Tsunami waves started at the epicenter of the earthquake off the east coast of Japan (in black surrounded by magenta) and traveled as far as the west coast of North America, seen here at the top right of the image.
The science surging behind the tsunami
by Chelsea Whyte
Tsunami waves devastated Japan early Friday as the earth’s fifth largest earthquake since 1900 struck off the eastern coast of the island nation.
The 23-foot waves left more than 300 people dead and another 350 missing as of Friday morning, according to the Associated Press.
“It’s very big,” said Seth Stein, professor of geology at Northwestern University. “It’s much bigger than any earthquake we’ve ever had in the lower 48 states.”
Mother Earth packs a punch with the seismic energy released from an earthquake like this one. At 8.9 on the Richter scale, “it’s much bigger than the world’s biggest nuclear bomb,” Stein said.
This earthquake is the largest recorded shock to hit Japan, containing nearly 60 times more energy than the 2010 earthquake in Haiti. The tsunami it generated roared across the Pacific Ocean, but lost much of its bite by the time it reached the beaches of Hawaii nearly 4,000 miles away.
The Pacific Plate, which makes up most of the foundation of the Pacific Ocean, normally operates in a constant state of tension against the continental plate that sits under Asia. When the plates slip out of their usual alignment, earthquakes result.
“What’s happening off the east coast of Japan is there’s an ocean plate which is dipping down underneath the plate that’s making up Japan and Asia. One is sliding underneath the other,” said Andrew Campbell, an associate professor of mineral physics at the University of Chicago. “And that sliding action causes a lot of earthquakes.”
Tectonic plates are made of rock that is usually a few dozen miles thick. The rock is basalt in Japan. The plates rest on the asthenosphere, the first layer of Earth’s mantle, made of hot rock that is softer than the surface crust.
“It’s not molten, but it’s hot. As you heat rocks up, they can sort of ooze,” said Campbell. “They become soft. This plate is able to move along and the soft rock underneath it sort of oozes.”
This viscous layer of the Earth's core is the texture of silly putty, said Campbell. It acts sort of like a solid and you can make it into a solid shape, but if you sit and watch it long enough, it will just ooze flat.
When the harder rock of the Pacific Plate slid over the Earth’s mantle in Japan, it caused not only an earthquake but a tsunami.
“The sea floor pops up and, as a result, it displaces the water and pushes it up,” Stein said. This action accelerates a massive rush of water. “The tsunami travels across the water pretty fast and then as it gets close to the shore, it slows down and as it slows down it gets bigger,” he said.
The tsunami waves bunch up like cars at a highway off-ramp, condensing and resulting in a growing surge of energy and pressure. So, as the wave slows and grows in size, it builds its power. Tsunami waves are not that different from any other wave, said Stein. They’re just lots bigger.
The National Weather Service warned that waves from the tsunami in Japan would reach the west coast of North and South America throughout the day Friday. The waves travel relatively fast and lose power as they cross the ocean.
“The tsunami wave travels about the speed of a jet plane, about 500 miles per hour,” Stein said. “So it takes as long to fly there as it would for the wave to get here."
By the time the waves reached the west coast of the United States, they had diminished in power, leaving Hawaiian and Californian coasts relatively undamaged, according to Reuters. The devastation is left mostly to Japan, where Reuters is projecting a death toll of at least 1,000 people.
http://news.medill.northwestern.edu/chicago/news.aspx?id=183077
