A tsunami can exceed mph in the deep ocean but slows to 20 or 30 mph in the shallow water near land. In less than 24 hours, a tsunami can cross the entire Pacific Ocean.
Only as it approaches land does a tsunami become a hazard. As the tsunami approaches land and shallow water, the waves slow down and become compressed, causing them to grow in height. In the best of cases, the tsunami comes onshore like a quickly rising tide and causes a gentle flooding of low-lying coastal areas. See also radar satellites capture tsunami wave images.
The behaviour of a tsunami wave when caused by subduction [as happened with Sumatra]. With diagrams. Mt Fuji seen from the sea , Hokusai, woodcut, When a wave hits the shore, it tends to act like a motorway pile-up to varying degrees. A typical beach has a run up of 3; that is, a 5 foot wave will rise to 15 feet on beaching three times the starting height of the wave. This will, of course, depend on the beach slope.
Naturally, a steep slope causes greater run up than a gentle beach. Even greater run ups would be expected in an inlet like a river or delta. Regions with run ups of approximately 40 are known. Various types of tsunami give different patterns of waves formation and propagation , this is an area of study that is rapidly developing and involves computer modelling, real-world and experimental data.
Study of the Shoemaker-Levy comet, which crashed into Jupiter in , has resulted in increased confidence in computer models. A common convenient, if grim, measure of the energy is by megatons of TNT, or numbers of Hiroshima-type bombs. Hiroshima was rated at about 15, tons of TNT.
Image credits: digitalglobe. Note that previously gray roads and light-coloured compounds are now brown from floodwater. Reports [ Most people think that a tsunami is one giant wave, like the ones they see surfers sliding down.
The first water wall is followed by other waves that can occur over a period of hours. In all, a tsunami is more like a coastal flood. Unless the observer is close to the originating cause , they will probably receive little warning of the event; that is, there will be no associated change in the weather.
On a beach, the only regularly reported clear warning sign that an dangerous sea event is going to happen soon is. Another warning sign is to see a relatively high line of frothing, white water on or near your sea horizon.
That is the tsunami wave coming in. Move inland as fast as you can, and uphill if possible, taking others with you. Forget your belongings, save yourself. Remember, the water that retreated abnormally will not stay retreated, it will return and, even it if it is not of tsunami proportions, will be a considerable mass of water. Of course, major earthquake-related tsunami events occur rarely, but they are not the sort of phenomenon in which you would wish to participate.
As you will know, water en masse can often be dangerous or, at the very least, a nuisance. Every year, a few people are killed by flash floods, breaking dams, or even a fast-rising tide when they have become trapped on a beach without local knowledge. I had the special treat of watching a fairly large naval ship moving down the channel.
I then went back to my reading. Five or ten minutes later, I was up to my armpits in water and with my clothes floating away. Tsunamis are a likely consequence of an earthquake or a landslip see next section.
Tsunami waves in the deep ocean can travel at high speeds for long periods of time for distances of thousands of kilometers and lose very little energy in the process. The deeper the water, the greater the speed of tsunami waves will be. At such high speeds, a tsunami generated in Aleutian Islands may reach Hawaii in less than four and a half hours. In , great tsunami waves generated in Chile reached Japan, more than 16, km away in less than 24 hours, killing hundreds of people.
Current Warnings Most Recent Tsunami. The Pacific Tsunami Warning Center was built in It collects seismic data from 26 member countries and a number of other nonmember participants via satellite, and keeps watch 24 hours a day for any suspicious shaking that might trigger tsunamis.
A second warning center, in Palmer, Alaska, studies local seismic activity that could trigger tsunamis heading toward the coasts of British Columbia, Washington, Oregon, and California. When seismometers detect a quake of magnitude 7. Once the warning goes out, civil-defense or local authorities must decide whether and how to evacuate coastal areas. Meanwhile, the warning centers begin monitoring water levels, using the U. National Ocean Survey's system of tide-gauge stations, found along coasts and in harbors and bays.
Changes in local tides enable the warning centers to determine if a tsunami has indeed been generated, and how big it might be. Scientists are also working on developing methods to detect tsunami movement within the ocean. Deep-sea sensors that each measure the weight of the column of water above them are now being tested. Changes in this weight could indicate that a tsunami crest has passed by. Fortunately, there has not been a Pacific-wide, tsunami-causing quake since This is not to say there have not been any recent tsunamis.
Indeed, since , some 3, people have died in local tsunamis. In July , for instance, a magnitude 7. The waves killed at least residents, toppled a concrete lighthouse, and left kelp hanging from power lines. Create a List. List Name Save. Rename this List. Rename this list. List Name Delete from selected List. Save to. Save to:. Save Create a List. Create a list. Save Back. How Does a Tsunami Work?
Grades 3—5 , 6—8 , 9— Photo: Grolier Somewhere along the vast ocean floor, a colossal plate of oceanic crust strains to dip under another.
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