I (Charles Richter) 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.
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.
