`Moment' a Better Measurement Than Richter Scale, Experts Say

Every earthquake is defined by a numerical "magnitude on the Richter scale." It is the first question asked about an earthquake, and it is the first number that the press reports.

But ask real seismologists if they have deep conversations revolving around the Richter scale number of the earthquake, and most likely, they will say no.

Real seismologists like McKay Professor of Engineering Sciences and Geophysics James R. Rice and Professor of Geology Adam M. Dziewonski use the term, "moment of the earthquake," and explain it as follows.

"The moment of the earthquake" is the product of the ruptured area multiplied by the average slip multiplied by the sheer modulus.

Think of a fault as a surface with materials on both sides. During an earthquake, the materials move against each other.

The offset of the fault is called the slip. In a medium-sized earthquake measuring six on the Richter scale, the average slip would be about half a meter.

The slip varies in places along the fault, and so the average of the slip is used in computing the "moment."

Any place along the fault where the slip is not zero is considered a ruptured area.

The sheer modulus measures the rigidity of the medium. The less rigid the medium, the lower the sheer modulus. Generally speaking, this value stays constant.

In general, the "moment" is more accurate than the Richter scale measurement. The Richter magnitude estimate may be up to half a unit off from the "moment."

Another reason seismologists use the "moment" is because it is easier to compare the magnitudes of different earthquakes.

The Richter scale is logarithmic, so a magnitude three earthquake is not half as strong as a magnitude six quake.

The ruptured area of a magnitude six quake is 10 kilometers by 10 kilometers. The ruptured area of a magnitude three quake is 10 meters by 10 meters. The same is true for the average slip value.

When speaking in terms of the "moment" it is common to say that one earthquake is a thousand times stronger than another. Such a comparison is not as obvious using the Richter scale.