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Deadly waves explained

Q. What is a tsunami?

A. A tsunami is a series of ocean waves that are generated by a large-scale disturbance of seawater. Most tsunamis are generated from earthquakes, but they can also occur after volcanic eruptions, landslides, and meteor impacts. The most destructive tsunamis are created by large earthquakes with an epicenter or fault line near or on the ocean floor. Usually, it takes an earthquake with a strength above 7.5 on the Richter scale to generate a destructive tsunami.

Q. Where do they occur most frequently?

A. The Pacific Ocean. The ocean covers more than one-third of the earth's surface and is surrounded by a region with many earthquakes and volcanoes known as the ''ring of fire." Tsunamis in the Indian Ocean are much rarer -- the last big one was in the 19th century. While tsunamis occur in the Atlantic Ocean, they are infrequent.

Q. What caused this week's earthquake and tsunami in the Indian Ocean?

A. The earth's crust consists of slowly-moving tectonic plates, and two of these plates collided deep under the Indian Ocean about 155 miles southeast of the Indonesian island of Sumatra, with the Indian plate diving under the Eurasian plate. The collision suddenly lifted a strip of seafloor hundreds of miles long, by an estimated 20 to 50 feet, which displaced a massive amount of water and started the tsunami. The earthquake registered 9.0 on the Richter scale, making it the most powerful in 40 years and fourth strongest in the last 100 years.

Q. How does a tsunami travel so fast and far across an ocean?

A. After an underwater earthquake, waves spread in all directions, much like the ripples a rock creates when it is thrown into a lake. The widely spaced tsunami waves, carrying an enormous amount of energy, travel unobstructed and often unnoticeable in the deep ocean at speeds of approximately 500 miles per hour. When these waves approach coastal areas, the sloping seafloor redirects the wave's energy upward. Some waves can reach 50 feet or higher and travel inland a mile or more. Some eyewitnesses reported this week's tsunami reached heights of 22 to 25 feet.

Q. Is a tsunami the same as a tidal wave?

A. Yes, although technically the term ''tidal wave" is a misnomer because tsunamis are not caused by the tidal cycle. The word tsunami is Japanese, a combination of ''tsu," meaning harbor, and ''nami," meaning wave.

Q. Can underwater earthquakes and tsunamis be predicted?

A. Not very well. While scientists have invested an enormous amount of effort in attempting to predict underwater earthquakes, many still occur with little or no warning. Tsunamis are also difficult to predict because not every large earthquake produces a strong tsunami. The false alarm rate for tsunamis is more than 50 percent.

Q. How far can the waves travel and still cause harm?

A. The waves are so powerful they can cause devastation thousands of miles from the earthquake's epicenter. This week's tsunami caused deaths in Somalia, 3,000 miles from the quake's epicenter.

Q. Why did the water recede right before the tsunami hit shore in Asia?

A. Because a tsunami is a series of waves, sometimes the trough -- the lowest point in a wave -- reaches shore first and the sea looks as if it is emptying, an effect often called a drawdown. Minutes later, the crest of the wave hits. This cycle can be repeated for several waves.

Q. Why wasn't anyone warned about the Indian Ocean tsunami?

A. Because of the rarity of tsunamis in the Indian Ocean, a warning system had not been set up in that region.

Q. What are the diseases most feared after this kind of disaster?

A. Typhoid is a major concern because the bacterium that causes it can spread easily in fetid water. Such water can also harbor cholera and hepatitis A. Authorities are also watching for outbreaks of malaria, a mosquito-borne disease. Conditions could be disastrously ripe for an outbreak of that viral illness: Refugees often will have neither the shelter of home nor netting that can prevent mosquitoes from reaching their human targets, and brackish standing water can be an ideal breeding ground for mosquitoes.

Sources: International Tsunami Information Center, National Oceanographic and Atmospheric Administration; City and county of Honolulu; University of Washington; World Health Organization; US Centers for Disease Control and Prevention; Dr. Irwin Redlener, director of the National Center for Disaster Preparedness at Columbia University's Mailman School of Public Health; George Maul, Florida Institute of Technology; news reports.

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