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Why The Chile Earthquake Deformed The Earth And Shortened Our Days

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The devastating 8.8 quake that hit Chile last week was a rare "megathrust" earthquake, among the most powerful known, and it affected the very shape of the planet. How did it do that, and will you notice the difference?

A megathrust earthquake is caused when one one tectonic plate is shoved violently underneath another in a process called subduction. These earthquakes are very rare; they tend to be 7.5 magnitude or higher, and there are only about 14 in recorded history (including the 2004 Indonesian quake that caused huge tsunamis).


In Chile, the Nazca plate was pushed against the South American Plate (see above). These two plates have a history of smashing together alarmingly: The Andes mountain range was created by their subduction action, and 3 of the 5 megathrust quakes recorded in the last century centered on Chile.

Megathrust quakes like Chile's are so huge, and cause such a giant release of energy, that they change the shape of the Earth. In the case of Chile's subduction quake, the planet became slightly denser and more compact. Mass was pulled closer to the Earth's center as one plate was thrust under the other. And that affected the Earth's spin. It made the planet spin slightly faster, to be precise, and shortened the length of the Earth day.


Geophysicist Richard Gross told the Los Angeles Times that he estimated each day from now on will be 1.26 millionth of a second shorter. Why? Gross said, "[The planet] got a bit more compact. Just like a spinning skater brings her arms in closer to her body to rotate faster."

The Indonesian quake shortened our days by 2.68 microseconds, according to NASA scientists.

Where might the next megathrust quake hit? Scientists believe it could be in a subduction zone off the Pacific coast of the Northern US and Canada, where the Juan de Fuca plate meets the North American plate. Geophysicists have used computer modeling to show what such a quake would look like, based in part on data they've reconstructed about a 9 magnitude quake there in 1700. The researchers estimate that megathrust quakes hit the region about every 400-500 years, so we're about due for another.


Additional research by Cayman Unterborn.