Glossary for Recent Earthquakes Pages Recent Central U.S. Earthquakes Glossary of Terms Version Computers automatically update the WWW pages as more reliable information about the earthquake is computed, particularly in the first 10 minutes following the earthquake. The highest version number is always considered authoritative. Magnitude Seismologists indicate the size of an earthquake in units of magnitude. There are many different ways that magnitude is measured from seismograms because each method only works over a limited range of magnitudes and with different types of seismometers. However, all of the methods are designed to agree well over the range of magnitudes where they are reliable.
Earthquake magnitude is a logarithmic measure of earthquake size.
In simple terms, this means that at the same distance from the earthquake, the shaking will be 10 times as large during a magnitude 5 earthquake as during a magnitude 4 earthquake. The total amount of energy released by the earthquake, however, goes up by a factor of 32. The original magnitude relationship defined by Richter and Gutenberg for local earthquakes in 1935. It is based on the maximum amplitude of a seismogram recorded on a Wood-Anderson torsion seismograph. Although these instruments are no longer widely in use, Ml values are calculated using modern instrumentation with appropriate adjustments.