Moment Magnitude Scale
Measures Great Earthquakes

Introduction
The moment magnitude scale (MMS) was devised by scientists after the 1960 Chilean earthquake and the 1964 Alaskan earthquake that were so large that the Richter scale did not adequately show the magnitude and size of what happened during these great earthquakes.

Cascadia Subduction Zone, USGS

Great earthquakes occur in Cascadia
Subduction Zone
, USGS


What the moment magnitude measures
The moment magnitude includes the area of the fault's rupture and slippage along the fault. It also includes the size of the seismic waves recorded by seismographs on seismograms. Formerly the United States Geological Survey used the Richter Scale when describing an earthquake's magnitude.

Use for great earthquakes
Today the moment magnitude scale supersedes this scale and is used to estimate the magnitudes for all moderate to large earthquakes. Other scales are used for earthquakes less than 3.5 magnitude, which is the majority of earthquakes felt worldwide. 

Richter scale
The reason the Richter scale was so popular was that the earthquake's magnitude could be determined by looking at the largest wave recorded on a seismogram right after the earthquake. News stories about the size of the earthquake could then be quickly given to the public on the radio and television stations.

Comparing magnitudes of two scales
After the 1960 Chile earthquake the Richter scale registered the earthquake size at magnitude 8.5. Scientists using the MMS revised the earthquakes size up to magnitude 9.5. This is the highest earthquake ever recorded since seismographs were invented.

Comparing other large earthquakes
This is a comparison between the two scales for four other large earthquakes that occurred in the past.

  • New Madrid, MO 1812 - Richter scale 8.7 -- MMS 8.1
  • San Francisco, CA 1906 - Richter scale 8.3 -- MMS 7.7
  • Prince William, AK 1964 - Richter scale 8.4 -- MMS 9.2
  • Northridge, CA 1994 - Richter scale 6.4 -- MMS 6.7


More Earthquake Links

1964 Alaska Earthquake The Great Alaskan Earthquake find out why Good Friday saved many people's lives!

New Madrid Earthquakes The New Madrid earthquakes were a series of three very large earthquakes felt by people over almost 2 million square miles.

Earthquake Waves Earthquake waves are produced by the more than three million earthquakes each year.

1960 Chile Earthquake The 1960 Chile earthquake was a magnitude 9.5 the largest earthquake ever recorded!


Earthquake Faults Find out more about fault that go up, down and sideways!

Moment Magnitude Scale Find out why this scale is used to measure great earthquakes.

What Causes Earthquakes. Find out what causes earthquakes and other fascinating facts about when our planet shakes, rattles, and rolls.

Kids Fun Science The links on our home page include information about volcanoes, science activities, plate tectonics, the rock cycle and much more.