"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
Number that characterizes the relative size of an earthquake
"Richter-type," or "ground-motion," magnitude based on measurement of the maximum motion recorded by a seismograph
Three types of magnitude scales:
local magnitude (ML), commonly referred to as "Richter magnitude,"
surface-wave magnitude (Ms)
body-wave magnitude (Mb),
Ground-motion based magnitudes (ML, Ms, Mb) have limited range and applicability
Underestimate of largest earthquakes
Moment magnitude (Mw) scale, based on the concept of seismic moment
Uniformly applicable to all sizes of earthquakes but...
More difficult to compute than the other types
All magnitude scales designed to yield approximately the same value for any given earthquake, at least over a certain size range
"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 |