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The massive
earthquake off Indonesia surprised scientists: Usually this type of jolt isn’t
this powerful. The biggest earthquakes tend to occur in subduction zones 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 fault line similar to California’s
San Andreas Fault. 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 seismologist 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.
Associated Press
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