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How quakes measure up

DigitalGlobe
The WorldView-2 satellite photo at left shows an oil refinery near Concepcion, Chile,
on Feb. 27 after the area was hit by an 8.8 earthquake. A pre-quake image of the
same area, seen at right, was taken by the QuickBird satellite on Feb. 21.


How did the magnitude-8.8 earthquake in Chile compare with January's 7.0 earthquake in Haiti? Was Chile's quake 60 times stronger? 600 times stronger? And exactly how much time will we be losing every day because the 8.8 quake affected Earth's rotation?

If it's just a question of running the numbers, scientists can provide extremely precise answers. But the realities of earth science aren't always easy to pin down on a yardstick or a stopwatch.

For example, consider the question about seismic strength: If the magnitudes of two seismic events are known, comparing the energy release is so simple a calculator could do it. The U.S. Geological Survey's "How Much Bigger" calculator shows that Chile's quake was 501.187 times stronger than Haiti's in terms of energy release.ŠHowever, if you're interested in the magnitude - that is, the rise and fall as measured by a seismograph - the waves produced by Chile's quakeŠwere 63.095 bigger in amplitude thanŠHaiti's.

The logarithmic scale used to measure seismic magnitude is set up so that a two-number step represents a thousandfold increase in the release of energy from the shock.

As I pointed out a couple of years ago, energy release is not the only factor that determines how devastating a quake will be. The impact of a quake also depends on the depth of the event and the character of the soil around the epicenter: Generally speaking, a shallow quake causes more damage than a deep quake, and structures built on soft soil will sustain more damage than structures founded on solid rock.

Another factor has to do with the proximity of the epicenter to populated areas andŠthe construction standards used in those areas. On all those counts, the Chileans had it better off than the Haitians, as this Associated PressŠstory explains. Then there's the tsunami factor. On that count, the Chileans weren't so lucky. The coastal town of Constitucion was hit by three giant waves that battered buildings and caused many deaths.

Seeing the quake zone from space
Although the impact of Saturday'sŠearthquake on Chile wasn't nearly as horrific as the impact of Haiti's quake, the signs of damage can be seen from space: DigitalGlobe released before-and-after images of an oil refinery near Concepcion,Šand the post-quake picture clearly shows leaking oil and black smoke. Reuters reports that Chile's biggest oil refinery could be down for a month.

The International Space Station's crew membersŠalso took pictures of the quake zone from orbit, and one snapshot shows collapsed sections of a bridge of the Bio-Bio River and dark smoke rising from the area surrounding an oil refinery. That refinery looks identical to theŠoneŠin the before-and-after images fromŠDigitalGlobe's QuickBird and WorldView-2 satellites.

Japanese astronaut Soichi Noguchi is continuing to send down images of Chile from the space station via Twitpic. One picture shows an Andean volcanoŠeast ofŠConcepcion. "YES, it is erupting!" Noguchi writes. "Can you see the plume?"

The shorter and longer day
A researcher at NASA's Jet Propulsion Laboratory said the quake was so powerful that it gave a kick to Earth's figure axis. The computer model used by geophysicist Richard Gross suggests that the quake shortened the length of an Earth day by 1.26 microseconds (or millionths of a second). "This change should be permanent," Gross told Space.com.

Changing the length of a day may sound like an impressive feat, but in fact thatŠmagnitude of change is so small that it's lost over time (so to speak). According to the computer model, the Sumatran earthquake of 2004ŠshortenedŠthe length of a day more than four times asŠmuch,ŠbyŠ6.8 microseconds. Seasonal changes in ocean currents and the atmosphere have a still more significant effect, moving the length of a day back and forth over a range of 1 millisecond, or 1,000 microseconds.

In the long run, all those changes will be canceled out by the gradual lengthening of Earth's day that is caused by the transfer of angular momentum to the moon. Apollo-era experiments indicated that the moon is gradually receding from Earth, at a rate of about 3.8 centimeters per year, and that implies that Earth's day should be getting 17 microseconds longer every year.

By that measure, the day was 15 minutes shorter when dinosaurs walked the earth. Geological studies of rock dating back to the Proterozoic era, 900 million years ago, suggest that back thenŠEarth's day lastedŠmerely 18 hours (rather than the current 24 hours) and that it took 481 of those days (rather than the current 365 days or so) for Earth to make its yearly orbital circuit.

If the day-lengthening trendŠcontinues far, far into the future, the moon and Earth wouldŠeventually go into mutual tidal lock. The moon is already locked in the sense that it presents the same face (more or less) to Earth. In 50 billion years or so, the moon would appear to be stationary in Earth's sky - and would no longer be seen from half of Earth's surface. Earth's day would last 47 times longer than it currently does.

However, we'll probably never make it to that stage. In just a few billion years, the sun is expected to puff up into its red-giant phase. Our planet and our moon will be burned to a crisp, and our distant descendants would have to find refuge farther out in the solar system. As I write in my book, "The Case for Pluto," one of those refuges might well be Pluto and its moon Charon, which have been locked together tidally for eons.

Now there's a happy thought for the week ahead.

Speaking of the week ahead ... and speaking of Pluto ... I'll be away from the office for the next week for a book tour that will take me through Iowa, Wisconsin and Illinois. Postings to the Weblog will not be as regular as usual, depending on news developments, time and bandwidth. But if you're around Mount Vernon, Iowa, at 7:30 p.m. on March 6 ... or Madison, Wis., at 7 p.m. on March 10 ... or Streator, Ill., at 7 p.m. on March 12 ... please look me up and say hello. I'll even sign a book for you.


Join the Cosmic Log corps by signing up as my Facebook friend or hooking up on Twitter. And if you really want to be friendly, ask me about "The Case for Pluto."