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Indonesian earthquake makes the history books

Wednesday's earthquake off the coast of Indonesia is making it into the history books for its size and rarity.
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Residents of the remote Simeuleu island near Aceh observe the sea level in their bay on April 11, 2012, after a powerful earthquake hit the western coast of Sumatra in Aceh province on April 11, 2012. A massive earthquake struck off Indonesia's Sumatra island, US and Indonesian monitors reported, prompting an Indian Ocean-wide tsunami alert. (STR/AFP/Getty Images)

Wednesday's massive 8.6-magnitute earthquake that hit off the coast of Indonesia was so powerful and rare that it will go down in history.

By Thursday morning, life was back to normal in Indonesia and other Indian Ocean countries. And yet, the earthquake was the eleventh largest since 1900. 

How is it then that a mere day after an earthquake of that magnitude, life has moved on? 

Wednesday's earthquake was as rare as rare can get according to seismologist Susan Hough of the US Geological Survey in Pasadena, Calif., who told the Associated Press, "It's clearly a bit of an odd duck."

Usually earthquakes of this magnitude are subduction quakes, but the one that struck Wednesday is known as a slip-strike, making it much less likely to form a tsunami. However, slip-strike earthquakes are usually far less powerful.

More from GlobalPost: Indonesia: Tsunami warning lifted after powerful earthquake strikes

Kevin Furlong, a professor of geosciences at Penn State University, is quoted in the same AP article saying, "A week ago, we wouldn't have thought we could have a strike-slip earthquake of this size. This is very, very large."

Subduction quakes occur when one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle. Indonesia's 2004 quake, which was said to have a magnitude as high as 9.3 according to the US Geological Survey, was a subduction quake.

The National Geographic explains: "The earthquake was the result of the sliding of the portion of the Earth's crust known as the India plate under the section called the Burma plate. The process has been going on for millennia, one plate pushing against the other until something has to give. The result on December 26 was a rupture the USGS estimates was more than 600 miles (1,000 kilometers) long, displacing the seafloor above the rupture by perhaps 10 yards (about 10 meters) horizontally and several yards vertically."

The 2004 earthquake caused a tsunami wave as high as 50 feet, according to some reports, and killed as many as 230,000 people. 

The earthquake Wednesday only raised the ocean floor about three feet. That is because slip-strike quakes move vertically, not horizontally. 

Even more surprising, the 2012 earthquake is most likely related to the 2004 subduction quake. Bradford Hager, a geophysics professor at MIT said, "I would call this an aftershock," saying it's not surprising to have slip-strike earthquakes long after an earthquake of the 2004 magnitude, adding the 2012 earthquake was, "in response to an even bigger quake."

Hager went on to explain that the motion of the Indonesian slip-strike is similar to what occurs at the San Andreas Fault in California and is usually far less dangerous because it involves less vertical motion. 

Wednesday's earthquake rattled more nerves than it did buildings with survivors of the 2004 tsunami staying alert, long past the retraction of the tsunami warnings. One woman told the Hindustan Times, "I just want to stay alert because I fear there will be more quakes coming. We are human, it is only natural that we have fear, but I really wish we will all be safe."

More from GlobalPost: Mexico: 6.3 earthquake in Guerrero state is felt in Mexico City

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