Our small, sensitive planet


In the hours after last month's Sumatra earthquake, an astonishing report began to circulate: So potent was the undersea jolt that it disrupted the planet's rotation, causing the day to shorten ever so slightly.

But did it?

"The answer to this question is a definite yes," write NASA scientists Richard Gross and Benjamin Chao in an article published Tuesday in Eos, the weekly newspaper of the American Geophysical Union. After plugging measurements of the Sumatra quake into a computer model, the scientists calculated that the quake sped up the planet's rotation - thus shortening the length of the day - by as much as 2.68 millionths of a second.

They also determined that the quake nudged the North Pole an inch closer to Northern Japan.

Perhaps the most surprising thing about these numbers is not their impact on daily life - essentially zilch, the scientists acknowledge - but that a single geophysical event like an earthquake can throw off an entire planet.

It turns out earthquakes are just the beginning. Chao and Gross, who have spent years cataloging and quantifying forces that affect the Earth's rotation and orientation, say that forces ranging from seasonal snowfall to the filling of China's Three Gorges Dam to the surging currents of the Earth's oceans exert a subtle, yet often measurable, influence on the twirl and tilt of the planet.

Theoretically, even an event as mundane as "a bus driving around town," they note, can have planetwide effect. "Anything that moves on the Earth will affect it," says Chao, a geophysicist at the Goddard Space Flight Center in Greenbelt.

Some forces are more powerful than others, of course. The Sumatra quake was what geologists call a "megathrust" event, which occurs when one massive crustal plate dives beneath another. In this case, the India plate subducted beneath the overriding Burma plate.

When the crustal plates curled together, they unleashed a lethal tsunami and a huge burst of energy. According to the U.S. Geological Survey, the magnitude 9.0 Sumatra quake was equivalent to the detonation of 75 million tons of TNT, or 23,000 atomic bombs. They also made the planet more compact, and thus caused it to rotate faster.

"Just like a spinning ice skater who pulls her arms in," says Thomas O'Brian, head of the time and frequency division of the National Institute of Standards and Technology in Boulder, Colo.

Gross and Chao may never know if their prediction about the Sumatra quake's impact on the length of the day is correct.

Scientists today measure the Earth's rotation using Global Positioning System satellites and a radio astronomy technique called very long baseline interferometry, or VLBI. However, these techniques are accurate to a mere 20 millionths of a second, not nearly sensitive enough to measure the subtle effect of the Sumatra quake, says Chao. Even if the prediction did turn out to be true, it wouldn't matter much: The quake was small potatoes compared to all the other forces that constantly jostle the planet.

The movement within the Earth's molten core, for example, can torque the planet's rotation by several thousandths of a second.

Weather has a similar influence. A few years ago, Goddard scientists showed that El Nino - a recurring tropical air and water disturbance in the Pacific - could slow the Earth's rotation and shorten the day. The reason: The event causes global air masses to swirl faster. The laws of physics dictate that the planet's rotational speed must slow in response.

As snow blankets the mountains each winter, it shifts enormous weight away from the Earth's core, causing the planet to rotate more slowly. (Think of the spinning ice skater spreading her arms.)

Even the filling of the Three Gorges Dam in China will have an effect: According to Chao's and Gross' calculations, it will lengthen the day by 60 billionths of a second.

All of these influences, however, are dwarfed by the moon's impact on the length of the day.

The moon's gravitational influence is so powerful that it has resulted in a net lengthening of the day over the ages. The effect - known as "tidal braking" - reduces the planet's rotation by a little more than two thousandths of a second per day every century.

It may not sound like much, but consider: Thanks to tidal braking, a clock in 1905 would have read 42 seconds behind a clock today. The phenomenon also explains the occasional need for a "leap second"- to help official atomic clocks keep pace with the planet's slowing rotation.

Chao and Gross, who works at the Jet Propulsion Laboratory in California, say that while their work appears esoteric, it has practical import for NASA.

To know the precise location of all the agency's far-flung spacecraft, it must know the precise location of the antennae it uses to track them. And that, in turn, requires knowing the Earth's rotation and orientation.

NIST's O'Brian says the NASA scientists' work in pinpointing the impact of events like the Sumatra quake on the planet's rotation also offers a lesson.

"It's good for all of us to remember that we live, in some senses, on a really small planet," he says.

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