4:07pm

Fri March 11, 2011
Earthquake Off Japan Coast Unleashes Tsunami

In Tsunami's Wake, A Wash Of Destruction

Originally published on Wed May 23, 2012 10:13 am

The largest earthquake known to have hit Japan has killed hundreds of people and caused widespread damage. The quake also created a large tsunami whose waves washed away boats, roads and buildings. The tsunami continued eastward across the Pacific and has struck Hawaii and now parts of the west coast of the U.S.

As the tsunami rolled past Hawaii Friday morning, some places reported surges of 6 or 7 feet, according to experts at the National Oceanic and Atmospheric Administration.

Hours later, Crescent City, Calif., at the northern end of the state, got hit harder. Rich Young, harbor master at the fishing town, said he and everyone else had been evacuated early Friday morning, as have residents of many Western seaside communities. Young says the harbor took a beating.

"The entire inner boat basin is destroyed," he says. "There are approximately 35 boats that are sunk. Boats are jumbled on top of each other, and it's just a hell of a mess."

Young, speaking from an emergency center near the town, says no injuries had been reported.

The first wave measured at about 6 feet, and though it didn't push water up onto land, the huge amount of water did destroy docks and boats. A tide of that size is not abnormal — but it usually rises slowly over eight hours.

"In a tsunami, it happens in 15 or 20 minutes," Young says. "You just get this enormously strong current. It's like Niagara Falls in a harbor."

As Young was describing the damage, he got word of yet another wave — this one, 8.1 feet high.

Rapid Response From The Warning Systems

Those waves started on the other side of the Pacific and took about 12 hours to cross the ocean. The amount of energy released was about 30 times that of the 1906 San Francisco earthquake.

Japan's earthquake was a subduction zone quake, meaning a section of the seafloor suddenly thrusts up and down. That, explains geophysicist Eric Geist of the U.S. Geological Survey, lifts the water above the seabed all the way up to the surface.

"You're lifting the whole water column up above where it's not supposed to be, and then the tsunami waves spread out from there," Geist says.

Japan has what scientists say is the most comprehensive tsunami warning system in the world. Engineers have stretched cables out from the land and attached instruments that measure water pressure and tides.

NOAA's Pacific Tsunami Warning Center also operates its own set of warning sensors — 32 of them in the Pacific alone.

"Within nine minutes, our Pacific Tsunami Warning Center had a warning out for Japan, Russia, Marquis Island and the Northern Marianas," says Laura Furgione, a NOAA scientist. "And 12 minutes later, our West Coast and Alaska Tsunami Center had a message out."

Uncertainty In Predicting Wave Height

But Geist says that's not a lot of time for Japanese coastal communities. The nearest city to the quake's epicenter was just about 80 miles away, and Geist says there may have only been about 15 minutes' warning.

"Even with a good system like that, there is just a limited amount of time you can get out of harm's way in terms of a tsunami," Geist says. "Some people will always be caught up in that kind of short time frame."

As the waves traveled east toward Hawaii and North America, the warning system's pressure sensors on the seafloor monitored the tsunami's speed and predicted when the wave might hit.

Once a tsunami reaches shallower water, the mass of water bunches up — there's nowhere for it to go but up, and the wave bounces off land masses, which creates a chaotic pattern of new waves.

"It's very difficult to predict exactly how high the tsunami is going to be, so that uncertainty is something we're still having to live with, I think," Geist says.

Another uncertainty: how many more waves are still to come. Scientists are warning coastal residents that waves could continue to arrive for hours after the first one.

Copyright 2012 National Public Radio. To see more, visit http://www.npr.org/.