Sumatra Quake in April Tied to Sea Floor Changes

Sumatra Quake in April Tied to Sea Floor Changes
A man wheels away a fridge removed from building debris in Pagandaran following the 2006 tsunami on Java in Indonesia. Photographer: Arie Basuki/Bloomberg News

An April earthquake near the island of Sumatra resulted from as many as five faults in the tectonic plate that serves as the floor of the Indian Ocean, according to researchers that say the geologic event may show the massive slab of rock is splitting into two.

A fault is a fracture that can push upward causing mountains, downward causing oceans or even side-to-side as the tectonic plate floats on a lower layer of molten rock. In the past, scientists didn’t know that a series of faults could act in concert, in this case sliding sideways, spurring a dangerous 8.7 magnitude quake, but no tsunami, the wall of ocean water that can result from such an occurrence.

The faults tied to the earthquake may become part of a boundary as the Indo-Australian plate splits, according to papers published today in the journal Nature. The finding may also help scientists better understand the San Andreas fault in California, which moves similarly, the researchers said.

“For decades, we’ve known this Indo-Australian plate is deforming internally and it’s not really acting like a rigid body,” said Keith Koper, a study author and an associate professor of geophysics at the University of Utah, in a telephone interview. “We think this will ultimately become a plate boundary, but we have to see what the earth decides.”

The Sumatra quake occurred on April 11, running for 2 minutes and 40 seconds. If each of the individual faults had spurred a quake on their own, none would have been as big, the research found. A fifth fault that broke open later caused a magnitude-8.2 aftershock.

Rare Force

An earthquake of magnitude 8.0 or greater is rare and can totally destroy anything near the epicenter, according to the U.S. Geological Survey. The east coast earthquake last year, of 5.8 magnitude, was almost 800 times weaker than the one in the ocean, since the scale, which goes to 10, increases logarithmically.

Because the plates didn’t move up or down in the Sumatra quake, ocean water wasn’t greatly displaced and no tsunami occurred, Koper said.

This was key in an area in which more than 170,000 people had died or went missing in 2004 as the result of a tsunami spurred by a similar-sized event. After the April quake, local officials canceled a tsunami alert within about four hours and it was linked to only about a dozen deaths, mostly from heart attacks, Bloomberg News reported at the time.

Three of the faults, which ranged from about 90 miles long to 120 miles, were laid out in parallel, the other was perpendicular and, at one point, crossed one of the others in an unusual pattern, Koper said.

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