Comparing Quakes

FOR EVERY earthquake, there is always a Richter number. In Southern California, Richter magnitudes have become almost as familiar as the Dow Jones average. One feels a bump in the night; one checks the next morning’s newspaper for the Richter.

It is ironic, then, that this scale is also one of the least understood scientific measurements. Invented in 1935 by Dr. Charles F. Richter at Caltech, the scale is based on the lines drawn on graph paper by a seismograph; the Richter magnitude is calculated from the amplitude of waves produced by an earthquake.

The misunderstanding comes from the fact that the Richter system is logarithmic. Thus, a Richter 2 quake is not twice as large as a Richter 1, but 10 times as large. A Richter 3, in turn, is 10 times as large as a 2, or 100 times as large as a 1. In the accompanying fictional scenario, for example, a 7.5 quake is hypothesized for the Newport-Inglewood Fault on the west side of Los Angeles. If the story had supposed a 6.5 event, it would have been only one-tenth as powerful.

The graph at right illustrates the dramatic differences in power between common earthquakes--those in the range of Richter 4 to 6--and the much more rare “great” earthquake of Richter 8 or higher. A great earthquake not only causes more intense shaking near its source; it also spreads the shaking over a much larger area. In one recent example of this, Mexico City was devastated in 1985 by a great earthquake whose source was more than 250 miles away.

Quake Magnitude 1. 1906 San Francisco 8.3 2. 1985 Mexico City 8.1 3. 1952 Kern County 7.7 4. 1980 Eureka 7.0 5. 1983 Coalinga 6.7 6. 1979 Imperial Valley 6.6 7. 1971 San Fernando 6.4 8. 1925 Santa Barbara 6.3 9. 1933 Long Beach 6.3 10. 1986 Palm Springs 6.0 11. 1987 Whittier 5.9

Source: U.S. Geological Survey

Source: Caltech