means of separating small, medium, and large earthquakes, but he found that his scale was capable of making much finer distinctions. Most magnitude estimates made with a variety of instruments at various distances from earthquakes agreed to within a few tenths of a magnitude. Richter formally published a description of his scale in January, 1935, in the Bulletin of the Seismological Society of America. Other systems of estimating magnitude had been attempted, notably that of pioneering Japanese seismologist Kiyoo Wadati, published in 1931, but Richter’s system proved to be the most workable scale yet devised and rapidly became the standard.

Over the next few years, the scale was refined. One critical refinement was in the way seismic recordings were converted into magnitude. Earthquakes produce many types of seismic waves, but it was not known which type should be the standard for magnitude. So-called surface waves travel along the surface of the earth. It is these waves that produce most of the damage in large earthquakes; therefore, it seems logical to let these waves be the standard. On the other hand, earthquakes deep within the earth produce few surface waves. Magnitudes based on surface waves would be too small for these earthquakes. Deep earthquakes produce mostly waves that travel through the solid body of the earth, or so-called body waves. Actually, two scales are needed: one based on surface waves and one on body waves. Richter and his colleague Beno Gutenberg developed scales for the two different types of waves that are still in use. Magnitudes estimated from surface waves are symbolized by a capital M, and those based on body waves are denoted by lowercase m.

From a knowledge of movements of the earth associated with seismic waves, Richter and Gutenberg succeeded in relating the energy output of an earthquake with its magnitude. Each increase of one Richter magnitude corresponds to about a thirtyfold increase in energy. A magnitude 6 earthquake releases about as much energy as a one-megaton nuclear explosion; a magnitude 0 earthquake releases about as much energy as a small car dropped off a two-story building.

An additional refinement to the Richter scale was developed in the 1970’s. Extremely large earthquakes release their energy over time spans as long as several minutes and over a fault break of hundreds of kilometers. The highest seismograph trace for the earthquake, however, measures the energy received at only one instant. It is also possible to estimate energy released from the length of the fault rupture and the amount of fault displacement, and by this measure, conventional magnitudes for the largest earthquakes are too small. A magnitude corrected for the long duration and great spatial extent of the largest earthquakes is called a seismic-moment magnitude. Japanese seismologist Hiroo Kanamori devised a seismic-moment magnitude scale during the 1970’s; on this scale, unlike the conventional Richter scale, some of the greatest earthquakes exceed magnitude 9.

Significance

The Richter scale is a good example of an accidental scientific discovery. Richter’s original intent had been to develop a scale for very rough measurements, but the scale proved to be useful on a completely unforeseen scale. Richter scrupulously avoided using the term “Richter scale” in his writings and went to some length to give credit to other researchers who also had similar ideas. The Richter scale has now become firmly established in popular and professional usage.

The Richter scale had been in use among scientists for about fifteen years before Richter scale readings began to be widely quoted by the press in the 1950’s. Just as scientists had, the press found the scale a useful tool for defining the strength of earthquakes. Initially, there was much confusion among laypersons over the difference between magnitude and intensity. Also, the logarithmic nature of the scale, clear enough to scientists, was widely misunderstood. It was not until about 1970 that the press generally stopped describing the Richter scale as running from 1 to 10.

The scientific utility of the Richter scale stems from its logarithmic nature. Wave phenomena occur over a great range of intensities and are best described by logarithmic scales; other examples include the speed and shutter settings on cameras (each step doubles or halves the original amount of light) and the decibel scale for sound. Virtually all the important physical quantities associated with waves are proportional to the amplitude of the wave raised to some power; such relationships are very simple to express in logarithmic terms. Because the magnitude of an earthquake is defined as the logarithm of its ground motion, many of the physical dimensions of earthquakes, such as their energy release, are directly and simply related to magnitude. Also, it is simple to develop equations to relate ground motion, magnitude, and distance from an earthquake.

The Richter scale made it possible for scientists to compare the energy outputs of earthquakes in a quantitative way and to derive new understandings of earthquake phenomena. For example, most of the energy released by earthquakes is released by the few most powerful ones. Also, earthquakes are more powerful in some geologic settings than in others. The greatest earthquakes (above magnitude 6.5) are associated with faults on or bordering the continents, whereas earthquakes along the midocean ridges rarely exceed magnitude 6.5. Deep earthquakes are rarely as strong as magnitude 7. These differences provide scientists with important insights into the forces at work in the earth.

Bibliography

Bolt, Bruce A. Earthquakes. 5th ed. San Francisco: W. H. Freeman, 2003. Presents a generally nontechnical account of earthquakes, their effects, and methods of studying them. Includes a good description of magnitude and intensity.

Boore, David M. “Motion of the Ground in Earthquakes.” Scientific American 237 (December, 1977): 68-87. Describes the types of waves generated by earthquakes and the instrumental techniques used in measuring them. Includes a description of the seismic-moment magnitude scale.

Gutenberg, Beno, and Charles F. Richter. Seismicity of the Earth. 2d ed. New York: Hafner, 1965. A pioneering study of earthquake locations and magnitudes around the world. Includes extensive tables of large earthquakes.

Hough, Susan Elizabeth. Earthshaking Science: What We Know (and Don’t Know) About Earthquakes. Princeton, N.J.: Princeton University Press, 2002. Presents information on earthquake science for lay readers, including discussion of earthquake measurement and the Richter scale.

Press, Frank. “Earthquake Prediction.” Scientific American 232 (May, 1975): 14-20. Interesting summary of research efforts and seismic clues used in attempts to predict earthquakes. Somewhat dated, in that short-term prediction of earthquakes has turned out to be much more difficult than was believed in the mid-1970’s.

Richter, Charles F. Elementary Seismology. San Francisco: W. H. Freeman, 1958. College-level textbook on seismology is dated but valuable for Richter’s personal description of the origin of the Richter scale.

U.S. Geological Survey. Earthquakes and Volcanoes 21 (January/February, 1989). Special issue devoted to the measurement of earthquakes and an excellent short survey of seismology. Includes maps of seismic activity, locations of seismographs, and explanations of how seismographs work.

Wesson, Robert L. “Predicting the Next Great Earthquake in California.” Scientific American 252 (February, 1985): 35-43. Presents a summary of modern field and theoretical techniques used in estimating earthquake risk and predicting earthquake magnitudes. Of particular interest is the fact that the article accurately predicts the location and approximate magnitude of the 1989 Loma Prieta earthquake.