An Interview with Charles F. Richter

by
Henry Spall
U.S. Geological Survey, Reston, Va.

Charles F. Richter, renowned seismologist, is a professor emeritus at the California Institute of Technology (Caltech). He is best known to the public for the Richter magnitude scale; but he is equally recognized in the scientific community for many other contributions to seismology including his books Elementary Seismology (1958) and Seismicity of the Earth (coauthored in 1954 with Beno Gutenberg).

HS: How did you become interested in seismology?
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.

HS: What were the origins of the instrumental magnitude scale?
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. I suggested that we might compare earthquakes in terms of the measured amplitudes recorded at these stations, with an appropriate correction for distance.

Wood and I worked together on the latest events, but we found that we could not make satisfactory assumptions for the attenuation with distance. I found a paper by Professor K. Wadati of Japan in which he compared large earthquakes by plotting the maximum ground motion against distance to the epicenter. I tried a similar procedure for our stations, but the range between the largest and smallest magnitudes seemed unmanageably large. Dr. Beno Gutenberg then made the natural suggestion to plot the amplitudes logarithmically. I was lucky because logarithmic plots are a device of the devil. I saw that I could now rank the earthquakes one above the other. Also, quite unexpectedly the attenuation curves were roughly parallel on the plot. By moving them vertically, a representative mean curve could be formed, and individual events were then characterized by individual logarithmic differences from the standard curve. This set of logarithmic differences thus became the numbers on a new instrumental scale. Very perceptively, Mr. Wood insisted that this new quantity should be given a distinctive name to contrast it with the intensity scale. My amateur interest in astronomy brought out the term "magnitude," which is used for the brightness of a star.

HS: What modifications were involved in applying the scale to worldwide earthquakes?
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.

HS: In looking at worldwide earthquakes, did Gutenberg explore other possible magnitude scales?
Richter: Yes, without any significant contribution from me, he later worked out a version of the magnitude scale that used the measured amplitudes and periods of body waves, that is P (primary), S (secondary), and PP (P waves that are reflected at the Earth's outer surface and then continue on as P waves). He was in favor of using this scale in preference to the surface-wave scale. Theoretically, it is better. I had doubts at the time, and I still feel that the body-wave scale is not satisfactory for general use because it gives results comparable with Gutenberg's only if his procedure is closely followed. Experience has shown that misunderstanding and oversimplified misapplications can occur. For instance, magnitude is sometimes assigned on the first few waves of the P group rather than the largest P waves as Gutenberg did. For deep-focus earthquakes, it has been shown that Gutenberg's results were distorted by the comparatively large loss of energy in body waves from shallow earthquakes when they pass out of and back into the crust.

HS: Many people have the wrong impression that "the Richter magnitude is based on a scale of 10."
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.

HS: No doubt you are often asked about the difference between magnitude and intensity.
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.

HS: There has been interest recently in reassessing what is meant by the "size of an earthquake."
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.

The most remarkable feature about the magnitude scale was that it worked at all and that it could be extended on a worldwide basis. It was originally envisaged as a rather rough-and-ready procedure by which we could grade earthquakes. We would have been happy if we could have assigned just three categories, large, medium, and small; the point is, we wanted to avoid personal judgments. It actually turned out to be quite a finely tuned scale.

The work of Dr. Kanamori and others is developing into a more detailed perspective of an individual earthquake. We can now arrive at different values of "size" by varying the quantity being calculated, the type of instrument used, and the kinds of observations made. This is very similar to astronomy where different magnitudes are assigned to the brightness of an astronomical object, depending on the range of wavelengths being measured. For example, some stars put out large amounts of energy in the infrared part of the spectrum, so that this can produce a different relative magnitude rating than using light energy from the middle of the spectrum. This is quite analogous to the investigation of earthquake size that Dr. Kanamori and his colleagues are pursuing.

HS: You are now more involved in assessing seismic risk?
Richter: Yes; a good deal of my consulting work consists of site analysis for proposed developments. The problem of assessing seismic risk starts at the geological-geophysical end and winds up at the engineering end. We have a given site, and we have to ascertain what disturbances will affect the structures on that site as a result of an earthquake. These are usually described in the form of a design earthquake with a time history of the motion and shaking characteristics that are expected. From here, it becomes an engineering problem; the engineer considers the ground motion that will occur and evaluates the requirements of the proposed structure in the light of the local foundation conditions.

The first steps toward earthquake-resistant construction were made by assuming a fixed acceleration due to the earthquake. Emphasis was usually put on the horizontal acceleration factor, for the simple reason that ordinary structures have a built-in safety factor for the vertical component; that is, gravity. As seismologists gained more experience from earthquake records, it became obvious that the problem could not be reduced to a single peak acceleration. In fact, a full frequency of vibrations occurs. The key factor is the length of time involved in shaking; in other words, how long is the acceleration applied.

HS: Should more effort be put into measures that actually reduce the earthquake hazard?
Richter: I usually point out that most loss of life and property has been due to the collapse of antiquated and unsafe structures, mostly of brick and other masonry. In every area of the world where there is earthquake risk, there are still many buildings of this type; it is very frustrating to try to get rid of them. At least there is progress of California toward building new construction according to earthquake-resistant design. We would have less reason to ask for earthquake prediction if this was universal.

HS: I realize your views on prediction. But would you be willing to predict what you see for seismology over the next decade?
Richter: Nothing is less predictable than the development of an active scientific field. In seismology, we have an increasing number of people and resources at work, so that it's entirely imaginable that there will be a breakthrough in some quite unforeseen area. We now have the capability to directly compare the Earth with other bodies in the solar system; for instance, the violently erupting volcanoes on Io (one of Jupiter's moons) must radically affect our whole thinking about geophysical and geochemical process in the interior of the planets and their satellites.

Abridged from the Earthquake Information Bulletin. Vol. 12, No. 1, January - February, 1980.