Very big earthquakes can be global events, studies find

Anything that advances the science of seismology is news in Southern California, or should be. This week's lead entries in the journal Nature qualify. The studies looked at data from this past April's massive undersea earthquakes off Sumatra, in the Indian Ocean. It's in the same seismic region as the 9.1 magnitude quake on December 26, 2004 that generated the Indian Ocean tsunami that killed more than 230,000 people in 14 countries. The April 2012 quakes did not cause a sea wave, since they were strike-slip earthquakes in which the shifting pieces of the Earth slide horizontally, rather than rise and displace vast volumes of ocean water.

But the April quakes that were studied were the biggest strike-slip quakes ever recorded: 8.7 and 8.2 on the magnitude scale, two hours apart. Scientists didn't realize that this type of earthquake could release that much energy. OK, now's a good time to point out that the San Andreas Fault that continually re-shapes California is, you guessed it, a strike-slip fault zone.

Seismologists would be less surprised by the April events if such big quakes had broken on the edges of the giant, moving tectonic plates that make up the earth's top layer. Most big quakes occur where plates meet, often because one of the plates is being forced under the other in an epic, messy collision. But April's whopper quakes off Sumatra occurred far from the Indo-Australian plate's edge. The scientists now suspect they are seeing the huge plate, which carries both India and Australia, begin to break into two or more smaller plates.

If that is what's happening, there's a good reason for it. Most of the plate is floating northwest, bumping up against our Pacific Plate and a few others. But the corner of the Indo-Australian Plate on which India sits has hit a snag: the Himalayan Mountains. That part of the plate can't budge, so the pressure of the rest of the plate trying to drift away from the stuck part is ripping fissures in the structure of the plate. Those April quakes were new cracks, or faults, opening up, across multiple underwater faults. The bigger quake involved four faults and lasted 2 minutes and 40 seconds.

That's all interesting enough, but there's also this. The April quakes were followed by an unprecedented number of smaller earthquakes great distances away as the energy waves circled the Earth. Several of those quakes were here, off the coast of North America. If there was any doubt that giant quakes could trigger other susceptible faults to move, the question seems mostly settled now. From Smithsonian Magazine's science site:

“Until now, we seismologists have always said, ‘Don’t worry about distant earthquakes triggering local quakes,’” co-author Roland Burgmann, professor of earth and planetary science at UC Berkeley, said in a statement. “This study now says that, while it is very rare – it may only happen every few decades – it is a real possibility if the right kind of earthquake happens.”
Burgmann calls this quake “one of the weirdest…we have ever seen.... It wasn’t a single fault that produced the quake, it was a crisscrossing of three or four faults that all ruptured in sequence to make such a big earthquake, and they ruptured deep."

Graphic:Keith Koper, University of Utah Seismograph Stations