The changing of microseconds

I don’t mean in the least to diminish the terror and severity of the Chilean earthquake in 2010, but I have long enjoyed horological and geophysical trivia.

Richard Gross, a geophysicist at NASA’s Jet Propulsion Laboratory says that the earthquake that struck Chile a week ago may have shifted the Earth’s axis by about 8cm and permanently shortened each day by about 1.26 microseconds. (That’s about 46 hundredths of a second every 1,000 years!) A large quake shifts massive amounts of rock and alters the distribution of mass on the planet, changing the rate at which the planet rotates. And the rotation rate determines the length of a day.

The magnitude 9.1 Indian Ocean earthquake in December 2004 shortened the length of days by 6.8 microseconds. On the other hand if the Three Gorges reservoir in China were filled, it would hold 40 cubic km (40 billion tonnes) of water. The shift of mass would lengthen days by 0.06 microsecond.