Magnitude vs. Intensity
Intensity scales measure the amount of shaking at a particular location. Therefore, the intensity of an earthquake will vary depending on where you are. Sometimes earthquakes are referred to by the maximum intensity they produce.
Magnitude scales, like the Richter magnitude and moment magnitude, measure the size of the earthquake at its source. Thus, they do not depend on where the measurement of the earthquake is made. On the Richter scale, an increase of one unit of magnitude (for example, from 4.6 to 5.6) represents a 10-fold increase in wave amplitude on a seismogram or approximately a 30-fold increase in the energy released. Except in special circumstances, earthquakes below magnitude 2.5 are not generally felt by humans.
Often, several slightly different magnitudes are reported for an earthquake. This happens because different measurement procedures will often give slightly different magnitudes for the same earthquake.
The ShakeMap for the 1994 magnitude 6.7 Northridge, CA earthquake shows the epicenter at the location of the green star. The intensity of shaking created by the earthquake is shown by the different color gradients on the map – red is more intense shaking, yellow and blue are less intense. The magnitude of the earthquake is 6.7 no matter where you are, but the intensities vary by location.