What is a tsunami
A tsunami (soo-NAH-mee) is an ocean wave or series of waves caused by a sudden disturbance of the ocean floor that displaces a large amount of water. Tsunamis are caused generally by earthquakes, less commonly by submarine landslides, infrequently by submarine volcanic eruptions and very rarely by large meteorite impacts in the ocean.
Persons caught in the path of a tsunami are at extreme risk from being crushed or struck by debris, or drowning. Children and the elderly are particularly at risk, as they often have less mobility, strength and endurance to evade or resist the onslaught.
How tsunamis are formed
Most tsunamis are caused by large offshore earthquakes. In order to understand how these tsunamis are caused we must first understand how earthquakes occur. The Earth’s surface is made up of huge slabs of rock called plates which fit together like an uneven jigsaw puzzle. The region where two or more plates meet is called a plate boundary. The plates are constantly moving but this plate movement is not smooth and continuous, rather the plates often lock together causing a build-up of energy. When the plates eventually move out of this locked position the energy that is released may be felt as an earthquake.
Map of the Earth's plate boundaries. The outer layer of the Earth is broken into a dozen or so slabs of rock that are moving relative to one another. These are called Tectonic Plates.
Much of the world’s earthquake and volcanic activity takes place along plate boundaries. At these plate boundaries the plates interact with each other in different ways; some of them slide past each other, others spread apart and others move toward each other with crumpling or one dipping beneath the other. This last type of plate boundary is called a subduction zone and it is at these subduction zones where most of the Earth’s tsunamis are generated.
The type of plate boundary where one plate dips beneath the other is called a Subduction Zone. Tsunamis may be triggered by earthquakes generated from this type of plate movement.
|Earthquakes & tsunamis
Earthquakes occur within fault zones (fractures in the Earth’s crust) associated with plate boundaries. Most strong earthquakes occur at subduction zones where one plate slides beneath another. If a strong enough earthquake occurs in the ocean at a subduction zone the abrupt vertical shifting of the sea floor can send tons of rock shooting upward with tremendous force, the energy of that force is transferred to the water. The energy pushes the water upward above normal sea level. Once the water has been pushed upward, gravity acts on it, forcing the energy out horizontally along the surface of the water.
This is similar to the ripple effect you get from throwing a pebble in the water, but in reverse: The energy is generated by a force moving out of rather than into the water. The energy then moves through the depths of the water and away from the initial disturbance. These waves are called tsunamis.
All earthquakes do not cause tsunamis. There are four conditions necessary for an earthquake to generate a large tsunami:
(1) The earthquake must occur beneath the ocean or cause material to slide in the ocean.
(2) The earthquake must be strong, at least magnitude 6.5.
(3) The earthquake must rupture the Earth’s surface and it must occur at shallow depth – less than 70km below the surface of the Earth.
(4) The earthquake must cause vertical movement of the sea floor (up to several metres).
For an animated depiction of how the December 26, 2004 Indian Ocean tsunami was caused go to the BBC’s website at http://news.bbc.co.uk/1/hi/in_depth/4136289 .
How an earthquake generates a tsunami.
Tsunamis on the move
In the deep ocean, potentially destructive tsunamis can be small – often only a few tens of centimetres or less in height – and can neither be seen nor felt on ships at sea. Where the ocean is over 6,000m deep, unnoticed tsunami waves can travel at the speed of a commercial jet plane, over 800km per hour (~500miles per hour). They can move from one side of the Pacific Ocean to the other in less than a day.
Often, the first sign of an approaching tsunami is usually a dramatically exposed shoreline. As the tsunami approaches land the speed of the front of the wave decreases considerably. As a result, the trailing waves pile on top of the waves in front of them (like a rug crumpled against a wall), thereby significantly increasing the height of the wave before hitting the shore.
Before - Kata Noi Beach , Phuket, Thailand at normal levels before withdrawal of the sea.
After - The sea recedes drastically at Kata Noi Beach, Phuket, Thailand before the third and strongest tsunami wave. The withdrawal of the sea may be a sign that a tsunami is near. (26.12.04)
Tsunamis arrive as a series of waves anywhere from 10-60 minutes apart. Although a tsunami advances much slower as it nears land, these great waves are powerful enough to flatten houses, buildings and trees and can carry ships far inland. The major withdrawal of the sea should be taken as a natural warning sign that tsunami waves will follow. It should be noted, however, that there have been instances where there was no withdrawal of the sea before the arrival of the tsunami e.g. some areas of Sri Lanka for the 2004 mega event.
Offshore and coastal features can determine the size and impact of tsunami waves. Reefs, bays, entrances to rivers, undersea features and the slope of the beach all help to modify the tsunami as it approaches land. When the tsunami strikes the coast, often as a wall of water, sea levels can rise many metres. The first wave may not be the largest in the series of waves. One coastal community may see no damaging wave activity while in another nearby community destructive waves can be large and violent. The flooding can extend inland by one km (~0.5 mi) or more, covering large expanses of land with water and debris.
The tsunami on 26th December, 2004 claimed over 250,000 lives and caused damage throughout the Indian Ocean making it the worst tsunami catastrophe in recorded history. It was also the first known basin-wide destructive tsunami in the Indian Ocean. Additional information and photos on this event can be found on the Links section of this website.
A village near the coast of Sumatra lays in ruin after the tsunami that struck South East Asia. (02.01.05)
|Tsunamis vs. Ocean waves
A tidal wave/ocean wave and a tsunami are not the same thing. In order to understand tsunamis, let us first look at waves in general. Waves are made up of a crest (the highest point of the wave) and a trough (the lowest point of the wave). Waves are measured in two ways:
- The wave height is the distance between the crest and trough.
- The wave length is the horizontal distance between two consecutive wave crests.
Anatomy of a tsunami wave.
The frequency of waves is measured by the time it takes for two consecutive waves to cross the same point. This is called the wave period. Tsunamis and normal waves have all of the same parts and are measured in the same ways, but there are many differences between the two. The chart below shows some of the differences.
|Typical Tsunami Wave vs. Typical Ocean Wave
|Wave Period (time required for two waves to pass a single point in space)
||5 to 20 seconds apart
||10 minutes to 2 hours apart
|Wave Length (horizontal distance between two waves)
||100-200 metres apart (300-600 feet apart)
||100-500 km apart (60-300 miles apart)
The primary differences are size, speed and source. Large tsunamis are also different from tidal waves and normal ocean waves in the following ways:
- Tidal waves and surf waves are related to changes in the atmosphere while tsunamis are related to changes within the Earth.
- Most tsunami waves do not break like normal surf waves at the beach that curl over as they approach shore rather they come in much like a very strong and very fast wall of water. Those that do break often form vertical walls of turbulent water called bores.
- Tsunami waves cannot be surfed. Tsunamis are an extremely powerful and dangerous phenomenon that should be avoided at all times.
Contributing Sources: Nathan Halabrin and Robert Valdes. "How Tsunamis Work". January 04, 2005 Contributing Sources: Nathan Halabrin and Robert Valdes. "How Tsunamis Work". January 04, 2005 http://science.howstuffworks.com/tsunami.htm (February 22, 2007) UNESCO IOC International Tsunami Information Centre www.tsunamiwave.info W. Jacquelyne Kious and Robert I. Tilling, This Dynamic Earth (online version) http://pubs.usgs.gov/gip/dynamic/dynamic.html (February 22, 2007)
UNESCO IOC International Tsunami Information Centre
W. Jacquelyne Kious and Robert I. Tilling, This Dynamic Earth (online version)