Questions and Answers about Tsunami

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1.) What is a Tsunami train, how is it caused and how will it affect the height of waves?

A: Tsunami are caused by disturbances of the volume of water above the seabed. The bigger the disturbance the higher the wave. The disturbance can be caused by an earthquake moving the Earth's crust over a wide area or by volcanoes, submarine landslides and meteorite impacts with the ocean at a point source. A series of waves will quickly form and spread out from the source area. This series is called a wave train. If the source area is broad then many waves make up this train. In some cases, for Pacific Ocean tsunami, there have been as many as a hundred waves arriving at a coast over a period of 24 hours with diminishing wave heights. If the source is from a point, then only 3-4 waves form in the wave train. The latter is very much like what you see if you threw a pebble into a pond.

2.) How do shallow coastal waters affect Tsunami?

A: Exactly as they do with any other wave. Tsunami behave like storm waves as they cross the shelf. They have approximately the same energy and velocities. In shallow water if there are obstructions then the wave crest will bend around the obstructions. This is called wave refraction. However for storm waves you might see the crest of waves passing by a stationary observer every 10-15 seconds. For tsunami the spacing between wave crests are every 5-20 minutes. Hence, while you notice storm wave crests because you can see many over a short distance, with tsunami it is difficult to see the crests passing over the shelf because they are spaced so far apart. The real difference occurs closer to shore. Storm waves break and dissipate their energy in surf zones. The energy that does reach a beach is still great enough to erode the coast. Tsunami waves more than likely do not break but surge up the beach. Thus with tsunami most of their energy reaches the coast without dissipation. This is why tsunami are more destructive than storm waves on land.

3.) How does location affect damage of Tsunami?

A: This is not going to be a short answer because the question is so important. Because tsunami surge over the shoreline without breaking they tend to flood the coast with water over a very short period of time. Their long length ensures that there is a substantial volume of water under the crest reaching the coast. There are eight types of topography or coastal settings that are particular prone to tsunami.

Most obviously tsunami favour exposed ocean beaches. If you are on a beach and feel an earthquake, you are particular vulnerable to any resulting tsunami. The United States National Oceanic and Atmospheric Administration (NOAA) emphasises this in its publication Tsunami! The Great Waves. It states, "If you are at the beach or near the ocean and you feel the earth shake, move immediately to higher ground, DO NOT wait for a tsunami warning to be announced." Sometimes a tsunami causes the water near the shore to recede, exposing the ocean floor. Anyone who frequents the ocean should be aware that a rapid withdrawal of water from the shore is 99% of the time a clear signature of the impending arrival of a tsunami wave crest. The time until arrival may be less than a minute or, in the case of the coast near Concepción Chile following of Great Chilean earthquake of 22 May 1960, up to 50 minutes later.

Tsunami travel best across cleared land or smooth topography near the coastline, because frictional dissipation is low. The residences of Hilo, Hawaii were dramatically made aware of this fact following the Aleutian Island earthquake of 1 April 1946 and the Chilean earthquake of 22 May 1960. On many flat coastlines that have been cleared for agriculture or development, authorities are now planting stands of trees to minimise the landward penetration of tsunami. If you like to live on the coast and are worried about tsunami, become green; don't chop down the trees for the view; and be gracious to the neighbours that build in front of you, especially if they have an architecturally designed house with lots of corners and rough textured walls.

Tsunami flood across river deltas especially those that are cleared and where the offshore bathymetry is steep. On these coasts, and they are numerous,-for example the east coast of Japan and the southeast coast of Australia,-tsunami waves approach shore rapidly and with most of their energy intact. Delta surfaces lying only a few meters above sea-level can allow tsunami to penetrate long distance inland because once the wave gets onto the surface it propagates as if it was still travelling across shallow bathymetry. There are records of tsunami in small seas travelling 10 km inland across a delta because of this reason.

Tsunami favour harbours. Tsunami is a Japanese word meaning harbour wave, and when they get into harbours, especially ones where the width of the entrance is small compared to the length of the harbour's foreshores, they become trapped and can't escape back out to sea easily. Inside a harbour or bay, long waves such as tsunami tend to travel back and form for hours dissipating their energy, not across the deeper portions of the harbour, but against the infrastructure built on the shoreline. Boats in harbours are particularly vulnerable, and should put out to sea and in deeper water following any tsunami warning. Ria coastlines, such as those along the coast of Japan or southeastern Australia are ideal environments for this process.

Tsunami treat rivers exactly like long harbours. When a tsunami gets into a tidal river or estuary where water depths can still be tens of meters deep, the wave can travel easily up the river to the tidal limits or beyond. Along some coasts, tide limits may be tens of kilometers upriver, and residents living along the riverbanks may be totally unaware that a threat from tsunami exists. If the river is deep and can allow the penetration of the wave upstream, the height of a long wave can rapidly amplify where depths shoal or the river narrows. At these locations water can spill over levees and banks, flooding any lowlying topography. NOAA is likewise aware of these facts and in its publication Tsunami! The Great Waves warns, "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."

Tsunami have an affinity for headlands that stick out into the ocean, mainly because their wave energy is dissipated less by frictional attenuation and even concentrated there by wave refraction. Storm waves can increase in amplitude on headlands two- or threefold relative to an adjacent embayed beach because of these and other processes. Tsunami are no different.

Tsunami are not blocked by cliffs. Compared to the long wavelengths of a tsunami, that can still measure over 12 km between wave crests at the base of cliff dropping into 20 m depth of water, the height of a cliff is minuscule. Steep slopes are similar to cliffs. Tsunami waves 1-2 m in height have historically surged up cliffs or steep slopes to heights of 30 m or more above sea-level. While the view from a cliff is great, anyone standing there during a tsunami may have a unique, non-repeatable experience.

Finally, tsunami are enhanced in the lee of circular-shaped islands. Not only do they travel faster behind islands, but their run-up heights can also be higher, especially if the initial wave is large. The 12 December 1992 tsunami along the north coast of Flores Island, Indonesia devastated two villages in the lee of Babi, a small coastal island lying 5 km offshore. Wave heights actually increase from 2-7 m. Similarly, the 12 July 1993 tsunami in the Sea of Japan, destroyed the town of Hamatsumae lying on a sheltered part of Okusihir Island. The tsunami run-up reached 30 m above sea-level,-more than three times the elevation opposite some communities fronting the wave on the more exposed coast. Over 800 people were killed in the first instance and 300 in the latter. Lee sides of islands are particularly vulnerable to tsunami because they wrap around these small obstructions as solitary waves, become trapped, and increase in amplitude

4.) How can the distance from the epicentre affect damage and warning issued to dangerous areas?

A: First you have to be able to detect the wave. Instruments now exist which can be placed on the seabed anywhere and notice the movement of a long wave passing overhead. This occurs because the rotational motion of long waves like tsunami reaches to the bottom of any ocean. If I life in San Francisco and an earthquake occurs in Chile then it will take the wave about 12 hours to reach me. If an earthquake is registered in Chile, the size of the earthquake can be determined within minutes by seismographs around the globe. If we then wait for the resulting tsunami to reach the first closest tide gauge we can determine the tsunami's presence and height. Hence a warning could be issued for San Francisco hours in advance. (During the Great Alaskan earthquake of 1964, this occurred with enough warning that thousands of people raced down to the cliffs along the coast to watch the tsunami arrive. Fortunately the wave was not very big at San Francisco because we know from the above answer that it is dangerous to stand on cliffs waiting for a tsunami to arrive.) Now lets think about people living along the coast of Chile during that earthquake. They survive the earthquake which generates a tsunami. Because the coast is very close the wave takes only twenty minutes to reach their coast. If there is no warning, all people can do is go by experience. The scenario would go like this: it was a big earthquake; the same sized earthquake fifty years ago produced a tsunami; this one could probably produce a tsunami; let's run for it. Chile in fact has installed sensors on the seabed just offshore. These sensors can detect the passage of a long wave. When they do they set off sirens at shore. People have twenty minutes to run for safety. I live along a coast, and if I had twenty minutes warning I could certainly run to a part of the coast not included in the answer given at 3) and safely survive a tsunami.

5.) The difference between storm surges and Tsunami?

A: Dynamically they are similar except that a storm surge has only one crest whereas a tsunami has several wave crests. The fact that a storm surge can flood long distances inland is no different to the effect that a tsunami would have if it came into the same coast. Tsunami waves normally travel faster than storm surges; however during the Long Island Hurricane of 1938 people described the approach of a 13m high storm surge exactly like people have described the approach of a 13m high tsunami wave. Storm surges are generated by meteorological conditions whereas tsunami aren't.

6.) Describe how they have tremendous energy because of the great volume of water?

A: I probably have given the answer to this above. It has to do with their long wave length, hence their potential to flood over a coastline. When you think about it it makes sense. I have seen 7 m high storm waves breaking in a surf zone. I would not want to be at the breaker point because there is tremendous energy being dissipated over bars. If you moved that storm wave to shore and dissipated that energy over a short distance at shore then you would have the effect of a tsunami.

If you could stand at shore and watch a tsunami pass over you, you would also notice that the water is travelling at high velocity. This velocity component gives the water enormous power. It is not the energy that does the damage but the rate at which this energy is transmitted. The difference between energy and power can be illustrated by comparing a bullet and an elephant. If a bullet moves at the speed of an elephant it wouldn't do much damage. In fact that bullet would touch you and drop at your feet. However the elephant would hit you and you would drop at her feet. Now increase the speed of the bullet. The bullet still has the same energy as before but it is transmitted every quickly. Its effects are deadly. (I won't describe what would happen if the elephant travelled as fast as a bullet. It's mind boggling.)

Storm waves have the same energy as tsunami. Storm waves break in a surf zone and dissipate most of that energy before reaching shore. Any part of the wave that reaches shore is travelling very slowly. The tsunami wave reaches shore without losing much of its velocity or energy. It has tremendous power at shore.