Explained: Measuring earthquakes
The powerful earthquake that struck Japan in March was a 9.0-magnitude event. But this was not, as some people may assume, as registered on the Richter scale, the famed measuring system dating to the 1930s. Seismologists today do not use the Richter scale as a universal tool for measuring earthquakes, because it does not accurately measure the energy emitted in jolts as big as the one that hit Japan. Instead, scientists use the moment magnitude scale, developed in the 1970s. An earthquake produces many types of waves, which radiate from its epicenter and move with a wide variety of frequencies. Compared to the Richter scale, the moment magnitude scale can account for more types of these waves, and at more frequencies. It is thus better able to estimate the total energy of earthquakes, and can also relate these observations to the physical features of a fault. “The moment magnitude...