Why is a magnitude 7 earthquake 1,000 times stronger than a 5?

A magnitude 7 earthquake is 1,000 times stronger than a 5 because magnitude is a logarithmic scale: each step up multiplies, rather than adds. On today's moment magnitude scale, one step means about 32 times more energy released, so two steps mean 32 times 32, roughly 1,000. A 7.0 releases about 32 times the energy of a 6.0 and 1,000 times that of a 5.0. Even a 0.2 bump roughly doubles the energy.

The idea goes back to Charles Richter, who in 1935 published a scale based on the size of the wiggles drawn by a seismograph, corrected for distance. Each whole number meant ten times the wave height. He borrowed the word "magnitude" from his childhood love of astronomy, where it measures the brightness of stars. His colleague Beno Gutenberg, who suggested the logarithmic idea, was left off the name, partly because he disliked giving interviews.

Richter's scale had a flaw: for really big earthquakes it saturates, getting stuck instead of climbing. Giant quakes rupture for longer than the short waves it measured. The 1960 Chilean earthquake was rated 8.5 on an older scale but was closer to 9.6. So in 1979 Thomas Hanks and Hiroo Kanamori defined the moment magnitude scale, based on the physics of the break itself: how large an area of fault slipped, how far, and how stiff the rock was. It is now the standard for large earthquakes, even though news reports still often call it "the Richter scale".