PASADENA, Calif. » The massive earthquake that struck off the coast of Japan Friday ranks as the fifth largest in the world since 1900, scientists said.

The magnitude-8.9 "megathrust" quake is similar to what happened during the 2004 Sumatra quake that spawned a killer tsunami and the earthquake last year in Chile. In all these cases, one tectonic plate is shoved beneath another.

Such earthquakes are responsible for the most powerful shifts in the Earth’s crust.

Japan is at particular risk, sitting in the "Ring of Fire" — an arc of earthquake and volcanic zones stretching around the Pacific where about 90 percent of the world’s quakes occur.

"The energy radiated by this quake is nearly equal to one month’s worth of energy consumption" in the United States, said U.S. Geological Survey scientist Brian Atwater.

More than 80 aftershocks greater than magnitude-5 have been felt since the Japanese rupture — a number that scientists say is normal for a quake this size.

USGS seismologist Lucy Jones said a friend who was in Tokyo for a tsunami planning meeting noted the shaking after the initial shock lasted for about five minutes.

Scientists said the quake erupted 6 miles below the ocean and about 80 miles off Japan’s eastern coast. It caused a 186-mile rupture in the sea floor — longer than the coastline of Washington state, said USGS geophysicist Paul Earle.

The rupture triggered deadly tsunami waves that washed away homes and boats along coastal Japan. Waves rippled across the Pacific, but so far had caused little damage in Hawaii. Alerts were posted even to the west coast of the United States.

Two days earlier, the region was rattled by a magnitude-7.2 quake that scientists now consider a foreshock. Foreshocks are basically earthquakes and are only identified as precursors after another quake follows them. After such an event, there’s only a 5 percent chance of an even bigger quake coming later.

"This was one of the rare instances where a big earthquake is followed by a bigger earthquake," said USGS geophysicist Doug Given.

Japan’s worst previous quake was a magnitude 8.3 temblor in 1923 in Kanto that killed 143,000 people, according to the USGS. A 7.2-magnitude quake in Kobe in 1995 killed 6,400 people.