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Geology

Japan sits atop deadliest section of Ring of Fire Add to ...

The recent earthquakes that have pounded Japan, New Zealand and Chile all have one thing in common: They were caused by the movement of a massive piece of Earth known as the Pacific Plate.

The Pacific Plate is one of nine giant tectonic plates that cover the Earth's surface. But it's bigger, faster and more deadly than the others. And Japan sits right on top of the most active section.

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"In a sense, Japan is a country that's got a huge problem," said John Clague, a professor at Simon Fraser University in Burnaby, B.C., who is also the Canada research chair in natural hazards research. Friday's quake "was the largest earthquake in Japan's history, but it's not the last by any means."

While earthquakes are common in all parts of the world, they are more frequent and more severe along several ridges of the Pacific Plate. Nine of the 10 largest earthquakes have happened along the edges of the Pacific Plate and the area, known as the Ring of Fire, accounts for roughly 80 per cent of the world's earthquakes.

What makes the Pacific Plate different is its size. The Earth's crust is divided into dozens of plates with six of the largest called "continental plates" because they comprise the continents. They are the North American, South American, Eurasian, African, Indo-Australian and Antarctic plates. The other three big ones - Pacific, Nazca and Cocos - are known as "oceanic plates" because they are located almost entirely underwater. There are many smaller plates, many of which have broken off from the larger ones.

The Pacific Plate covers almost the entire floor of the Pacific Ocean, making it the largest plate on the planet. As a result, whenever it rubs or collides with other plates the impact can be felt from New Zealand to Japan, Alaska, Canada, the United States and down to Chile.

It also moves much faster than the others, which results in more frequent earthquakes. All plates shift because the Earth's crust sits on top of a layer of hot, malleable rock. The soft rock layer is constantly moving as bits of it heat up, rise close the surface and then fall down as they cool. That heating and reheating pushes the plates around the Earth's crust slowly, at about the same rate a human fingernail grows. Because the Pacific Plate is so large, it travels with less drag. As a result, parts of it shift as much as 10 centimetres annually, about twice as fast as most other plates.

Earthquakes happen when two plates either rub alongside each other or collide, forcing one plate on top of the other. Neither process is smooth and pieces often become stuck together. As the plates keep moving, pressure builds in the locked area, sometimes for decades or centuries. An earthquake occurs when the stuck section suddenly gives way. Some plates pull apart from each other, which generally doesn't cause earthquakes. Iceland, for example, is being slowly torn apart because it's on top of a ridge between the North American and the Eurasian plates that are moving in opposite directions.

Quakes are more severe along the Pacific Plate in part because it is denser than the others. When the Pacific Plate hits a continental plate, it tends to slide underneath, something geologists call "subduction." There are several "subduction zones" along the ridges of the Pacific Plate, including a couple off the coast of British Columbia.

The sheer size of the Pacific Plate means these zones are massive, sometimes stretching hundreds of kilometres. So when a piece gets stuck and finally gives way, the force is often far more powerful than when other plates interact. Japan's earthquake on Friday measured 8.9 on the Richter scale, making it about 1,000 times more intense than the quake that struck Haiti last year. Reports also suggest the force of Friday's earthquake was so powerful it moved the island of Hensho 2.4 metres to the east.

The Pacific Plate is also grinding alongside the North American Plate in California along the San Andreas fault, which geologists call a "transform fault." Los Angeles lies on the Pacific Plate while San Francisco is on the North American Plate. The plates are moving in opposite directions, which means that in about a million years Los Angeles will end up in Alaska (providing Alaska doesn't move). California has had several earthquakes, the strongest hit San Francisco in 1906 and measured 7.9 on the Richter scale.

Japan is vulnerable because it sits on top of the intersection of the Pacific Plate and at least three others - the Eurasian Plate, a sub plate and the Philippine Plate. Geologists aren't exactly sure how many plates and sub plates are in the area, but they are all colliding and grinding against each other. In fact, Japan was created millions of years ago by all of this plate shifting.

Friday's quake is believed to have been centred in a subduction zone where the Pacific Plate collides with part of the Eurasian Plate. The Pacific Plate has been sliding underneath the other plate for centuries and earthquakes are common. The Pacific Plate is also sliding under the Philippine Plate farther south, and a major quake there would impact Tokyo.

While geologists have long expected a major earthquake to hit Japan, the power of Friday's earthquake left many of them stunned. "It's hard for us to grasp just how big an earthquake this is," said Tom Brocher, an earthquake specialist at the U.S. Geological Survey. "This is just ginormous."

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