Under the force of gravity, the water wants to return to a stable position. All tsunamis involve the entire column of water from the sea floor to the ocean surface. The deeper the water, the more potential energy and driving force for the tsunami. In the deep ocean, a tsunami is about as fast as a jet airplane, traveling around miles per hour. In the very deepest parts of the oceans, the speed can be over miles per hour. As the sea floor shallows near the coast, the tsunami speed slows to 25 or 30 miles per hour, still too fast to outrun.
Tsunamis are always largest at the coast. In the deep ocean, the height of a tsunami is usually too small to be noticed, even by small boats in the water. As the tsunami approaches shallower water, the speed slows and the water begins to build up. A tsunami that is only a few feet high in deep water can build to tens of feet along the closest coasts. A tsunami spreads out as it travels, losing energy, but if the tsunami is very large, it can still be big enough to cause damage thousands of miles away.
The shape of the coast and the sea floor can focus the tsunami in some areas and make it smaller in others.
Scientists studying tsunami hazards consider all these factors — the size and nature of the source, and the coastal and sea floor shape in determining the size of a tsunami. The term tsunami refers to the mechanism of generation not how big or small the waves are.
Every year between 10 and 20 tsunamis are detected. And all tsunamis, whether large or small, travel at the same speed. The speed of a tsunami depends only on how deep the water is and is independent of how large or the type of source that caused it.
There are true tidal waves. They are caused by the gravitational attraction of the sun and moon and can be predicted many years in advance, just by knowing orbital positions and local site conditions. There are some places where these conditions can produce a truly tsunami-like waves. The Qiantang River in East China is the site of a famous tidal bore, a surge of water that rushes from the river mouth inland.
When the tidal conditions are just right, the bore can reach heights of 30 feet and rush inland at speeds of 25 miles per hour. It has become a tourist attraction and some even try to surf it. A: Unfortunately, scientists cannot predict when the next large earthquake will occur, what its size will be, and whether a tsunami will be generated. From history, however, we do observe that large or great earthquake don't occur often. And, since not all earthquakes generate tsunamis, it is a very infrequent occurrence.
While there have been many aftershock earthquakes in Indonesia, Nicobar and Andaman Islands in the last day 27 so far of magnitude greater than 5. Of some concern is that sometimes, smaller earthquake might trigger underwater landslides which could generate tsunamis, but these tsunami generally only affect the area very near to the source. A: The Krakatau volcanic eruption has generated a destructive tsunami higher than 40 m on the Indonesian coast where more than 36 lives were lost.
A: Yes, but only if the aftershock is very large. Such a large aftershock is now very unlikely. No aftershocks of the magnitude 9. The great Chile earthquake of magnitude 9.
A: Although tsunamis are commonly depicted as a giant breaking wave with a crest towering over the land, this image is hardly if ever the case. Instead tsunamis can more accurately be described as a rapidly-rising tide without a developed wave face, which quickly and forcefully floods low-lying coastal areas.
Ironically, in deep, open-ocean water, tsunami waves are often less than a meter high and can travel at speeds up to 1, kilometers per hour. The crest of this wave can be several meters high by the time it reaches the shoreline. In this case, instead of extremely high water levels, the first sign of a tsunami is what appears to be an unusually low low-tide. Although onlookers might be intrigued by this unusual site, this major withdrawal of the sea should be taken as a warning that a tsunami wave will soon follow.
This is a natural warning; people should move inland away from the shoreline. When you can see the wave you are too close to escape. The Indian Ocean tsunami traveled as much as 3, miles nearly 5, kilometers to Africa, arriving with sufficient force to kill people and destroy property. Scientists say that a great earthquake of magnitude 9 struck the Pacific Northwest in , and created a tsunami that caused flooding and damage on the Pacific coast of Japan.
Scientists are able to calculate arrival times of tsunamis in different parts of the world based on their knowledge of when the event that generated them occurred, water depths, and distances. But the powerful shock wave of energy travels rapidly through the ocean as fast as a commercial jet. Once a tsunami reaches shallow water near the coast it is slowed down.
The top of the wave moves faster than the bottom, causing the sea to rise dramatically. 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 feet 30 meters. Most tsunamis cause the sea to rise no more than 10 feet 3 meters. The Indian Ocean tsunami caused waves as high as 30 feet 9 meters in some places, according to news reports.
In other places witnesses described a rapid surging of the ocean. Flooding can extend inland by a thousand feet 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 a future tsunami event.
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. News reports so far suggest that more than , people may have lost their lives, many of them washed out to sea. The most damaging tsunami on record before was the one that killed an estimated 40, people in following an earthquake in the South China Sea.
In some 36, people were killed by tsunamis in the South Java Sea, following the eruption of Indonesia's Krakatoa volcano. In northern Chile more than 25, people were killed by a tsunami in 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 Lisbon earthquake that killed as many as 60, people in Portugal, Spain, and North Africa. This quake caused a tsunami as high as 23 feet 7 meters in the Caribbean. 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, 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 killer waves thousands of miles across the ocean many hours after the event generated a tsunami.
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. Many people were killed by the Indian Ocean tsunami because they went down to the beach to view the retreating ocean exposing the seafloor.
Apparently they were unaware that this phenomenon precedes a killer wave. Experts believe that a receding ocean may give people as much as five minutes' warning to evacuate the area. An approaching tsunami is not something to be admired unless you are safely on high ground.
0コメント