The Seattle earthquake registered 6.8 on the … well, not on the Richter scale, which the New York Times reported has been displaced. Is the Richter scale obsolete, and what is used instead?
While the Richter scale is not obsolete, the universal measurement today is the moment magnitude scale. The Richter scale was developed by seismologist Charles Richter (1900-1985) in the 1930s to bring consistent, objective criteria to evaluating the size of earthquakes. Richter, who worked in southern California, using data from seismographs–which measure earth movement–devised a method to calculate where an earthquake began, or its epicenter, and its magnitude. The way the scale works, each whole number increase, say from 4.8 to 5.8 to 6.8, represents a 10-fold increase in the size of the ground motion. As effective a tool as the Richter scale is, it has some drawbacks. For one, it is a relative measure; that is, it was developed to compare the size of one earthquake to another. But scientists wanted something that measured an earthquake not just in comparative terms, but in absolute terms. They wanted to get a physical snapshot of how much energy was released when a fault slipped. For another, although the Richter scale was revised to be used to measure earthquakes in other parts of the world, it had limitations since it was based on conditions in California. Scientists wanted a measurement that was universally applicable.
With the creation of more sophisticated seismology equipment in the 1970s, scientists could determine the actual area where the rupture in the fault took place and measure how much energy was released there during the earthquake, a calculation they call the seismic moment. Because the Richter scale had become so commonly understood, they devised a method to convert the information from the seismic moment into a scale comparable to Richter’s, or what’s now known as the moment magnitude scale. It is also possible to compare earthquakes using the new scale to those measured in the past on the Richter scale. Although the two scales may use different methods, they should end up producing virtually the same number.Many news organizations have dropped the “Richter” but not adopted the phrase “moment magnitude” scale. Instead the common shorthand is to describe an earthquake as being a “magnitude [fill in the number].”
Explainer thanks Paul Bodin and Gary Patterson of the University of Memphis. For more on earthquakes and their measurement, click here.