8.6 Earthquake in Indian Ocean Triggered Aftershocks in L.A.'s Backyard

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Wed, Sep 26, 2012 at 1:29 PM

click to enlarge MARTIN LUFF / FLICKR
  • Martin Luff / Flickr
If you've noticed that the Ring of Fire around the Pacific Rim seems to rock like a frat party every time there's a giant earthquake, new science might just back up your observation.

A new study by U.S. Geological Survey seismologist Fred Pollitz documents how the Pacific Rim went bonkers following the 8.6 East Indian Ocean quake of April 11.

Quakes were set off as far away (and as close to us) as ...

... Baja, California, Mexico, aka our own backyard. The Baja action south of the fishing village of Bahia Los Angeles included a "100-fold increase in seismicity" and "13 events in the following two days" after the Indian Ocean rocker, according to the study.

The shakers there included a 6.0 and 7.0, which would have been massive had they occurred in L.A.

click to enlarge After the Indian Ocean quake, this. - USGS
  • USGS
  • After the Indian Ocean quake, this.

The USGS calls the findings "unprecedented." The conclusions are certainly something to think about the next time a Big One hits across the Pacific. The study was published this week in the journal Nature.

The East Indian temblor was the largest quake ever recorded on a strike-slip fault (those where one surface plate is traveling horizontally in the opposite direction of another, like SoCal's own San Andreas), according to the USGS, which states:

click to enlarge USGS
  • USGS

An extraordinary number of earthquakes of M4.5 and greater were triggered worldwide in the six days after the M8.6 East Indian Ocean earthquake in April 2012. These large and potentially damaging quakes, occurring as far away as Mexico and Japan, were triggered within days of the passage of seismic waves from the main shock that generated stresses in Earth's crust.

Seismologist Pollitz:

No other recorded earthquake triggered as many large earthquakes elsewhere around the world as this one ...

The research has helped change the very definition of "aftershock," according to the Survey:

While aftershocks have traditionally been defined as those smaller earthquakes that happen after and nearby the main fault rupture, scientists now recognize that this definition is wrong. Instead, aftershocks are simply earthquakes of any size and location that would not have taken place had the main shock not struck.

After that 8.6, there were a week's worth of aftershocks.

So next time the Big One hits thousands of miles across the Pacific, don't be surprised if Southern California ends up rocking, too.

[@dennisjromero / djromero@laweekly.com / @LAWeeklyNews]

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