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The Japanese Quake Explained


Published March 11, 2011 |

A powerful tsunami spawned by the largest earthquake in Japan’s recorded history slammed the eastern coast Friday, sweeping away boats, cars, homes and people as widespread fires burned out of control. Tsunami warnings blanketed the entire Pacific, as far away as South America, Canada, Alaska and the entire U.S. West Coast.

The U.S. Geological Survey (USGS) said the 2:46 p.m. quake was a magnitude 8.9, the biggest earthquake to hit Japan since officials began keeping records in the late 1800s. Geologist Harley Benz told Fox News it was the fifth largest earthquake in the last 100 years.

What caused the gigantic Japanese earthquake, and will it be followed by aftershocks? Here are answers to these and other questions.



What caused the earthquake?

The massive earthquake near the east coast of Honshu, Japan, occurred as a result of faults on or near the subduction zone — the interface plate boundary between the Pacific and North America plates, the USGS said.

The Pacific plate thrusts underneath Japan at the Japan Trench, and dips to the west beneath Eurasia; these rocky plates cover the planet like a giant jigsaw puzzle and creep past one another at a very slow rate. The Pacific plate moves approximately westwards with respect to the North America plate at a velocity of 3.2 inches per year. The release of energy as the the two plates move past each other is what causes the earthquakes.

The Japan Trench subduction zone is relatively volatile, experiencing 9 earthquakes of magnitude 7 or greater since 1973. The largest of these was a 7.8 magnitude earthquake approximately 162 miles (260 km) to the north of the March 11 event, in December 1994, which caused 3 fatalities and almost 700 injuries, the USGS said. In June of 1978, a 7.7 magnitude earthquake in the area caused 22 fatalities and over 400 injuries.

What does magnitude 8.9 mean?

“It’s comparable in size to the earthquake in Chile last year, that was a magnitude 8.8. And very similar kinds of ruptures in both cases,” Robert Williams, a geologist with the USGS told Fox News.

Magnitude measures the energy released at the source of the earthquake. Since magnitudes are given on a logarithmic scale, a 7.0-magnitude earthquake would release 10 times as much energy as a 6.0-magnitude temblor. Geoscientists also look at an earthquake’s intensity, which measures the strength of shaking produced by the earthquake at a certain location and is determined from the effects that shaking has on people, structures and the environment.



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The Japanese Quake Explained

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