Earthquake Intensity and Magnitude

Intensity: The Modified Mercalli Scale

Magnitude: The Wadati-Richter Scale

"In 1935, Charles Richter developed the local magnitude, ML scale for moderate-size (3 < ML < 7) earthquakes in southern California. The ML scale is often called the “Richter scale” by the press and the public. All of the currently used methods for measuring earthquake magnitude (ML, duration magnitude mD, surface-wave magnitude MS, teleseismic body-wave magnitude mb, moment magnitude M, etc.) yield results that are consistent with ML. In fact, most modern methods for measuring magnitude were designed to be consistent with the Richter scale. There is some confusion, however, about earthquake magnitude, primarily in the media, because seismologists often no longer follow Richter's original methodology. Richter's original methodology is no longer used because it does not give reliable results when applied to M >= 7 earthquakes and it was not designed to use data from earthquakes recorded at epicentral distances greater than about 600 km. It is, therefore, useful to separate the method and the scale in releasing estimates of magnitude to the public." USGS, http://earthquake.usgs.gov/docs/020204mag_policy.html

Moment Magnitude

"The preferred practice is to use M = (log Mo)/1.5-10.7, where Mo is in dyne-cm (dyne-cm=10-7 N-m), the definition given by Hanks and Kanamori in 1979. An alternate form in Hanks and Kanamori’s paper, M=(log Mo-16.1)/1.5, is sometimes used, with resulting confusion. These formulae look as if they should yield the same result, but the latter is equivalent to M = (log Mo)/1.5-10.7333. The resulting round-off error occasionally leads to differences of 0.1 in the estimates of moment magnitude released by different groups. All USGS statements of moment magnitude should use M = (log Mo)/1.5-10.7 for converting from scalar moment Mo to moment magnitude." USGS, http://earthquake.usgs.gov/docs/020204mag_policy.html

Seismic moment of an earthquake is given by

The actual moment magnitude is calculate using

Comparison between the Richter and Moment Magnitude Scales

Earthquake Richter Scale Moment Magnitude
New Madrid, MO, 1812 8.7 8.1
San Francisco, CA 1906 8.3 7.7
Prince William, AK 1964 8.4 9.2
Northridge, CA,1994 6.4 6.7

From UALR, Arkansas Center for Earthquake Education and Technology Transfer (ACEETT), http://quake.ualr.edu/public/moment.htm

Compare the fault area of the magnitude 7.3 (top) with that of the magnitude 5.6 (smallest one near the bottom). From USGS, http://earthquake.usgs.gov/
image_glossary/magnitude.html

Duration Magnitude