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This article is from the In-Depth Report The Japan Earthquake, Tsunami and Nuclear Crisis

How Does an Earthquake Trigger Tsunamis Thousands of Kilometers Away?

As Japan suffered the worst earthquake in the country's recorded history, tsunami waves fanned out across the Pacific Ocean at the speed of a jetliner
Japan, tsunami, earthquake
Japan, tsunami, earthquake



COURTESY OF NOAA CENTER FOR TSUNAMI RESEARCH

The massive magnitude 8.9 earthquake that struck near the east coast of Honshu, Japan's main island, at 2:46 P.M. local time and unleashed a fierce tsunami claiming hundreds of lives is already being felt as far away as the west coast of North America, about 8,000 kilometers away. Much of this has to do with the depth of the ocean that the tsunamis waves traversed as well as the sheer size of the quake, which was the strongest recorded in Japan's history.

The tsunami hit Hawaii about seven hours after it washed away entire towns along Japan's northern coast. Whereas the waves that struck Japan have been reported as high as seven meters, Hawaii was spared serious damage. Still, the threat of the tsunami closed ports in Honolulu and Guam and led to warnings, watches and coastal evacuations in 20 countries, including the U.S., Indonesia and Chile.

To find how a tsunami could pose a serious threat from such a great distance, Scientific American spoke with Greg Valentine, a geology professor and director of the University at Buffalo, The State University of New York Center for GeoHazards Studies. As it turns out, Valentine had been scheduled to fly to Japan Friday morning for a meeting to discuss a collaborative program on earthquake and volcano hazards. The meeting was cancelled as Japanese officials deal with the aftermath of this disaster and the possibility that more tsunamis will follow.

[An edited transcript of the interview follows.]

How can an earthquake create a tsunami?
This tsunami was probably generated by an earthquake where the crust from beneath the Pacific Ocean is diving down beneath Japan. What's happening is the uppermost 50 to 100 kilometers of the solid earth in the Pacific Ocean is generally moving to the northwest at a speed that's very slow by our standards, maybe several centimeters per year. But the crust around Japan is less dense and lighter than the crust in the Pacific. When the two come together, the ocean crust goes down, and those two plates really grind against each other. Along this area where the two are grinding, forces build up over time. And then it will suddenly snap, where the Pacific plate will go down very suddenly and the Asian plate that Japan is part of will sort of bounce up a little bit. The sudden motion of those two plates displaces a huge volume of water, and that's what causes the tsunami.

How are tsunami waves different from normal waves?
Tsunami waves travel very, very rapidly. The normal waves that we're accustomed to on the ocean are mostly driven by the wind, so they travel at the speed that the wind blows. With a tsunami, the speed depends on the depth of the water. Out in the open ocean, for example, where it's 5,000 meters deep, the speed of a tsunami's waves will be about 220 meters per second. The speed of the waves in 500-meter deep ocean drops to about 70 meters per second. If there's a lot of deep ocean between where a tsunami starts and where it's going these waves will get there extremely fast.

How does a tsunami wave change as it reaches the coast?
Part of what makes the tsunamis so damaging is that, as this mass of water is approaching a shoreline, it's also slowing down, so the water at the front of the wave is moving slower than the water coming in at the back of the wave. As a result, you get this huge piling up effect of the water.

As this approaches the shoreline, the sea level there can increase by several meters or even 10 meters—maybe 20 meters in an extreme case—and stay that way for some long period of time because these waves have a very long wavelength. So it's really different than a wind-driven wave that crashes on the shoreline and rolls back into the sea. In the case of a tsunami, the sea level at that shoreline is increasing for some period of time. Anything below that level on the shore will be flooded for a length of time.

Can the height of tsunami waves be predicted before they reach shore?
We can predict the path and the speed pretty well, but the height at a given location can be pretty hard to predict. It has to do with understanding the details of the fluid dynamics within the waves and the way the seafloor is shaped—how quickly it gets shallow.

Is there a danger of aftershocks from this earthquake creating additional tsunamis in the coming days and weeks?
Yes, there is. Often when an earthquake releases some stress along a subduction zone, such the one where the ocean crust is pushing below Japan, it can throw other parts of that zone out of equilibrium. This subduction zone is a very large-scale feature that is maybe hundreds to thousands of kilometers long, where the Pacific plate is going down beneath the Asian plate. Only a small part of that snapped to generate this earthquake and tsunami, but the fact that part of it snapped now changes the whole balance of forces along that system. So there could be another one, although most aftershocks are not as strong as the original earthquake.

How strong would an aftershock need to be to create a tsunami?
In general a magnitude 6 or higher would give you substantial enough motion to trigger a tsunami, but it also depends on the local situation, so I wouldn't say that's written in stone in any way. The higher the magnitude, the more likely an earthquake is to cause a tsunami because the magnitude reflects the amount of motion of the crust when it snaps.

But it also depends on the type of earthquake. Some earthquakes are caused when two plates are sliding horizontally past each other. The San Andreas Fault in California is a good example of this. If a horizontal slide were to happen on the seafloor, there wouldn't be as much vertical motion of the crust so that might generate a smaller tsunami. The reason subduction zones like the one near Japan are so bad for generating tsunamis is that it's a lot of up-and-down motion that really moves a lot of water.

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