'Odd duck' Indonesia quake surprises scientists

Apr 11, 2012 By ALICIA CHANG , AP Science Writer

(AP) -- The massive earthquake off Indonesia surprised scientists: Usually this type of jolt isn't this powerful.

The biggest earthquakes tend to occur in where one plate of the Earth's crust dives under another. This grind produced the 2004 magnitude-9.1 Indian Ocean disaster and the magnitude-9 Japan quake last year.

Wednesday's magnitude-8.6 occurred along a strike-slip similar to California's . Scientists say it's rare for strike-slip quakes, in which blocks of rocks slide horizontally past each other, to be this large.

"It's clearly a bit of an odd duck," said Susan Hough of the U.S. Geological Survey in Pasadena, Calif.

As one of the world's most seismically active places, Indonesia is located on the Pacific "Ring of Fire," an arc of volcanos and fault lines encircling the Pacific Basin. Pressure builds up in the rocks over time and is eventually released in an earthquake.

Wednesday's quake was followed by a magnitude-8.2 aftershock. Both were strike-slip quakes.

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

So large, in fact, that the main shock went into the history books. Record-keeping by the USGS National Earthquake Information Center ranks Wednesday's shaker as the 11th largest since 1900. It's probably the largest strike-slip event though there's debate about whether a similar-sized Tibet quake in 1950 was the same kind.

A preliminary analysis indicates one side of the fault lurched 70 feet past the other - a major reason for the quake's size. By contrast, during the 1906 magnitude-7.8 San Francisco earthquake along the San Andreas - perhaps the best known strike-slip event - the ground shifted 15 feet.

The Sumatra coast has been rattled by three strong strike-slip quakes since 2004, but Wednesday's was the largest.

Explore further: NASA gets two last looks at Tropical Cyclone Jack

More information: Earthquake page: http://earthquake.usgs.gov/

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not rated yet Apr 11, 2012
What dident suprise me, was that it was closly followed be another large earthquake allmost at the antipode.
Magnitude 7.0 - MICHOACAN, MEXICO.
I will now wait for another expert to tell me yet again that they are not connected.

not rated yet Apr 11, 2012
You could expect the seismic waves to be concentrated at the atntipodes, but dd the Mexican quake occur as the waves from Sumatra arrived?
not rated yet Apr 12, 2012
Followed by yet another. Ive stated before about 7.0 plus magnitude earth quakes seemingly triggering large quakes at the antipodes. Ive correctly forcast several in the last few years, only to have an expert belittle me or claim there was no connection.
Obviously certain conditions need to be met, such as the area needs to be sismicly active, and allready have sufficient built up stress.
They are starting to realise that earthquakes trigger faults farther and farther away, but if we can have an ocian wide tsunami allert, I think its time at least issue possable hightend earthquake risk warning for large areas, at the antipodes, after a 7.0 plus event.
At least then people can have an action plan forfront in thier mind, and not be taken by suprise. Maybe keep an emergency kit with them, or if circumstances allow even avoid the possability of becoming trapped in collapsing structures.
I have watched it to many times for it to be dissmised again.
1 / 5 (1) Apr 12, 2012
I guess, because my speeling is so bad, they assume I must be thick. One of the perks of dyslexia.

not rated yet Apr 16, 2012
This region continues to be shaken by strong quakes. It's very interesting because there appear to be two central areas - one focus for the quakes is along a strike-slip fault on the Indian Ocean floor. The other appears to be at the northern end of 90 East Ridge. While I understand the strike slip activity and aftershocks, I wonder what's happening along 90 East Ridge?

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