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 the 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 InterviewThe 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.