Map and projection courtesy NOAA/NWS/West Coast and Alaska Tsunami Warning Center
for National Geographic News
Published February 27, 2010
Moving across the ocean at jetliner speeds, the first tsunami waves from Chile's magnitude 8.8 earthquake struck Hawaii about 12 hours after the temblor, whose death toll in Chile has now grown to 214. (Chile Earthquake Pictures: Quake Spurs Tsunami Threat.)
Meanwhile, tsunami warnings remain in effect for Russia and Japan (Video: Tsunami 101).
In the open sea, tsunami waves are low swells that do no damage. But when they hit land, they can rise up and strike with unexpected power.
Their energy comes from the earthquake itself, said Solomon Yim, interim director of the Hinsdale Wave Research Facility, at Oregon State University. "The power is generated because a large column of water has been lifted up or dropped down," he said. "This huge volume contains a lot of energy."
When the earthquake occurs offshore, as was the case in Chile, the waves propagate in two directions: toward the mainland, which can be struck in minutes, and across the open ocean.
They travel much, much faster than normal waves. That's because the entire column of ocean water (from sea surface to ocean bottom) is moving together horizontally, even in the deepest parts of the ocean, with a very long wave length. Wind waves, on the other hand move only water particles near the surface.
That difference means tsunami waves travel at 450 to 500 miles per hour (725 to 800 kilometers per hour), with wave crests more than 100 miles (160 kilometers) apart.
The first waves to hit Hawaii measured about 3 feet (one meter), news reports said. But that doesn't mean larger waves couldn't follow--or strike elsewhere, Yim said.
That's because predicting tsunami wave height is difficult. In part that's because some regions tend to focus incoming waves, making their heights unexpectedly large, "whereas nearby locations do not have much of a runup at all," Yim said.
But the magnitude of the waves also depends on details of the earthquake's motion. "If you have large uplift or drawdown, you will create a tsunami," Yim said. But earthquakes can also move the seabed back and forth horizontally. "If all the movement is horizontal, you will have zero tsunami," Yim said.
Furthermore, both an up-and-down, tsunami-producing earthquake and sideways motion can look very similar on a seismometer. The only way to determine exactly what happened is to scan the seabed, days, weeks, or even months after the event--not useful for predicting dangers from waves that can cross an entire ocean in 24 hours. "So we [have to] give predictions with a very large amount of uncertainty," Yim said.
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