Japan quake among most powerful on record

The undersea earthquake that propelled massive waves from a fault line hundreds of miles long was among the five most powerful ever recorded, scientists said Friday.

The quake was caused by two giant, opposing tectonic plates slipping past each other under the Pacific Ocean off the northern coast of Japan.

Dan Davis, a geophysicist and professor in Stony Brook University's geosciences department, illustrated the process by placing one hand atop the other, tilting them together at a slight upward angle, then pushing them together using his elbows and forearms.

The buildup of tension and eventual slip of one object above and the other below represents what happened on a massive scale off the coast of Japan.

A tsunami resulted because, "any vertical displacement in the sea floor forces water up and down and creates release at the surface," Davis said.

The large swath of Japanese coastline affected suggests a very large fault line, he said.

"It's a huge area," Davis said. Though the movement involves a slip of just several meters, the area involved "could be 200 kilometers [124 miles] wide and 600 kilometers [373 miles] long." The power released was about 25,000 times that of the first atomic bomb used on Hiroshima in World War II, he said.

The largest earthquake recorded since scientists began measuring them in 1900 was the magnitude 9.4 off Prince William Sound on the coast of Alaska on March 28, 1964.

Last year's magnitude 8.8 Chilean quake was more comparable to Japan's, said Susan Hough, a seismologist at the U.S. Geological Survey and an adjunct professor at the California Institute of Technology in Pasadena. The January 2010 earthquake in Haiti had a magnitude of 7.0.

What made this quake particularly unusual was its duration, she said.

"The earthquake happened for minutes versus 15 seconds," which is more typical of quakes, she said.

"The fault was actually moving for minutes. It's the actual fault that's moving, just sending energy down into the earth."

And up into the water.

"This incredibly powerful wall of water, that's what gave us loss of life and all the damage," Hough said. "In the open ocean, the wave spreads out in all directions at about 500 miles an hour."

Davis of Stony Brook said there is almost zero chance of a comparable earthquake and tsunami striking Long Island.

"We are nowhere near this kind of plate boundary," he said. "We could never have this kind of earthquake."