If we discuss our own size, we can talk about height, weight, frame size, or musculature. We can even combine height and weight measurements into a body mass index, which tries to describe the relationship of height and weight. These various ways of measuring size each have utility. However, they each differ in the specific quality which they are measuring. They also differ in sophistication.
There are different ways of describing the strength of an earthquake as well. The Mercalli scale essentially tries to describe the amount of destructive effect an earthquake shows at a given location. This effect often varies with the distance from the epicenter though, along with the type of soil and the type of structures affected by the shaking. Therefore, with this scale, it is hard to compare the actual energy of different quakes, though the destructiveness of various events can be compared.
The Richter Scale is a more direct measurement of the force of a temblor, an earthquake. It comes closer to measuring the actual force of a quake. It measures the waves earthquakes set off, by measuring their effect on a sensitive instrument that they impact. This is a better measurement, in some ways, but it is not perfect.
When the Richter scale assigns the magnitude of an earthquake, it is describing the maximum single ground wave amplitude projected back to the earthquake focus. It does allow for the way the signal is weakened by traveling through the earth. However, it gives a peak value, and so it does not measure the average mechanical power of an earthquake anymore than the high tide mark measures the force of a set of waves upon a beach.
Scientists like to use a measurement that more accurately reflects the overall size of the force exerted by the earthquake. Called the Seismic Moment, it measures the leverage exerted on faults which produces movement during an earthquake.
Moment is proportional to the slip (the amount of movement) on the fault, times the area of the part that moves. Geologists can estimate the amount of slip, lateral movement, from a seismogram, which is the record of an earthquake traced by a seismograph. They can measure ground shift by geodetic means, that is, by using devices that measure and locate ground. In other words, this measurement describes the stuff that got moved, times the distance that it got moved.
The seismic moment thus describes the force of an earthquake by directly measuring the work the earthquake did. Indirectly, moment reflects energy release. This is then converted into a number that can be compared to any earthquake, using a standard formula. Once converted, it is called the moment magnitude. This is a number that is easily compared to other the measurements of other earthquakes, and is valid over all sizes of temblors.
Kei Aki was first to measure seismic moment, and Thomas Hanks and Hiroo Kanamori developed the magnitude scale based upon it. These are scientists whose names are not as generally known as Mr. Richter’s is, but theirs is the measurement generally used by seismologists.
This is not the only measurement in use by seismologists, but it is an understandable one. Just as the Richter Scale replaced the Mercalli in common use, it is quite likely that the Moment Magnitude Scale will someday supplement the Richter as the public becomes more sophisticated about earthquake measurement.