The Modified Mercalli and Richter scales are used to rate and compare the intensity of earthquakes. The Modified Mercalli scale is somewhat subjective, because the apparent intensity of an earthquake depends on how far away from its center the observer is located. Rating intensities from I to XII, it describes and rates earthquakes in terms of human reactions and observations. The Richter scale measures the motion of the ground 100 km (60 mi) from the earthquake’s epicenter, or the location on the earth’s surface directly above where the earthquake occurred. The rating scale is logarithmic; each increase of 1 on the scale represents a tenfold increase in the motion of the ground.
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