What is Earthquake Magnitude?

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences
Boston College

Earthquake magnitude is a concept that can be quite confusing. Although it is well-recognized as a measure of the size of an earthquake, the principles underlying earthquake magnitude are not as simple as one might expect. Here is a short, and hopefully not too technical, explanation of the essential characteristics of earthquake magnitude:

Magnitude is a measure of the size of an earthquake at the location where it occurred. This is not the same thing as the amplitudes of the seismic waves where they are recorded, because (as can be seen in the figures below) the amplitudes of those waves decay with distance as they travel from the epicenter to the station. To estimate the magnitude of the earthquake from seismograms recorded at stations around the world, it is necessary to adjust the amplitude you observed at your station for the amount that the waves decayed as they traveled from the earthquake to your station.

A magnitude scale is a scheme for making these amplitude adjustments to determine a number that represents the size of the earthquake. Since exactly how to make these adjustments is a science in itself, there are many formulas that have been developed by seismologists to make the necessary adjustments. The original magnitude formula was developed by Charles Richter (in 1935), which is why the magnitude of an earthquake is often loosely referred to as the size of the earthquake on the “Richter Scale.”

Since the development of the Richter Scale, there have been many other magnitude formulas developed by other seismologists. This can lead to quite a bit of confusion, but all of these formulas should give roughly the same result. To minimize this confusion, the U.S. Geological Survey (USGS) publishes an “official” magnitude for each earthquake they report. This official magnitude is the USGS’s estimate of the magnitude that is most appropriate to release to the public given all of the complications discussed above. These official magnitudes reported by the USGS are often the values that seismologists are referring to when they discuss a given earthquake publicly. But sometimes a seismologist’s reported magnitude of an earthquake might be that of a seismic observatory or research institution other than the USGS. This sometimes happens when an earthquake occurs in an area where seismologists operate seismographs close to the epicenter, and thus they feel they have a more accurate estimate of the magnitude of that particular earthquake than the magnitude reported by the USGS.