How Do Tsunamis Work?

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Tsunami and earthquake damage seen from the air above Sendai, Japan, one day after an earthquake measuring 8.9 on the Richter scale hit the northeast coast on March 11, 2011.

Today, a magnitude 7.3 earthquake struck off the coast of Japan’s Miyagi prefecture, generating a 3-foot-high tsunami. That, of course, is nothing compared to the tsunami that struck the same region last year, which was as high as 65 feet in some areas. That tsunami left 19,000 dead or missing, destroyed miles of coast line, and caused meltdowns at the Fukushima Dai-Ichi nuclear plant.

Tsunamis have have been wreaking similar havoc for centuries. Since 1850 alone, tsunamis have been responsible for taking 420,000 lives and causing billions of dollars of damage. How do these waves work?

Don’t Call It A Tidal Wave

Tsunamis have nothing to do with the wind-generated waves we're used to seeing, or the tides—they’re a set of ocean waves caused by the rapid displacement of water. Most commonly, this happens when large underwater earthquakes push up the seabed; the larger and shallower the earthquake, the bigger the potential tsunami. Once generated, the waves split: A distant tsunami travels out into the open ocean, while a local tsunami travels toward the nearby coast. The speed of the waves depends on the depth of the water, but typically, waves roll across the ocean at speeds between 400 and 500mph.

It’s not only the method of generation that differentiates tsunamis from wind-generated waves. On average, wind waves have a crest-to-crest wavelength—the distance over which the wave’s shape repeats—of approximately 330 feet and a height of 6.6 feet. A deep ocean tsunami will have a wavelength of 120 miles and amplitude (the distance from the peak of the wave to its trough) of only about 3.3 feet. This is why tsunamis are difficult to detect in the open ocean.

As a tsunami approaches the shore, the wave compresses: Its speed and wavelength decrease while its amplitude grows enormously. Most waves arrive on-shore not as a huge wave but as a fast-moving tidal bore that floods the shoreline. However, if the trough of the wave arrives before the ridge, or peak, the sea will recede from the shore, exposing normally submerged areas, as the trough builds into a ridge. This can serve as a brief warning that a tsunami is about to occur.

Other causes of tsunamis include underwater landslides and explosions. Another type of wave, called a mega-tsunami, is caused by above-water landslides or glacier calving. The largest recorded mega-tsunami struck in Alaska’s Lituya Bay in 1958; an earthquake triggered a landslide that displaced so much water that the waves created were 470 feet taller than the Empire State Building.

Monitoring

Like earthquakes, tsunamis can’t be predicted—but that doesn’t mean scientists aren’t trying to figure out ways to warn people before the flooding starts. Using a system of buoys called DART—Deep-Ocean Assessment and Reporting of Tsunamis—researchers can monitor ocean wave height in real time. When an earthquake occurs that scientists believe is likely to trigger a tsunami, these strategically placed buoys send reports on sea level change back to tsunami warning centers. There, scientists use that data to create a model of the potential tsunami’s effects and decide whether or not to issue a warning or make populations evacuate.

Recently, the DART system took a star turn: Director Peter Berg used it in the film Battleship as a method of creating the game’s iconic grid. (The Hollywood version of DART is much more robust than the real world version, which has just 39 buoys.)

Location, Location, Location

Because they’re mostly generated by quakes associated with the movement of oceanic and tectonic plates, tsunamis most frequently occur in the Pacific, where oceanic plates slide under lighter continental plates, fracturing them and creating vertical movement. However, scientists recently reported that East Coast tsunamis are possible; the most likely cause would be a submarine landslide.

And then, of course, there's the fear of a mega-tsunami generated by a landslide off La Palma in the Canary Islands, which some scientists believe is a real possibility; others, however, don't think there's reason to worry.

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December 7, 2012 - 10:00am
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