The magnitude of an earthquake measures its size: the energy released at the source. It is worked out from the seismograms. Each earthquake has a single magnitude, written with one decimal, such as 6.3 or 9.1.
示例
Example 20.9 (Magnitude 9 and magnitude 7)
The 2004 earthquake off Sumatra had a magnitude of 9.1. An earthquake of magnitude 7.1 is two steps smaller: it released about , so about 1000 times, less energy.