Last Friday was the Giant Jump in Great Britain. British schoolchildren were sent out onto playgrounds across the country at 11:00 a.m. and told to jump up and down for a minute, in an effort to make a detectable squiggle on seismometers. This bit of fun launched Science Year, a project of the U.K. Department of Education and Skills intended to boost interest in science among 10- to 19-year-olds.

But if you read the Reuters story about the event, you might wish it were aimed at reporters instead, or even better, at the scientists who talk to them. According to Reuters,

Scientists said a million children with an average weight of 110 pounds jumping 20 times in a minute would release two billion joules of energy and trigger the equivalent of an earthquake measuring three on the Richter scale.

It's hard to imagine professional seismologists making this claim. Where would it have come from?

Richter's original scale calculates a number based on seismic energy. Here's the formula.

M = (log₁₀ E - 4.8) / 1.5

You plug in a number for energy (E) and get out a number for magnitude (M).

"Seismic energy" was whatever the seismometers of Richter's time were capable of measuring. You get a magnitude by measuring the peaks on the chart paper, estimating the amount of energy it would take to cause the peaks, and plugging that into the formula. The energy estimates are based on calibrations of the seismometers in the lab.

If you drop a 1 kilogram mass from a height of half a meter (20 inches, about how high a kid can jump), you release 5 joules. Some of that energy will be carried away by friction, sound and other effects. Say 40% (2 joules) is coupled to the ground to create seismic waves. Multiply by a million kids, 50 kg per kid (pretty heavy kids, but it makes the numbers easy), and 20 jumps, and you get 2 billion  joules. Plug that in, and out comes 3.0 on the Richter scale.

But there are lots of energy releases that seismometers don't measure. Light bulbs use a lot of energy. You could add all of it up and plug it into the Richter formula. That doesn't mean that light bulbs cause earthquakes. An inch of rain weighs 66 million kilograms per square mile and falls from a height thousands of times greater than a kid can jump, but rainfall doesn't cause earthquakes.

No respectable seismologist would conclude that a million jumping kids would produce a magnitude 3.0 earthquake. It takes a reporter to make that leap.

According to the same story,

Early estimates suggested 75,000 tons of energy had been released during the minute of jumping.

"Tons" as a unit of energy refers to tons of TNT, the unit of weapon yield. But the kids certainly didn't release the equivalent of a 75 KT explosion, a million times more energy than the magnitude 3.0 equivalent. There are other, more obscure "ton" units of energy (toe and tce, ton oil equivalent and ton coal equivalent), but they imply an even higher energy release.

So what could the Reuters reporter be talking about?

The weight of the children, apparently. If a million and a half kids actually participated, and their average weight was 50 kg, their total weight would be 75,000 metric tons.

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