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What unleashed Japan's massive 8.9 earthquake?

Like the other 14 biggest earthquakes since 1900, the magnitude 8.9 event – that shook Japan and triggered tsunamis that swept the Pacific – was created by a piece of the Earth's crust shoving down into the planet's interior.

By Staff writer / March 11, 2011

Rescue workers hurry to a building in Tokyo's financial district after a magnitude 8.9 earthquake struck, off the coast of northern Japan, on March 11. Several strong aftershocks and tsunamis followed the quake, which also caused buildings to shake violently in the capital Tokyo.

Kim Kyung-Hoon / Reuters

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As Japan struggles to take the full measure of damage and loss of life following a devastating earthquake and tsunamis that struck the country just before 3 p.m. local time on Wednesday, scientists are closely tracking dozens of powerful aftershocks to help assess the quake's effect on adjacent segments of the fault that ruptured.

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"This is certainly something people are going to look at" in some detail, says Geoffrey Abers, a research professor at the Lamont-Doherty Earth Observatory in Palisades, N.Y.

Over the past 10 to 20 years, he says, "we've learned a lot about about loading and triggering, where one part of a fault breaks like this and stresses segments of the fault next to it that haven't had a big earthquake in a long time." The effect: One temblor can nudge other segments closer to failure.

It doesn't happen often, Professor Abers says. On average, one out of 20 quakes of this scale triggers ruptures along adjacent segments of the fault within a couple of years of the initial shock.

Yet, he adds, a 1-in-20 chance represents "a much elevated hazard" for people, their homes, and businesses nearby.

The 2004 earthquake that struck off the west coast of northern Sumatra and generates a devastating series of tsunamis is a case in point, researchers say. On Dec. 26, a magnitude 9.1 quake struck along a boundary between large plates in the Earth's crust where one plate slides beneath another.

Since that episode, segments along the 2,000-kilometer (about 1,200 mile) stretch of the "subduction" zone that included the epicenter of the 2004 quake have experienced a series of massive earthquakes, ranging in magnitude from 8.6 to 7.8.

Wednesday's earthquake, which also occurred in a subduction zone, is said to be the most powerful to strike Japan since the early 1800s, when the country began to keep records of its temblors.

It triggered tsunamis that reportedly reached 30 feet high along the stretch of Japanese coastline closest to the quake's epicenter. Tsunamis also raced across the Pacific to reach the west coast of North and South America. Hilo, Hawaii, recorded a tsunami that stood almost 5 feet above sea level, while Crescent City and Port San Luis, Calif., logged tsunamis ranging from 6 to 7 feet high.

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