8216;Odd duck8217; Indonesia quake surprises scientists

Big quakes tend to occur in subduction zones where 1 plate of earth's crust dives under another.

The massive earthquake off Indonesia surprised scientists: Usually this type of jolt is not that powerful.

The biggest earthquakes tend to occur in subduction zones where one plate of the earth8217;s crust dives under another. This grind produced the 2004 magnitude-9.1 Indian Ocean disaster and the magnitude-9 Japan quake last year.

Yesterday8217;s magnitude-8.6 occurred along a strike-slip fault line similar to California8217;s San Andreas Fault. Scientists say it8217;s rare for strike-slip quakes,in which blocks of rocks slide horizontally past each other,to be this large.

8220;It8217;s clearly a bit of an odd duck,8221; said seismologist Susan Hough of the US Geological Survey in Pasadena,Calif.

As one of the world8217;s most seismically active places,Indonesia is located on the Pacific 8220;Ring of Fire,8221; an arc of volcanoes and fault lines encircling the Pacific Basin.

Pressure builds up in the rocks over time and is eventually released in an earthquake.

Yesterday8217;s quake was followed by a magnitude-8.2 aftershock. Both were strike-slip quakes.

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8220;A week ago,we wouldn8217;t have thought we could have a strike-slip earthquake of this size. This is very,very large,8221; 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 Wednesday8217;s shaker as the 11th largest since 1900. It8217;s probably the largest strike-slip event though there8217;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 quake8217;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 yesterday8217;s was the largest.