Richter scale

The invention:



A scale for measuring the strength of earthquakes

based on their seismograph recordings.



The people behind the invention:



Charles F. Richter (1900-1985), an American seismologist

Beno Gutenberg (1889-1960), a German American seismologist

Kiyoo Wadati (1902- ), a pioneering Japanese seismologist

Giuseppe Mercalli (1850-1914), an Italian physicist,volcanologist, and meteorologist







Earthquake Study by Eyewitness Report



Earthquakes range in strength from barely detectable tremors to

catastrophes that devastate large regions and take hundreds of thousands

of lives. Yet 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 attempt to measure the power of earthquakes was the

development of intensity scales, which relied on damage effects

and reports by witnesses to measure the force of vibration. The

first such scale was devised by geologists 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 it was difficult to apply the scale 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 F. 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 geologic features, 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.





Measuring Earthquakes One Hundred Kilometers Away



An objective technique for determining the power of earthquakes

was devised in the early 1930’s by Richter at the California Institute

of Technology in Pasadena, California. The eventual usefulness of

the scale that came to be called 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, but detected only by instruments.

Richter was concerned about the possible misinterpretations

of the listing. 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. On this scale, earthquake force is expressed

in magnitudes, which in turn are expressed in whole numbers

and decimals. Each increase of one magnitude indicates a tenfold jump

in the earthquake’s force. These measurements were defined for a

standard seismograph located one hundred kilometers fromthe earthquake.

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.





Impact



Richter had hoped to create a rough means of separating small,

medium, and large earthquakes, but he found that the 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 KiyooWadati,

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 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

seemed logical to let these waves be the standard. Earthquakes deep

within the earth, however, produce few surface waves. Magnitudes

based on surface waves would therefore be too small for these earthquakes.

Deep earthquakes produce mostly waves that travel through

the solid body of the earth; these are the so-called body waves.

It became apparent that two scales were 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, which are still in use. Magnitudes estimated from surface

waves are symbolized by a capital M, and those based on body

waves are denoted by a lowercase m.

From a knowledge of Earth movements associated with seismic

waves, Richter and Gutenberg succeeded in defining the energy

output of an earthquake in measurements of magnitude. 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.











Charles F. Richter

Charles Francis Richter was born in Ohio in 1900. After his

mother divorced his father, she moved the family to Los Angles

in 1909. Aprecocious student, Richter entered the University of

Southern California at sixteen and transferred to Stanford University

a year later, majoring in physics. He graduated in 1920

and finished a doctorate in theoretical physics at the California

Institute of Technology in 1928.

While Richter was a graduate student at Caltech, Noble laureate

Robert A. Millikan lured him away from his original interest,

astronomy, to become an assistant at the seismology laboratory.

Richter realized that seismology was then a relatively new

discipline and that he could help it mature. He stayed with it—

and Caltech—for the rest of his university career, retiring as

professor emeritus in 1970. In 1971 he opened a consulting

firm—Lindvall, Richter and Associates—to assess the earthquake

readiness of structures.

Richter published more than two hundred articles about

earthquakes and earthquake engineering and two influential

books, Elementary Seismology and Seismicity of the Earth (with

Beno Gutenberg). These works, together with his teaching,

trained a generation of earthquake researchers and gave them a

basic tool, the Richter scale, to work with. He died in California

in 1985.