'Odd duck' Indonesia quake surprises scientists - CBS News 8 - San Diego, CA News Station - KFMB Channel 8

'Odd duck' Indonesia quake surprises scientists

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In this image made from Indonesian television TV One, two women react on a street shortly after they ran out from a building when a strong earthquake hit in Aceh in Indonesia, Wednesday, April 11, 2012. (AP Photo/TV One via AP Video) In this image made from Indonesian television TV One, two women react on a street shortly after they ran out from a building when a strong earthquake hit in Aceh in Indonesia, Wednesday, April 11, 2012. (AP Photo/TV One via AP Video)

LOS ANGELES (AP) — 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.

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Follow Alicia Chang's coverage at http://www.twitter.com/SciWriAlicia

Copyright 2012 The Associated Press.

THIS IS A STORY UPDATE. For an earlier version, read below.

BANDA ACEH, Indonesia (AP) -- Two massive earthquakes triggered back-to-back tsunami warnings for Indonesia on Wednesday, sending panicked residents fleeing to high ground in cars and on the backs of motorcycles. No deadly waves or serious damage resulted, and a watch for much of the Indian Ocean was lifted after a few hours.

Women and children cried in Aceh, where memories are still raw of a 2004 tsunami that killed 170,000 people in the province alone. Others screamed "God is great" as they poured from their homes or searched frantically for separated family members.

Patients were wheeled out of hospitals, some still lying in their beds with drips attached to their arms. And at least one hotel guest was slightly injured when he jumped out of his window.

The U.S. Geological Survey said the first 8.6-magnitude quake was a shallow 22 kilometers (14 miles), hitting in the sea 270 miles (435 kilometers) from Aceh's provincial capital.

An alert that followed from the Pacific Tsunami Warning Center in Hawaii advised countries all along the rim of the Indian Ocean, from Australia and India to as far off as Africa, that a seismically charged wave could head their way.

Two deadly tsunamis in the last decade - the most recent off Japan just one year ago - have left the world much better prepared.

Sirens sounded along coastlines and warnings spread like wildfire by mobile phone text messaging. Though often chaotic, evacuations began immediately with streets clogged with traffic, especially in Aceh.

The only wave to hit, though, was less than 30 inches (80 centimeters) high, rolling to Indonesia's emptied coastline.

Just as the region was sighing relief, an 8.2-magnitude aftershock followed.

"We just issued another tsunami warning," Prih Harjadi, from Indonesia's geophysics agency, told TVOne in a live interview.

He told his countrymen to stay clear of western coasts.

Residents in Aceh could hardly believe it.

"What did we do to deserve this?" cried Aisyah Husaini, 47, who lost both her parents and a son in the 2004 tsunami. "What sins have we committed?"

"I'm so scared, I don't want to lose my family again," she said, clinging to her two children in a mosque in Banda Aceh, where hundreds of people sheltered.

Again, though, the threat quickly passed.

Experts said Wednesday's quakes did not have the potential to create massive tsunamis because the friction and shaking occurred horizontally, not vertically. The earth's tectonic plates slid against each other, creating more of a vibration in the water.

In contrast, mega-thrust quakes cause the seabed to rise or drop vertically, displacing massive amounts of water and sending towering waves racing across the ocean at jetliner speeds.

Roger Musson, seismologist at the British geological survey who has studied Sumatra's fault lines, said initially he'd been "fearing the worst."

"But as soon as I discovered what type of earthquake it was ... I felt a lot better."

The tremors were felt in neighboring Malaysia, where high-rise buildings shook, and Thailand, India and Bangladesh.

Those countries, Sri Lanka and the Maldives evacuated buildings and beaches and readied relief efforts in case of disaster.

The World Meteorological Organization said communication systems set up after the 2004 tsunami appeared to have worked well.

"Our records indicate that all the national meteorological services in the countries at risk by this tsunami have received the warnings in under five minutes," said Maryam Golnaraghi, the head of WMO's disaster risk reduction program.

The alert was sent out by U.S. National Weather Service, which operates a tsunami warning station in Hawaii, she said.

Indonesia straddles a series of fault lines that makes the vast island nation prone to volcanic and seismic activity.

The giant 9.1-magnitude quake and tsunami on Dec. 26, 2004, killed 230,000 people in about a dozen nations.

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