If you feel like the days are going by a little quicker lately, you're right. Our days are now three millionths of a second shorter, because of the earthquake that caused the devastating tsunamis in southeast Asia. And that's not the only global change.
Shaking up the earth. I'm Bob Hirshon and this is Science Update.
The massive December 26, 2004 earthquake off the coast of Indonesia caused more than the tsunamis. It also changed the shape of the earth, shifted the North Pole, altered the global gravity field, and shortened the length of our days.
Benjamin Fong Chao of NASA's Goddard Space Flight Center says these changes were extremely small, but measurable. He says the quake redistributed some of the earth's mass, making it less round overall.
This phenomenon is basically the same as when you see a spinning skater, when he or she draws arms closer to the body, it spins faster. And this is what happened for the earth as a whole due to this earthquake.
He says the findings will help scientists test their theories of earthquakes, by comparing their predictions to what actually happened. I'm Bob Hirshon for AAAS, the Science Society.
Making Sense of the Research
This research reminds us that the earth is a physical entity. It's constrained by the same laws of physics that affect a spinning skater or a spinning basketball.
The fact that the Indonesian earthquake changed the shape of the earth may sound remarkable. But the truth is, you yourself can change the shape of the earth just by digging a hole in the ground. The difference between your shovel and a gigantic earthquake is that the effect of the earthquake can actually be measured by looking at the earth as a whole.
Once the shape of the earth changes, the other factors come into play. As the earth spins over time, it slowly redistributes its mass toward the middle, making it rounder (or fatter). When the Indonesian earthquake came along, it shifted some of that mass away from the middle again. How much mass? According to Chao's results, one part in one hundred billion. That's not much, but again, it's measurable.
As Chao explained, changing the shape of the earth changes the speed at which it rotates. Now that it's rotating slightly faster, our days are slightly shorter (three millionths of a second shorter, to be precise). Furthermore, since gravity depends on an object's mass, the redistribution of mass has altered the earth's gravitational field. (The earth's gravitational pull varies slightly from point to point, depending on the density of the earth at the point in question. This is true of any object, but it's noticeable only in very large objects like moons and planets.)
By changing the center of mass of the planet, the earthquake also shifted the position of the poles—in other words, the ends of the earth's axis. This effect was stronger than the other changes: the mean North Pole shifted about an inch. We say the "mean North Pole" because the earth wobbles slightly as it rotates, so its axis can't be pinned down to one exact location.
There hasn't been a quake this strong since the late 1950's and early 1960's, when two quakes in Alaska and one in Chile topped 9.0 on the Richter scale. Because this earthquake was large enough to produce detectable results, scientists are using it to test their models of how earthquakes behave. Over the coming months and years, they'll be comparing the actual measurements Chao describes with the measurements predicted by their models. If the models conflict with the actual measurements, they'll know the models need a little more work.
Now try and answer these questions:
- Why did the Indonesian quake change the length of our day and the position of the North Pole?
- How is the earth like a spinning skater?
- Why is this earthquake useful for scientists?
- What would happen if another earthquake distributed more mass to the Equator?
Wikipedia, the free online encyclopedia, has created a detailed, multi-page entry on the 2004 Indian Ocean earthquake.
Steven Dutch, a professor of Natural and Applied Sciences at the University of Wisconsin-Green Bay, has created a page called Changing a Planet's Rotation from Within, which delves into some of the physical issues discussed in this story. Dr. Dutch also directly addresses the effects of the Indonesian quake on The 2004 Indonesian Earthquake and Earth's Rotation page.
The Science of Jumping and Rotating , by Montana State University's Burns Telecom Center, analyzes the physics of rotation on a human scale.