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Monday, March 15, 2010

Shorter days after earthquakes

As I’m sure you all know, Chile suffered a devastating 8.8 magnitude earthquake on Feb. 27th. This was during the Vancouver Winter Olympics, leading to a failed attempt by one Chilean skier to return home early. Some 500 people were killed, with many more missing or homeless.

By comparison, the recent Haitian earthquake registered only 7.0, but was responsible for several hundred thousand deaths. Remember, earthquakes are rated on a logarithmic scale. In other words, the Chilean earthquake was almost a hundred times stronger than the Haitian quake. The proximity to populated centers and the lack of infrastructure led to the staggering death toll.

What you may not have known is that the February earthquake apparently shifted Earth’s figure axis (the axis about which Earth’s mass is balanced) by about 8 centimeters, or 3 inches. In consequence, the temblor shortened the length of our days by about 1.26 milliseconds. That’s millionths of a second for those of you resetting your alarm clocks.

How did this happen? The Chilean earthquake was what is known as a ‘thrust’ earthquake, in which one tectonic plate dives beneath another. The slightly reduced surface area caused the Earth to speed up like an ice skater drawing in her arms, in an analogy drawn by Keith Sverdrup, a seismologist at the University of Wisconsin-Milwaukee.

Only extremely powerful thrust earthquakes can alter the Earth’s figure axis in this manner, which is why the Earth’s rotation is not constantly changing. The position of the earthquake along the globe also determines the severity of the effect, with equatorial quakes being less effective.