Why do tsunamis move so fast

The 27th February, Chilean main event, at , was originally assessed at 8. It was detected in Japan after a little over 24 hours, a distance of about 17, km or over 10, miles. So the tsunami appears to have been propagated at approximately kph or or mph; that is, about as fast as an airliner. This is probably the main source tracking the Sumatra tsunami. An animation of the first few hours only, so far.

Bangladesh appears to have escaped lightly, despite having 17 million people living less than one metre above sea level.

Another animation of the first three hours. These show weaker waves going North and South, than the waves going East and West. This is probably why much of India and Bangladesh has been little affected. Seismic diagram of the Sumatran quake of 26 December A tsunami animation of the Pacific region [2. For example, in an earthquake in Chile caused a tsunami that swept across the Pacific to Japan. Tsunami waves are distinguished from ordinary ocean waves [1] by their great length between peaks, often exceeding miles in the deep ocean, and by the long amount of time between these peaks, ranging from five minutes to an hour.

The speed at which tsunamis travel depends on the ocean depth. 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. Tsunami speed can be computed by taking the square root of the product of the acceleration of gravity, which is At rates like this, a tsunami will travel from Alaska's Aleutian Islands to Hawaii in about five hours; or from the Portugal coast to North Carolina in eight and a half hours.

Please Contact Us. Toggle navigation JetStream. A: I know of two historic tsunamis in the Pismo Beach area. On November 22, , a tsunami occurred along the central California coast.

It was observed at six communities, including Pismo Beach. This tsunami doesn't appear to have been associated with a felt earthquake and is usually attributed to a submarine landslide.

On November 4, , a magnitude 7. The oldster was absolutely correct, and I'm glad he passed his information on to you. Q: Regarding the development or improvement of a tsunami warning system, have the major hotel chains been asked to participate?

It seems as though international hotel chains would make ideal candidates for local warning systems, because:. It is only one small part of what needs to be done, but given how much more likely it is to see a hotel than a government safety office on waterfront property, it seems like a natural way to augment the government contacts who could help quickly spread the word when there is a tsunami alert.

A: Large hotels are very involved with tsunami evacuation and dissemination of tsunami warnings in Hawaii. Many of the hotels are designed with tsunamis in mind, with lower floor areas that are open and the waves can easily travel through without affecting the building's structural integrity. Hotel staff help manage evacuations, and in areas where the hotel may be cut off from higher ground inland, the upper floors may be used for vertical evacuation.

In the Pacific Northwest, a number of hotels have also become involved with tsunami planning efforts.

In Crescent City, California, you will find tsunami information often posted in hotel rooms, and many hotels in Oregon, Washington, and British Columbia are also active in promoting tsunami awareness and appropriate behavior when an earthquake hits. It would be great to get more hotels on the safety bandwagon. Unfortunately, there are still a number who are concerned that mentioning tsunamis may scare their customers. Others are concerned about legal ramifications. I hope one outcome of the Indonesian earthquake is a greater acceptance in the tourist industry of their responsibilities in reducing the risk for their guests.

Q: When all is said and done, it appears to this north coast estuary dweller that the only recourse for a local tsunami is self-help, i. Please comment. A: I know Seaside well and have attended several meetings in your lovely community. There are tsunami hazard maps posted in your community and an active public education program about tsunami hazards. Awareness and personal understanding of what to do the next time a strong earthquake occurs are the main way to reduce losses when the next tsunami hits.

For you, the earthquake is the warning and you may have no more than 10 to 15 minutes between ground shaking and the arrival of the first significant waves. Don't take a car, because the roads may be damaged from the earthquake shaking and others may have created a massive log jam.

Do be aware of the tsunami hazard maps that are available for your community and know where you are going to head. The last Cascadia earthquake occurred on a night in January, so be prepared to respond in the dark and in the rain.

Yes, you will need to rely on yourself, but the technical information on areas at risk should be able to guide you in your response. There is a FEMA-sponsored pilot project that is using Seaside as the case study for combining inudation studies, structural response of buildings, and human response in a tsunami situation. We are all eager to hear the results of that study, and I hope it will provide you with more information. Until the NOVA episode, I was under the impression that a long, shallow, sloping beach like ours would attenuate a tsunami wave, thereby potentially causing less damage.

Would such a wave not crest farther out and be lower than one slamming against a sharply sloping beach? A: Actually, a long, uniformly sloping beach is more likely to allow coherent buildup of tsunami amplitudes.

Tsunami waves don't "crest. In deep water, wind waves don't feel the seafloor. We call them deep-water waves, and their speed depends on the driving force of the winds.

When they reach the coast, they start to feel the bottom once the water is shallow enough. This causes the wave to crest and break it sort of falls over on itself. Tsunami waves are never deep-water waves, because even in the deepest ocean they still feel the bottom. As the sea bottom becomes shallower, the velocity of the tsunami slows down, and to conserve energy the wave amplitude builds up.

Changing the offshore slope affects how quickly the tsunami slows and builds in amplitude. The wave crests of a tsunami can range from a few to a hundred kilometers or more apart as they travel across the ocean.

As the waves approach the coast, their wavelength decreases and wave height increases. On the open ocean, the wavelength of a tsunami may be as much as two hundred kilometers, many times greater than the ocean depth, which is on the order of a few kilometers. In the deep ocean, the height of the tsunami from trough to crest may be only a few centimeters to a meter or more - again depending on the generating source.

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.