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Powerful aftershocks of Indian Ocean 8.6 earthquake reached Alaska

Ben Anderson

A new study published in the journal Nature Wednesday says that a powerful undersea earthquake in the Indian Ocean earlier this year caused powerful aftershocks around the globe, including in Alaska and on the West Coast of the U.S.

On April 11, a magnitude-8.6 quake trembled along the Indo-Australian tectonic plate, which stretches from India, running underneath the Philippine Islands, and encircling Australia. That quake, while powerful and noteworthy on its own, was a different kind of temblor altogether.

Most earthquakes are caused when one tectonic plate slides underneath another. But some, what are known as “strike-slip” events, come when one plate slides against another horizontally. These events typically create less powerful quakes, so the strength of the April 11 event came as a surprise. The study refers to it as the strongest strike-slip event “by far” in recorded history.

Another strong strike-slip earthquake came in November 2002 along the Denali fault in the Alaska Interior. That magnitude 7.9 earthquake is probably the best studied strike-slip event in the world, according to Dr. Carl Tape, a seismologist with the University of Alaska Fairbanks Geophysical Institute.

Study co-author Fred Pollitz, a geologist with the U.S. Geological Survey in California, said that researchers looked at that event for comparison to the Indian Ocean quake.

"We looked at a couple of other large-strike slip events, for example the 2004 magnitude 8.1 Tasman Sea event, and did not see elevated global seismicity rates after them," Pollitz said in an email to Alaska Dispatch. "The magnitude 7.9 Denali earthquake produced quite a few small quakes in western North America but not any dramatic global response."

In this year's case, one small point of contact between two plates in the East Indian Ocean resulted in two strike-slip quakes of 8.2 or higher within two hours of each other. And the result of that friction caused seismic waves to ripple out throughout the Pacific in a domino effect of quakes along other fault lines.

The first place on the globe to record an aftershock was the western portion of Alaska’s Aleutian Island chain, stretching far into the Pacific Ocean toward Russia. A magnitude 5.5 quake struck there at about 1 a.m. Alaska time, a mere 21 minutes after the earthquake thousands of miles to the southwest.

The Aleutian quake was the first, but by no means the last, of the aftershocks from the Indian Ocean shaker. The study notes that the Aleutian quake was a bit of an outlier among the aftershocks -- “all others are delayed by hours to days,” the study said.

So how did scientists know that the subsequent earthquakes, which were observed all around the globe, resulted from the original Indian Ocean disturbance? Well, for one, the overall number of earthquakes magnitude 5.5 and greater spiked in the days following.

“The basic point of the study is simple,” Tape said. “There weren’t many big earthquakes in the days before the offshore Sumatra earthquake, but there were a lot of big earthquakes in the days following it, even at very large distances from the Sumatra event.”

That much is clear. In the six days preceding the April 11 event, there were only two earthquakes magnitude 5.5 or greater occurred around the world. In the six days following the Indian Ocean quake, there were 24.

But, Tape said, nailing down exactly which aftershocks directly resulted from the April 11 quake is tougher.

“The question is that after the earthquake happened, there were all these other triggered earthquakes, and some of these probably would have happened anyway, but you can’t say for certain which ones would have happened or wouldn’t have happened,” he said.

That’s the case with the Aleutian quake. It may have been triggered by the initial “wave” radiating across the globe from the Indian Ocean on the morning of April 11, but the fact that it came so much earlier than the other aftershocks means it’s tricky to attribute it to the quake less than a half-hour before.

Tape and colleagues have submitted a paper of their own for review suggesting that another Alaskan earthquake can be attributed to the April 11 quake, a magnitude 3.9 temblor that shook much of the Interior Alaska city of Fairbanks.

While the study doesn’t answer all the questions about earthquakes and their predictability -- or lack thereof -- it does provide a strong indicator that some earthquakes have more effects than others. And that only helps geologists better understand the shifting earth beneath our feet.

Contact Ben Anderson at ben(at)alaskadispatch.com