31 March 2010
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.
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