Earth's pole is a pushover for quakes - earthquakes shift Earth's rotational axis - Earth Science - Brief Article

Science News, March 8, 1997 by Richard Monastersky

As Asian nations gain economic clout, they are shifting the world's political center of balance from the Atlantic region to the Pacific. Less conspicuously, earthquakes are changing Earth's physical balance by gently nudging the North Pole in the direction of the Pacific at a rate of about 10 centimeters per century. Earthquakes exert such a weighty effect because the globe's largest shocks occur mostly along the Pacific Rim, reports Giorgio Spada of the University of Bologna in Italy.

Spada studied the influence of earthquakes by using a computer to simulate random quakes and then calculating their effect on the planet's rotational axis. Quakes can shift the orientation of Earth's axis because they move large sections of the crust and subtly unbalance the planet.

In previous studies, Benjamin Chao of NASA's Goddard Space Flight Center in Greenbelt, Md., and his colleagues showed that quakes since 1977 have exerted a force that would push the pole in the direction of Tokyo. Chao's analysis did not address why quakes have such an effect.

In the March 1 Geophysical Research Letters, Spada reports studying many aspects of simulated quakes, including their longitude, latitude, depth, and fault orientation. Of these factors, the longitude of quakes influenced Earth's rotation most. Because most large quakes occur along the Pacific Rim, they combine to force the pole toward Tokyo.

Actual measurements since 1900, however, show the pole edging in the opposite direction at the rate of about 10 cm per year. Geophysicists explain this motion as a legacy of the ice age. Continents once weighted by ice have been rising for the last 10,000 years, overwhelming the quakes' influence.

By geologic standards, the glacial effect is short-lived. Over extremely long periods of time, more deep-seated processes come into play. In the Jan. 17 Science, Spada and his coworkers show that the process of subduction-which sends surface plates sinking into the mantle-has controlled the pole's position over the last 100 million years.-