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1811-12, New Madrid, Mo. 8.6, 8.5, 8.7 on the Richter scale. 1886, Charleston, S.C. 7.8 on the Richter scale. 1906, San Francisco. 8.3 on the Richter scale.

1971, San Fernando Valley. 6.5 on the Richter scale.

The Richter scale, which measures the magnitude of earthquakes, ranges from 0 to 8.9. Each whole-number increase in the scale represents a tenfold increase in a quake's magnitude. Thus, the magnitude of an earthquae measuring 8.0 on the scale is 10,000 times greater than one measuring 4.0 on the scale. Because each whole-number increase also represents roughly 60 times more energy released, a quake with a magnitude of 8.0 releases 10 million times the energy of a tremor measuring 4.0.