The Dec 26th 2004 Indonesian megathrust earthquake quickened Earth's rotation and changed our planet's shape.

by Dr Tony Phillips

NASA scientists studying the Indonesian earthquake of Dec. 26, 2004, have calculated that it slightly changed our planet's shape, shaved almost 3 microseconds from the length of the day, and shifted the North Pole by centimeters.

Dr. Benjamin Fong Chao of NASA's Goddard Space Flight Center and Dr. Richard Gross of NASA's Jet Propulsion Laboratory said all earthquakes have some effect on Earth's rotation. It's just that the effects are, usually, barely noticeable.

This one was not usual: The devastating megathrust earthquake registered nine on the new "moment" scale (modified Richter scale), making it the fourth largest 'quake in one hundred years.

more Geography of the Dec. 26th Indonesian earthquake

Chao and Gross routinely calculate earthquakes' effects on Earth's shape and rotation. They also study changes in polar motion--that is, the shifting of the North Pole.

According to their latest calculations, the Dec. 26th earthquake shifted Earth's "mean North Pole" by about 2.5 centimeters (1 inch) in the direction of 145 degrees east longitude, more or less toward Guam in the Pacific Ocean. This shift is continuing a long-term seismic trend identified in previous studies.

The quake also affected Earth's shape. Chao and Gross calculated that Earth's oblateness (flattening on the top and bulging at the equator) decreased by a small amount--about one part in 10 billion. This continues the trend of earthquakes making Earth less oblate. Less oblate means more round.

They also found the earthquake decreased the length of the day by 2.68 microseconds. (A microsecond is one millionth of a second.) In other words, Earth spins a little faster than it did before. This change in spin is related to the change in oblateness. It's like a spinning skater drawing arms closer to the body resulting in a faster spin.

None of these changes have yet been measured - only calculated. But Chao and Gross hope to detect the changes when Earth rotation data from ground based and space-borne sensors are reviewed.