What unleashed Japan's massive 8.9 earthquake?

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.

"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.

The rupture was centered some 15 miles beneath the sea floor and 76 miles east of Sendai, a city of slightly more than a million people on the east coast of the Japanese island of Honshu. The quake's epicenter also fell roughly 165 miles west of the Japan Trench, a long, deep gash in the sea floor marking the boundary between the Pacific and Eurasian plates. There, the Pacific plate is colliding with the more buoyant continental crust of the Eurasian plate and sliding beneath it.

Indeed, this collision built the Japanese islands.

Wednesday's temblor is the fourth "great" earthquake – a quake with a magnitude 8.0 or higher – to strike the Pacific Rim within the last seven years. If the estimate of 8.9 holds up as scientists continue to analyze the event, the quake would become the fifth most powerful to strike the globe since 1900, according to records compiled by the US Geological Survey.

With one exception, the 15 most powerful quakes during this period have occurred along subduction zones ringing the Pacific Basin.

The reason subduction zones generate such large quakes has to do with the size of the segments of crust moving past each other, explains Tim Masterlark, a geophysicist at the University of Alabama in Tuscaloosa.

The magnitude of a quake increases with the surface area of crust along the fault segment that breaks, he explains. On a fault like California's San Andreas, essentially a vertical crack in the crust, a quake's strength is limited by the length of the segment that ruptures and the fault's depth, which tends to find its ultimate limit in the thickness of the crust where the break occurs.

In subduction zones, however, the fault between the two plates dips at a shallow angle, significantly increasing the surface area subject to a build-up of stress and the energy that will be released when a rupture finally occurs.

When quakes like this occur, they have a global impact on the planet, adds Richard Gross, a geophysicist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. Based on initial estimates of the magnitude and location of Wednesday's quake, he calculates that the Earth's rotation slowed by about 1.6 millionths of a second, and the axis around which the planet's mass is distributed – some 33 feet away from the planet's spin axis – shifted by some 6 inches.

The changes occur because the quake redistributes mass across the Earth's crust.

These numbers represent more than answers to triva questions, Dr. Gross adds.

"Especially for really big earthquakes, the faults slips over quite a large area," he says, which makes it difficult for geophysicists to piece together details of the mechanism triggering a quake at a given location.

Seismographs and global-positioning-system (GPS) satellites can help, but they can also provide conflicting information, Gross says. Scientists draw inferences from that information to help piece together the details.