Tsunamis: Facts About Killer Waves

[silver_corolla]

National Geographic News
December 27, 2004

The Christmas weekend tsunami that was generated by the most powerful earthquake in decades is believed to have killed more than 20,000 people and displaced a million more.

The epicenter of Sunday's magnitude-9 quake was under the Indian Ocean near the Indonesian island of Sumatra. Within hours killer waves slammed into the shores of Indian Ocean countries, snatching lives and demolishing property from Africa to Thailand.

While relatively rare in recent centuries in the Indian Ocean, tsunamis have been generated in every ocean of the world, but none more so than the Pacific Ocean. Sooner or later every shoreline is struck by a tsunami.

Find out the essential facts about these giant waves: what causes them, how they work, the most disastrous on record, and signs that warn of their arrival. This is information you need to protect yourself.

• A tsunami is a series of great sea waves caused by an underwater earthquake, landslide, or volcanic eruption. More rarely, a tsunami can be generated by a large landslide into the ocean, or a giant meteor impact with the ocean.

Scientists have found traces of an asteroid-collision event that they say would have created a giant tsunami that swept around the Earth several times, inundating everything except the mountains 3.5 billion years ago. The coastline of the continents was changed drastically and almost all life on land was exterminated. (Read the story)

• Tsunami (pronounced soo-NAH-mee) is a Japanese word. Tsunamis are common in Japan and many thousands of Japanese have been killed by them in recent centuries.

• An earthquake may generate a tsunami if it is of sufficient force and there is violent movement of the earth, causing substantial and sudden displacement of water. But much also depends on the depth of the water at the earthquake site.

• A tsunami is not a single wave but a series of waves, also known as a wave train. The first wave in a tsunami is not necessarily the most damaging.

• Tsunamis can take from a few minutes to as much as a day to travel to a shore. The closer land is to the event that generated the tsunami the sooner it will be struck.

• Tsunamis are not tidal waves. Tsunamis can be very long (as much as 60 miles, or 100 kilometers) and be as far as one hour apart. They are able to cross entire oceans without great loss of energy.

One of the most striking facts about tsunamis is that an earthquake on one side of the Pacific Ocean can cause giant waves and destruction on the other side. Scientists say that a great earthquake of magnitude 9 struck the Pacific Northwest in 1700, and created a tsunami that caused flooding and damage on the Pacific coast of Japan. (Read the story)

• When the ocean is deep tsunamis can travel unnoticed at speeds up to 500 miles per hour (800 kilometers per hour), crossing the entire ocean in a day or less. Scientists are able to predict arrival times of tsunamis based on their knowledge of water depths and distances.

• A tsunami may be less than a foot (30 centimeters) in height in the open ocean, and may not even be noticed by sailors. But the powerful energy of the wave travels rapidly through the ocean. Once a tsunami reaches shallow water it is slowed down down, compressing the wave energy and causing the top of the wave to heighten dramatically.

• Geological features such as reefs, bays, river entrances, and undersea formations may dissipate the energy of a tsunami. In some places a tsunami may cause the sea to rise vertically only a few inches or feet. In other places tsunamis have been known to surge vertically as high as 100 feet (30 meters). Most tsunamis cause the sea to rise no more than 10 feet (3 meters).

Flooding can extend inland by a thousand feet (300 meters) or more. The enormous energy of a tsunami can lift giant boulders, flip vehicles, and demolish houses. Knowledge of the history of tsunamis in your area is a good indicator of what is likely to happen in the future.

• Tsunamis do not necessarily make their final approach to land as a series of giant breaking waves. They may be more like a very rapidly rising tide. This may be accompanied by much underwater turbulence, sucking people under and tossing heavy objects around. Entire beaches have been stripped away by tsunamis.

Many people have said a tsunami sounds like a freight train.

• The 2004 Indian Ocean tsunami could rank as one of the most devastating on record. News reports so far suggest that more than 20,000 people may have lost their lives, many of them washed out to sea.

The most damaging tsunami in history was the one that killed an estimated 40,000 people in 1782 following an earthquake in the South China Sea. In 1883 some 36,500 people were killed by tsunamis in the South Java Sea, following the eruption of Indonesia's Krakatoa volcano. In northern Chile more than 25,000 people were killed by a tsunami in 1868.

• The Pacific is by far the most active tsunami zone, according to the U.S. National Oceanic and Atmospheric Administration (NOAA). But tsunamis have been generated in other bodies of water, including the Caribbean and Mediterranean Seas, and the Indian and Atlantic Oceans. North Atlantic tsunamis included the tsunami associated with the 1775 Lisbon earthquake that killed as many as 60,000 people in Portugal, Spain, and North Africa. This quake caused a tsunami as high as 23 feet (7 meters) in the Caribbean.

• The Caribbean has been hit by 37 verified tsunamis since 1498. Some were generated locally and others were the result of events far away, such as the earthquake near Portugal. The combined death toll from these Caribbean tsunamis is about 9,500.

• Large tsunami waves were generated in the Marmara Sea in Turkey after the Izmit earthquake of 1999.

• What are the warning signs? An earthquake is a natural tsunami warning. If you feel a strong quake do not stay in a place where you are exposed to a tsunami. If you hear of an earthquake be aware of the possibility of a tsunami and listen to the radio or television for additional information. Remember that an earthquake can trigger a tsunami thousands of miles across the ocean many hours after the seismic event.

• Many people have reported that an approaching tsunami is sometimes preceded by a noticeable fall or rise in the water level. If you see the ocean receding unusually rapidly or far it's a good sign that a big wave is on its way. Go to high ground immediately.

• Because tsunamis can approach the shore as fast as 100 miles per hour (160 kilometers per hour) it is often too late to get away if you see one. An approaching tsunami is not something to be admired unless you are safely on high ground.

• Remember that a tsunami is a series of waves and that the first wave may not be the most dangerous. The danger from a tsunami can last for several hours after the arrival of the first wave. A tsunami wave train may come as a series of waves or surges that are five minutes to an hour apart. The cycle may be marked by a repeated retreat and advance of the ocean. Stay out of danger until you hear it is safe.

• A tsunami may be small at one point of the shore and large at another point a short distance away. Do not assume that because there is minimal sign of a tsunami in one place it will be like that everywhere else.

• Tsunamis can travel up rivers and streams that lead to the ocean. Stay away from rivers and streams that lead to the ocean as you would stay away from the beach and ocean if there is a tsunami.

• It's always a good idea to keep a store of emergency supplies that include sufficient medications, water, and other essentials sufficient for at least 72 hours. Tsunami, earthquake, hurricane—an emergency can develop with little or no warning.

• NOAA advises that since tsunami wave activity is imperceptible in the open ocean, vessels should not return to port if they are at sea and a tsunami warning has been issued for the area. Tsunamis can cause rapid changes in water level and unpredictable dangerous currents in harbors and ports. Boat owners may want to take their vessels out to sea if there is time and they are allowed to do so by port authorities. People should not stay on their boats moored in harbors. Tsunamis often destroy boats and leave them wrecked above the normal waterline.

• Heightened awareness of the potential for a tsunami to inundate the U.S. western coastline has caused NOAA, the U.S. Geological Survey, and the Federal Emergency Management Administration to initiate a program to predict tsunamis more accurately. As a tsunami traverses the ocean, a network of sensitive recorders on the sea floor measures pressure changes in the overhead water, sending the information to sensors on buoys, which, in turn, relay the data to satellites for immediate transmission to warning centers.

• The Tsunami Warning System (TWS) in the Pacific, comprised of 26 member countries, monitors seismological and tidal stations throughout the Pacific region. The system evaluates potentially tsunamigenic earthquakes and issues tsunami warnings. There is no international warning system for tsunamis in the Indian Ocean.

• Use your common sense. If you feel or hear of a strong earthquake do not wait for an official tsunami warning. Tell your family and friends to join you in leaving for high ground.

[BlueBimmer]

very informative!

[jedi]

sana dito sa pinas eh walang maligaw na Tsunami....

[roninblade]

syet, looking at the major tectonic plates, the philippines is in a dangerous position if something similar happens here.

[odell]

sa thailand nga un kabatch ko dati sa saint jude nawawala pa until now...

[alnr]

thanks for the post silver_corolla

huwag naman sana mangyari sa atin yun, at sana wala nang mauulit na ganun...

[Tifosi]

Grabe talaga itong earthquake na nangyari; even Somalia which is approx. 4,500 miles away was affected by the tsunami
http://www.cnn.com/2004/WORLD/africa....ap/index.html

Wag naman sana mangyari sa atin to. If we look at the pic roninblade posted, the whole eastern side of our country would be at risk kung may mangyaring earthquake like the one off of Sumatra!

[van_wilder]

sana dito sa pinas eh walang maligaw na Tsunami....

di ko lang maalala, pero once tinamaan ata tayo ng malakas ng tsunami or tidal wave, di ko lang maalala... kasi hindi ko dati alam yun difference, nabasa ko lang sa guiness book of world records. ngayon alam ko na ang difference...

[Prometheus]

Lubog na nga tayo sa tatlong bagyo na dumaan. Sana di naman mangyari sa atin to, wala ng pundo ang gobyerno, unless kunin sa pork barrel. Kutongan ang mga opisyal.

[Karding]

Another representation of the plate: