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Charles Richter in the 1930's. The figures were once based on this scale but now a scale based upon seismic moment has superceded the Richter scale.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.

1. Vibrations felt only by a few people under very special circumstances.
2. Vibrations felt a few people on upper floors of buildings and in position to observe the swinging of suspended objects.
3. Vibrations felt generally by people indoors but may not be any more intense than those due to a passing vehicle.
4. Vibrations felt almost all people indoors and some outdoors. Some objects displaced. Sounds produced in structures. Some vehicles perceptibly rocked.
5. Vibrations felt by everyone. Some objects rocked off of tables and shelves. Some objects overturned.
6. Vibrations and motion felt by everyone. Even heavy objects displaced. Some structural damage such as to plaster or wallboard.
7. Some structures, such as brick chimneys, damaged. Slight damage in other structures such as woodframe buildings.
8. Structural damage to even well-designed structures. Frame structure walls pushed out of shape. Masonry structures destroyed. Heavy objects overturned.
9. Considerable damage to structures, even partial collapse. Some structures shifted off of foundations.
10. Even woodframe structures destroyed. Foundations damaged. Rails bent.
11. Almost all masonry structures destroyed. Woodframe structures generally damaged and some destroyed. Metal structures such as bridges destroyed and rails severly bent.
12. Most structures destroyed. Elevation or subsidence of land forms. Some objects thrown into the air.

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 was reaching a set of seismographs. The Richter scale is not expressed in terms of the seismic wave amplitude. Instead it 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 signficantly from the Richter ratings. For larger scale quakes (7.0 and above) the seismic moment rating is moderately less numerically than the Richter rating.

---

HOME PAGE OF applet-magic HOME PAGE OF Thayer Watkins