Frequently asked questions about Tsunamis
Thanks to Ainullotfi Abdul Latif who post this article to islah-net yahoogroups.
What is a tsunami?
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.
Where do they occur most frequently?
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 more rare -the last big one was in the 19th Century. While tsunamis occur in the Atlantic Ocean, they are infrequent.
What caused this week's earthquake and tsunami in the Indian Ocean?
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 theEurasian 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, the most powerful in 40 years and fourth strongest in the last 100 years.
How does a tsunami travel so fast and far across an ocean?
After an underwater earthquake, waves spread out 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 around 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.
Is a tsunami the same as a tidal wave?
Yes, although technically the term 'tidal wave' is a misnomer because tsunamis are not caused by the lunar cycle. The word tsunami is Japanese, a combination of 'tsu," meaning harbor, and 'nami," meaning wave.
Can underwater earthquakes and tsunamis be predicted?
Not very well. While scientists have invested an enormous amount of effort in attempting to predict submarine earthquakes, many 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 over 50 percent.
How far can the waves travel and still cause harm?
Thousands of miles. The waves are so powerful they can cause devastation thousands of miles from the earthquake's epicenter. For example, a 1960 Chilean earthquake caused devastating tsunami waves as far away as Hawaii and Japan. This week's tsunami caused deaths in Somalia, 3,000 miles from the quake's epicenter.
Why did the water recede right before the tsunami hit shore in Asia?
Because a tsunami is a series of waves, sometimes the trough the lowest point in a wave -reaches shore first and the sea looks like 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, with each full wave separated by time spans between 10 minutes and many hours. The first wave is often not the most powerful.
What are the warning signs of a tsunami?
People might feel an earthquake that causes them to fall or hold onto something, although not all tsunamis are preceded by a noticeable tremor. If a tsunami-generating earthquake occurs a short distance from shore, waves could arrive within five or 10 minutes, with little time for warning. If an earthquake occurs thousands of miles away, it could take hours to reach shore, giving scientists time to issue warnings. Warning systems exist in high-risk tsunami regions such as the Pacific.
What should people do if they expect or see one?
Get to high ground immediately. Steel and/or concrete buildings of six or more stories in height are probably safe if you get to the third floor or above.
Why wasn't anyone warned about the Indian Ocean tsunami?
Because of the rarity of tsunamis in the Indian Ocean, a warning system had not been set up in that region.
Why are authorities worried about outbreaks of disease after the tsunami?
The calamity made clean water hard to come by, as water treatment systems and wells were flooded with salt water. That has left precious little water for drinking, cooking, and bathing. Decomposing bodies are also a peril because of the bacteria and viruses they can release. And survivors are often left without shelter from mosquitoes and other predators. Now, survivors face new threats as they jam into refugee camps where respiratory illnesses can be a major source of death, especially among children.
What are the diseases most feared after this kind of disaster?
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. In the aftermath of disasters such as Hurricane Andrew in 1992, the biggest worries included wounds from nails and other detritus left behind. Such injuries could expose tsunami survivors to tetanus and other diseases.
What is being done to reduce the threat of disease?
The World Health Organization is providing chlorine and water purification tablets in an attempt to make water suitable for drinking. Relief agencies are supplying mosquito nets and other materials to protect survivors from the elements. The global health agency is also activating a surveillance system to closely track outbreaks of disease, to provide prompt treatment and to contain spread.
Sources: International Tsunami Information Center, National Oceanographic and Atmospheric Administration; City and county of Honolulu; University of Washington; WHO; U.S. Centers for Disease Control; Dr. Irwin Redlener; news reports.