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Quakes by the numbers

— You can't always judge a quake by its numbers. Two of the magnitude-7-plus quakes recorded in the past six months illustrate the complexities behind scientific statistics. As terrible as it was, last November's magnitude-7.7 quake in Chile ended up killing two people. In contrast, the estimated death toll from today's magnitude-7.9 quake in China, which doesn't sound as if it should be that much stronger, is at 8,500 and rapidly rising.

Although magnitude figures are an easy way to quantify the power of a quake in a headline, it takes something more to tell the whole story of an earthquake's strength.

On one level, the magnitude scale measures what happens to the needle on a seismometer. When Ohio seismologist Charles Richter came up with his scale in the 1930s, the system was set up to standardize the measurement of ground displacement due to seismic waves. He even helped design the seismometers initially used for calculating the scale he invented.

That original scale has been tweaked through the decades, and nowadays calling it the "Richter scale" is an anachronism. The most common measure is known simply as the moment magnitude scale. It uses the same type of logarithmic scaling system that Richter applied to his original scale, even though the seismologist once said "logarithmic plots are a device of the devil."

The scale is set up so that two whole-number steps represent a thousandfold increase in energy released by a quake. Energy values rise in a geometric progression rather than a mere linear progression. A magnitude-7 quake releases 31.6 times more energy than a magnitude-6 quake, and 1,000 times as much as a magnitude-5.

So even though 7.9 doesn't sound like all that much more than 7.7, the bigger quake is almost twice as energetic as the smaller one.

A quake's energy is just one part of the equation, however. The damage done depends not only on that raw energy, but also how it propagates through the soil - and most importantly, the character of the region around the epicenter. The measure of a quake's effect is known as its seismic intensity, and quakes that have a similar magnitude could have widely varying intensity.

If a 7.9 quake occurs in a wasteland and nobody's around to feel it, does it really make an impact? Today's quake didn't happen in a wasteland, but was centered just 55 miles away from Chengdu, the capital of Sichuan province, with an estimated urban population of around 4 million (plus 6 million more people in the surrounding area).

In the past, the modified Mercalli scale has been used as an observational measure of an earthquake's intensity, based on a Roman-numeral scale ranging from I to XII. A Category III quake would merely set some hanging objects swaying, while a Category IX shaker would do heavy damage and provoke general panic. Nowadays, peak ground acceleration serves as a more objective measure of intensity, and is often written into the building codes.

How do you get the full story behind the science of killer earthquakes? You can start with our interactive graphics on earthquake science in general and the Chengdu quake in particular. My colleague Will Femia provides a roundup of earthquake coverage over at his Clicked blog.

The U.S. Geological Survey's Earthquake Hazards Program summarizes the Sichuan situation and presents a podcast as well as a fantastic poster explaining the event.

In the next day or so, we may see before-and-after satellite imagery of the earthquake zone, from outfits such as DigitalGlobe, NASA's MODIS team and GeoEye. The U.S. government's National Geospatial-Intelligence Agency is already checking its satellite data, Reuters reports.

Geoeye spokesman Mark Brender cautioned that the imagery has to be precisely targeted to spot the damage to buildings. "We need to know the 'big where' - latitude and longitude coordinates," Brender told me.

While we're on the subject, I should mention that orbiting satellites have been doing a great job of monitoring this month's other dramatic outbursts from Mother Nature. DigitalGlobe, for example, has put together a chilling analysis of before-and-after satellite views from Myanmar's cyclone-hit areas. Meanwhile, today's "Image of the Day" from the MODIS team shows the long plume of ash spewing forth from Chile's Chaiten volcano.

Have you come across other eye-in-the-sky views documenting the latest examples of nature's wrath? Feel free to offer your Internet links, as well as your observations on the seismic numbers game.