San José State University
Thayer Watkins
Silicon Valley
& Tornado Alley

Measures of Earthquake Magnitudes

Although there is a ready demand by the general public for figures on the magnitudes of earthquakes there is a misperception on the part of that general public as to the nature and meaning of those magnitude figures. Most believe that the broadcast figures are based on the scale devised by Charles Richter in the 1930's. The figures were once based on that scale but now a scale based upon seismic moment has superceded the Richter scale. The two scales do not differ significantly for moderate size quakes (magnitudes 2 to 6) so the distinction does not matter for the general public. The two scales do however differ for the more severe quakes (7.0 and above).

The first earthquake magnitude scales were simple designations such as major/minor. But soon people wanted a more refined classification scheme. Some observers added a {moderate, median, mean} category. Soon there were multilevel classifications based upon observed damage. One such classification was formulated by Giuseppe Mercalli around 1900 in Italy. It was modified by American seismologists in 1931. This Modified Mercalli Intensity (MMI) scale is given below. To emphasis that it is a qualitative ranking the categories as designated by Roman numerals rather than Arabic numerals.

The U.S. Geological Survey compiles a MMI rating for an earthquake from questionnaires sent to the postmasters in the area affected. This means that the MMI rating comes long after the earthquake has occurred. People wanted an immediate rating of earthquakes and Charles Richter in 1935 gave them one.

Richter's scale is based upon the maximum amplitude of seismic waves reaching a set of seismographs. The Richter scale is not expressed in terms of the seismic wave amplitude itself. Instead the Richter scale is a logarithmic scale so a 6.0 earthquake on the Richter scale involves wave amplitudes ten times that of a 5.0 earthquake. The Richter scale figure is only imperfectly correlated with the damage done by an earthquake. The level of damage done by an earthquake could be called its intensity. To emphasize that his scale does not measure intensity per se Richter adopted a term from astronomy magnitude. In astronomy are star's brightness is given by a logarithmic measure, magnitude. Another concept of earthquake intensity that might be used instead of level of damage is the amount of energy released in an earthquake. Energy release as a measure of earthquake intensity has the virtue that it is, in principle, more objective. Damage from an earthquake depends upon the extent and type of development in an affected area. It also depends upon the human assessment of the severity of the damage. The amount of energy released by an earthquake of Richter magnitude n+1 is about thirty to fifty times larger than the amount released by an earthquake of Richter magnitude n. Again there is only an imperfect correlation between Richter magnitude and energy release.

While the Richter scale was popular with the press and the public, seismologists had some reservations about it and eventually replaced it with another logarithmic measure based upon the seismic moment of an earthquake. The term moment in the name indicates that it involves force acting over a length. When this concept of earthquake magnitude was applied it turned out the ratings for small scale quakes (2.0 to 7.0) did not differ significantly from the Richter ratings. For larger scale quakes (7.0 and above) the seismic moment rating is moderately less numerically than the Richter rating.

The focus on overall magnitude measures leads people to ignore the factor of distance in earthquake damage. A magnitude 5.6 earthquake that is nearby may shake a building more than a 7.1 earthquake that is far away. Below is shown the distance-amplitude relationship for earthquakes.

At a significant distance from the epicenter the amplitude is inversely related to the square of the distance.

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