# Charles Richter, Inventor of the Richter Magnitude Scale

## Comparing the sizes of earthquakes

Seismic waves are the vibrations from earthquakes that travel through the Earth; they are recorded on instruments called seismographs. Seismographs record a zig-zag trace that shows the varying amplitude of ground oscillations beneath the instrument. Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world. The time, locations, and magnitude of an earthquake can be determined from the data recorded by seismograph stations.

The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included for the variation in the distance between the various seismographs and the epicenter of the earthquakes. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value.

At first, the Richter Scale could be applied only to the records from instruments of identical manufacture. Now, instruments are carefully calibrated with respect to each other. Thus, magnitude can be computed from the record of any calibrated seismograph.

Earthquakes with magnitude of about 2.0 or less are usually called microearthquakes; they are not commonly felt by people and are generally recorded only on local seismographs. Events with magnitudes of about 4.5 or greater—there are several thousand such shocks annually—are strong enough to be recorded by sensitive seismographs all over the world. Great earthquakes, such as the 1964 Good Friday earthquake in Alaska, have magnitudes of 8.0 or higher. On average, one earthquake of such size occurs somewhere in the world each year. The Richter Scale has no upper limit. Recently, another scale called the moment magnitude scale has been devised for more precise study of great earthquakes.

The Richter Scale is not used to express damage. An earthquake in a densely populated area which results in many deaths and considerable damage may have the same magnitude as a shock in a remote area that does nothing more than frighten the wildlife. Large-magnitude earthquakes that occur beneath the oceans may not even be felt by humans.

## NEIS Interview

The following is a transcript of an NEIS interview with Charles Richter:

How did you become interested in seismology?
CHARLES RICHTER: It was really a happy accident. At Caltech, I was working on my Ph.D. in theoretical physics under Dr. Robert Millikan. One day he called me into his office and said that the Seismological Laboratory was looking for a physicist; this was not my line, but was I at all interested? I talked with Harry Wood who was in charge of the lab; and, as a result, I joined his staff in 1927.

What were the origins of the instrumental magnitude scale?
CHARLES RICHTER: When I joined Mr. Wood's staff, I was mainly engaged in the routine work of measuring seismograms and locating earthquakes, so that a catalog could be set up of epicenters and times of occurrence. Incidentally, seismology owes a largely unacknowledged debt to the persistent efforts of Harry O. Wood for bringing about the seismological program in southern California. At the time, Mr. Wood was collaborating with Maxwell Alien on a historical review of earthquakes in California. We were recording on seven widely spaced stations, all with Wood-Anderson torsion seismographs.

What modifications were involved in applying the scale to worldwide earthquakes?
CHARLES RICHTER: You're quite rightly pointing out that the original magnitude scale which I published in 1935 was set up only for southern California and for the particular types of seismographs in use there. Extending the scale to worldwide earthquakes and to recordings on other instruments was begun in 1936 in collaboration with Dr. Gutenberg. This involved using the reported amplitudes of surface waves with periods of about 20 seconds. Incidentally, the usual designation of the magnitude scale to my name does less than justice to the great part that Dr. Gutenberg played in extending the scale to apply to earthquakes in all parts of the world.

Many people have the wrong impression that the Richter magnitude is based on a scale of 10.
CHARLES RICHTER: I repeatedly have to correct this belief. In a sense, magnitude involves steps of 10 because every increase of one magnitude represents a tenfold amplification of the ground motion. But there is no scale of 10 in the sense of an upper limit as there is for intensity scales; indeed, I'm glad to see the press now referring to the open-ended Richter scale. Magnitude numbers simply represent measurement from a seismograph record—logarithmic to be sure but with no implied ceiling. The highest magnitudes assigned so far to actual earthquakes are about 9, but that is a limitation in the Earth, not in the scale.

There is another common misapprehension that the magnitude scale is itself some kind of instrument or apparatus. Visitors will frequently ask to "see the scale." They're disconcerted by being referred to tables and charts that are used for applying the scale to readings taken from the seismograms.

No doubt you are often asked about the difference between magnitude and intensity.
CHARLES RICHTER: That also causes great confusion among the public. I like to use the analogy with radio transmissions. It applies in seismology because seismographs, or the receivers, record the waves of elastic disturbance, or radio waves, that are radiated from the earthquake source, or the broadcasting station. Magnitude can be compared to the power output in kilowatts of a broadcasting station. Local intensity on the Mercalli scale is then comparable to the signal strength on a receiver at a given locality; in effect, the quality of the signal. Intensity like signal strength will generally fall off with distance from the source, although it also depends on the local conditions and the pathway from the source to the point.

There has been interest recently in reassessing what is meant by the "size of an earthquake."
CHARLES RICHTER: Refining is inevitable in science when you have made measurements of a phenomenon for a long period of time. Our original intent was to define magnitude strictly in terms of instrumental observations. If one introduces the concept of "energy of an earthquake" then that is a theoretically derived quantity. If the assumptions used in calculating energy are changed, then this seriously affects the final result, even though the same body of data might be used. So we tried to keep the interpretation of the "size of the earthquake" as closely tied to the actual instrument observations involved as possible. What emerged, of course, was that the magnitude scale presupposed that all earthquakes were alike except for a constant scaling factor. And this proved to be closer to the truth than we expected.

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