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An Outline of Tsunamis and their Destructive Force

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"An Outline of Tsunamis and their Destructive Force"
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Try this experiment: Toss a coin into a kitchen sink filled with water. See the concentric circles rippling out from where the coin dropped into the water? That's the pattern of the giant killer ocean wave known as a tsunami.

"Tsunami" (soo-nah-mee) is a Japanese word meaning "harbor wave" or "big wave in port." They're also known as seismic sea waves or as tidal waves, although the latter term is inaccurate because these destructive forces have nothing to do with normal ocean tides. Earthquakes in the deep ocean cause tsunamis, and they've been around as long as the Earth's great oceans have existed. While their causes are fairly simple, their behavior and the destruction they can cause are more complicated.

According to an online brochure from the National Oceanic and Atmospheric Administration (NOAA), these giant sea waves result from earthquakes, landslides beneath the ocean, underwater volcanic eruptions and meteorite impacts. Most tsunamis occur in the Pacific Ocean, but they can happen anywhere that an earthquake or landslide displaces large amounts of water.

The most destructive event of this kind may be the stunning Indian Ocean tsunami of Dec. 26, 2004, which resulted from a catastrophic 9.0 underwater earthquake near the Indonesian island of Sumatra. A large area of land under the water moved about 10 meters (roughly 10 yards), causing massive displacement of the water above it. Within hours the tsunami struck the coastlines of 11 countries around the Indian Ocean. People were snatched out to sea by the force of the retreating wave, while others drowned on the beaches or in their coastal homes. Estimated to be moving at about 100 miles per hour when they came ashore, the tsunami's successive waves destroyed property from Indonesia north to Thailand and west all the way to Africa, a distance of some 3,000 miles (about 5,000 kilometers).

A Jan. 7, 2005, report by National Geographic News quotes the U.S. Geological Survey estimate that the earthquake that produced the 2004 tsunami released energy equivalent to 23,000 Hiroshima-type atomic bombs. The kinetic energy of that land movement was transferred to the displaced ocean water, causing it to drop and then rise up, moving with the speed of a commercial jetliner, some 500 miles per hour. The 2004 tsunami may be even more destructive than the legendary Krakatau (or Krakatoa) volcanic eruption of 1883, which produced waves topping 125 feet above sea level.

According to a USGS tsunami website, tsunamis have four stages to their life cycle: initiation, split, amplification and run-up. During the initiation stage, a portion of the seafloor lifts up and then drops, which pushes the water above it up and down. The energy from this up-and-down motion then is transferred horizontally (remember the coin and the circles?) to produce the tsunami wave. Next comes the "split" stage, when the first tsunami divides into a wave that heads out to deep ocean and one that travels toward the coast. Their respective speeds vary according to the water's depth, with deep-ocean tsunamis traveling faster than local waves.

Next comes the amplification phase, which occurs as the wave travels over the continental shelf. This phenomenon occurs with both local and deep-ocean tsunamis. As with sound waves, amplification makes tsunami waves become taller and closer together. However, the first part of the wave to reach shore is the channel between peaks known as the trough, which appears as the ocean receding from the shoreline. This is one of the first natural warnings that a tsunami is imminent, and too many people fail to heed the alarm.

The final stage is called run-up, when the wave's peak travels onshore. This stage serves as a measurement of the height of the water above sea level. Only the largest tsunamis, such as the 2004 Indian Ocean wave and the 2011 Japanese tsunami, produce giant breakers. Instead tsunamis typically act like strong, fast-moving tides with forceful surges and rapid sea-level changes. It's these strong currents, often carrying massive amounts of debris, which cause most damage.

A final and most dangerous characteristic of tsunamis is known as "persistence." Although a tsunami is imagined as a single, giant killer wave, in reality a seismic sea wave can generate multiple fronts, continuing even after it first strikes land. This happens because some of the tsunami's energy gets reflected back to the ocean. In addition, variations in coastline geography can change a tsunami's forward motion, just as water will flow around an obstruction. Sometimes a tsunami will produce a type of coastal-trapped waves known as "edge waves." These waves flow back and forth parallel to the shore. These differences in surge and amplitude can cause secondary waves to have greater height and force than the tsunami's first wave. This is why residents are urged to stay away from the coast for several hours after a tsunami strikes.

Tsunami forecasting took a big leap forward after the catastrophic Indian Ocean tsunami killed 237,000 people in Asia, according to the NOAA Center for Tsunami Research. Greater investment in technologies then under development resulted in the construction and deployment of DART – Deep-Ocean Assessment and Reporting of Tsunamis. Installed in 2009, the monitoring system, while not foolproof, has greatly improved the ability of forecasters to predict tsunamis and thereby save lives.

More about this author: Cynthia Astle

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