Two hundred years ago, Missouri was rocked by an earthquake so severe it made the Mississippi River flow backward and set off church bells in Boston more than 1,000 miles away.
These details help convey the staggering scale and reach of what was a remarkable geologic event. Today, along with those accounts, we would also get a number: the magnitude of the earthquake. But that number is based on a logarithmic scale, and can be hard to grasp.
Earthquakes aren't measured linearly, but in orders of magnitude. Which means a 6.1 magnitude quake like the one that shook Northern California over the weekend is about twice as big as the 5.8 earthquake that rattled Washington, D.C., in 2011—and nearly three times as strong in terms of the amount of energy it released. Some more context: The 7.0 earthquake that devastated Haiti in 2010 was eight times bigger than the Northern California quake, and released 22 times more energy. None of this jibes with the linear way people use numbers for most measurements in daily life.
Here's how Jesse Singal explained it over at New York magazine earlier this year:
On a linear scale, we know that four is twice as big as two and eight twice as big as four. This is what a casual observer of earthquake magnitude scales would expect: that an earthquake of 6.0 packs twice the punch of a 3.0. But no! In reality, a 6.0 quake releases 31,622.776 times as much energy as a 3.0 quake. And a 7.0 releases 31.622 times as much energy as a 6.0.
So why do geologists talk about earthquakes this way? Why not use a scale that operates more like the ones used to measure weight, or length, or temperature, or any number of other natural phenomena?