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Evan Horowitz

Taking the measure of an earthquake

A building damaged in Napa, Calif, damaged by Sunday’s earthquake was the subject of attention Monday.

Robert Galbraith/REUTERS

A building in Napa, Calif., damaged by Sunday's earthquake was the subject of attention Monday.

Saturday, a magnitude 7.8 earthquake struck Nepal, killing hundreds of people, causing widespread damage, and triggering avalanches on Mount Everest.

You probably know that the 7.8 number comes from something called the “Richter scale,” but what does that mean? And how much can it tell us about the impact?

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Considered on their own, those Richter scale numbers can be pretty misleading. For instance, a 7.8 earthquake like the one in Nepal isn’t just a little bigger than a 7.0. It’s 16 times as powerful.

Even so, the consequences of even the biggest quake depend on a range of other factors, like how far it is from major cities, and how close it is to the surface of the earth.

What does the Richter scale measure?

The Richter scale measures the amount of energy released during an earthquake, but it does so indirectly, using a mathematical adjustment called a logarithm (if the last time you heard about logarithms was in high school math class, don’t worry there’s no quiz).

The value of logarithms is that they help solve a basic problem with measuring earthquakes. The difference between small and large earthquakes is enormous, so big that it’s genuinely hard to compare them.

For example, let’s say you want to measure the length of lots of different things. Cats, for instance, and also Shaquille O’Neal, and the Washington monument, and the distance to the moon. If you try to compare these directly using feet (or inches or miles), you find that it’s very difficult to find a usable scale.

Logarithms have the useful feature of making large differences comparable. The trouble, though, is that you end up with a series of numbers that have little intuitive meaning. What does it mean, for instance, to say that Shaquille O’Neal is .85?

The same problem arises for the Richter scale. A 6.0 earthquake isn’t 6.0 of anything. And a 6.1 earthquake isn’t just a little more powerful. It’s 40 percent bigger.

Is there an alternative?

One approach is just to forget the logarithmic adjustment and look at the raw amount of energy released by various earthquakes. What you find is that a 1.0 point increase on the Richter scale translates to a 30-fold increase in the size of an earthquake.

The last big earthquake to shake the ground of Massachusetts was a magnitude 5.8 event that radiated out from Virginia in 2011. It released 31 TeraJoules of energy, which is roughly the same as 30,000 lightning bolts.

One of the biggest recorded earthquakes in Massachusetts occurred in 1755. It was four times as big as the Virginia quake.

The magnitude 7.8 earthquake that just struck Nepal was 500 times more powerful than the 1755 event.

In 1960, an earthquake measuring 9.5 on the Richter scale hit Chile. That’s 350 times bigger than today’s quake in Nepal.

Are bigger quakes necessarily deadlier?

All else being equal, more energy generally means more damage. But of course, all else is not equal. There are a lot of other factors to consider and the quakes that release the most energy aren’t necessarily the deadliest.

1) Nearby populations. Earthquakes centered in populated areas tend to affect more people. Had the 7.8 earthquake hit Delhi, instead of Nepal, the death toll could have been vastly higher.

2) Quality of infrastructure. Wealthy, earthquake-prone cities like Tokyo and San Francisco have building codes specially tailored to withstand such forces. The deadliest and most devastating earthquakes thus tend to afflict poorer areas, like Port-au-Prince in 2010 or Southwestern China in 2008.

3) Depth. Some earthquakes happen deeper underground than others, and the deeper they are the more limited their impact on those of us who live on the surface.

4) Indirect consequences. Hundreds of thousands of people were killed in Indonesia when a 2004 earthquake triggered a massive tsunami. A 1970 earthquake off the coast of Peru triggered landslides that killed tens of thousands.

Are we getting any better at predicting earthquakes?

Despite some growing optimism, scientists mostly lack the ability to effectively predict earthquakes.

It is possible, however, to do a better job explaining the size and power of earthquakes. And that means not just talking about the numbers on the Richter scale, but finding other, more intuitive ways to describe the huge amounts amount of energy being released.

Evan Horowitz digs through data to find information that illuminates the policy issues facing Massachusetts and the U.S. He can be reached at evan.horowitz@globe.com. Follow him on Twitter @GlobeHorowitz
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