Richter versus moment magnitude

When a news report says an earthquake measured a certain number on the Richter scale, it is almost always wrong about the scale, if not the number. For any sizeable earthquake, seismologists today report moment magnitude, written Mw, not the Richter magnitude that Charles Richter defined in 1935. The two usually agree for small and moderate events, which is why the old name has stuck, but they are built on different physics, and the difference matters most for exactly the earthquakes people care about most: the large ones.

What the Richter scale actually was

Charles Richter, working in Southern California, created a local magnitude scale, now written ML, to compare earthquakes recorded on a specific instrument, the Wood-Anderson seismograph. It took the largest wiggle on the seismogram, corrected for how far away the earthquake was, and turned that into a single number. It was a genuine advance, giving the first objective measure of earthquake size, and it worked well for the moderate, nearby California earthquakes it was designed for.

Its limitation is that it measures the amplitude of shaking on an instrument, not the true size of the rupture. For large earthquakes that turns out to be a serious problem.

The saturation problem

Every amplitude-based magnitude scale runs into a ceiling called saturation. Beyond a certain size, the measured wiggles stop growing even as the earthquakes keep getting bigger, because the instruments and wave types being measured cannot capture the very long, slow motions that giant ruptures produce. Local Richter magnitude saturates around magnitude 6.5 to 7. The body-wave scale tends to flatten near magnitude 6 to 6.5, and even the surface-wave scale saturates for the greatest earthquakes above magnitude 8. A scale that reports the same number for a magnitude 8 and a magnitude 9 is hiding a difference of roughly 32 times in energy.

How moment magnitude fixes it

In 1979 Hiroo Kanamori and Thomas Hanks introduced the moment magnitude scale, Mw, built on a physical quantity called the seismic moment. Seismic moment is the rigidity of the rock multiplied by the area of the fault that slipped and by the average distance it slipped. In other words, it measures how much rock moved, how far, over how large an area, rather than how big a wiggle reached a particular instrument. Because it is tied to the physical size of the rupture, it does not saturate. It was deliberately calibrated to line up with the Richter scale for moderate earthquakes, so the familiar numbers still mean roughly what people expect.

Scale	Symbol	What it measures	Works best for	Saturation
Local (Richter)	ML	Peak amplitude on a seismograph, distance-corrected	Small to moderate, nearby quakes	Around magnitude 6.5 to 7
Body wave	mb	Amplitude of fast body waves through the Earth	Distant moderate quakes	Around magnitude 6 to 6.5
Surface wave	Ms	Amplitude of slower surface waves	Large shallow quakes	Around magnitude 8
Moment	Mw	Seismic moment: rock rigidity, fault area, and slip	All sizes, especially the largest	Does not saturate

Each step is bigger than it looks

Magnitude scales are logarithmic, so the spacing between numbers hides how large the real differences are. Going up one whole magnitude multiplies the shaking amplitude recorded on a seismogram by about 10, and it multiplies the energy released by about 32. A magnitude 7 is not a little larger than a magnitude 6; it releases roughly 32 times more energy. A magnitude 8 releases about 1,000 times more than a magnitude 6. This is why the difference between the numbers on the news can be so much larger than it sounds, and why saturation, which quietly compresses the biggest events, was worth solving.

Why you still hear the word Richter

Old habits and a good story keep the name alive. For the everyday earthquakes most people feel, the Richter and moment values are close enough that the number is not misleading, and the public learned the term decades ago. But when you see an official magnitude from a monitoring agency for a large earthquake, it is a moment magnitude. You can watch those values update in real time on the live globe or see recent events near you. For a sense of what those numbers actually feel like on the ground, see the guide to what different magnitudes feel like, and for how the shaking itself is rated, the Modified Mercalli intensity scale.