WORDS on WEATHER & CLIMATE

What is The Earth’s Figure Axis?

by CRAIG JAMES

Meteorologist Craig James, new Squire columnist

Since the huge earthquake in Japan earlier this month, I have read in many places that it not only shifted the island of Japan eight feet, but it also shifted the axis of the Earth about six-and-a-half inches to the east. What hasn’t been made all that clear is that it shifted the figure axis of the Earth, not the north-south axis. That seems even less clear. What exactly is the Earth’s figure axis? I confess to not having heard the term before.

A seismologist from the University of Wisconsin explains that the figure axis is the imaginary line around which the world’s unevenly distributed mass is balanced. The figure axis deviates from the north-south axis by about 33 feet. You can visualize the difference this way by imagining “a spinning figure skater holding a rock in one hand. The rotational axis of the skater is still down the middle of the body, but the skater’s figure axis is shifted slightly in the direction of the hand holding the rock.” What he is saying is that we now have more mass on one side of the north-south rotational axis than we did before the earthquake, which will cause the Earth to wobble a bit differently as it rotates.

This wobble has apparently had the effect of speeding up the rotation of the Earth. I say “apparently” because the change has been so small as not to be measurable. Scientists can only measure the length of the Earth’s day with an accuracy of 20 millionths of a second. Using computer models, it has been calculated that this earthquake shortened the Earth’s day by less than two millionths of a second. The Earth’s crust moved inward toward the center of the Earth, which would be like the skater pulling her arms slightly inward, causing her to spin faster.

The earthquake in Japan is called a thrust earthquake, with an inward motion. This is the only type of earthquake that can speed up the Earth’s rotation and shorten the length of the day. The seismologist says, “Other types of earthquakes, such as horizontal strike-slip quakes, in which two plates slide horizontally past one another, don’t affect Earth’s rotation.”

The earthquake last year in Chile and the one in Sumatra in 2004 were also thrust earthquakes. The Chile quake shortened the length of day by a little over one millionth of a second while the Sumatra quake should have shortened the length of the Earth’s day by nearly seven millionths of a second.

Is this shift of the Earth’s figure axis cause for concern? Not according to a scientist from NASA’s Jet Propulsion Laboratory who says, “Earth’s rotation changes all the time as a result of not only earthquakes, but also the much larger effects of changes in atmospheric winds and oceanic currents. Over the course of a year, the length of the day increases and decreases by about 550 times larger than the change caused by the Japanese earthquake. The position of Earth’s figure axis also changes all the time, by about one meter [3.3 feet] over the course of a year, or about six times more than the change that should have been caused by the Japan quake.”

We are therefore assured that the changes in the Earth’s rotation and figure axis should not have any impact on our daily lives, unlike of course the impact caused by the earthquake itself, because the changes are perfectly natural and happen all the time. These changes are the least of our worldly worries. But I’ll bet you now know a few things you didn’t before you read this article.

Craig James has been retired since July 1, 2008, after 40 years of broadcasting television weather. He was chief meteorologist at WZZM-TV for 12 years and chief meteorologist at WOOD-TV for 24 years. He is a graduate of Penn State University, where he received a Centennial Fellowship Award. He was also honored as a Fellow of the American Meteorological Society.



 

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