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CK-12 Foundation
Ck 12: Earth Science: Scales That Represent Earthquake Magnitude
[Free Registration/Login may be required to access all resource tools.] Covers the scales scientists use to measure the size and intensity of earthquakes.
US Geological Survey
U.s. Geological Survey: How Much Bigger..? Calculator
A calculator to show the difference in magnitude between earthquakes. Understand that the Richter scale used to measure earthquakes is base-10 logarithmic scale.