- On July 12, 1993 a magnitude 7.8 earthquake in the Sea of Japan produced tsunami that totally destroyed the southern half of Okushiri Island. Waves were greater than 30 feet and some could have been 100 feet. The earthquake was about 50 miles offshore and the tsunami arrived within minutes. 120 people died.
- The states most at risk for Tsunami are California, Hawaii, Oregon and Washington. Hawaii is at greatest risk and they have about 1 tsunami a year and a dangerous tsunami about every 7 years.
- On March 28, 1964 an extremely large earthquake (magnitude 8.4) struck Alaska. It caused tsunami waves that were very destructive in southeastern Alaska, in Vancouver Island, Canada, and in the States of Washington, California and Hawaii. Waves ranged in size from 6 to 21 feet. The tsunami killed more than 120 people and damages costing more than $106 million. It was the costliest tsunami ever to strike the Western United States and Canada.
- Although a large asteroid impact is highly unlikely, scientists studying the possibility have decided that a moderately large asteroid or about 5-6 km in diameter falling in the middle of the Atlantic Ocean, would generate a tsunami that would travel all the way to the Appalachian Mountains in the upper two-thirds of the United States. Coastal cities would be wiped out by such a tsunami.
- Nuclear explosions could possible create tsunami but none have ever been generated from testing yet. Furthermore, such testing is currently banned by international treaty.
What causes a tsunami?
When an undersea earthquake or other major disturbance causes a section of the ocean floor to suddenly rise or sink, the mass of water above the affected area also rises or sinks. This unexpected movement of the water creates a series of powerful waves.
Undersea earthquakes that cause massive changes to the ocean floor and the displacement of a large volume of water are the most common cause of a tsunami.
Tsunami can also be caused by other undersea events such as volcanoes or landslides
A tsunami can also be caused from events above the ocean floor. These events could include a meteorite crashing into the ocean, major landslides near a coastline or material from an erupting volcano forming a landslide. The impacts of tsunami triggered this way tend to be localised.
More than 75 per cent of tsunami are caused by undersea earthquakes.
Where do tsunami occur?
Most tsunami occur in the Pacific and Indian Oceans. The boundary of the Pacific Ocean experiences frequent earthquakes. This boundary is commonly known as the Ring of Fire. There are two major subduction zones in the Indian Ocean that can also generate tsunami.
Subduction zone earthquakes are the most common source of destructive tsunami. These earthquakes are generated when two tectonic plates meet and one goes under the other, usually at a rate measured in centimetres. The sinking (subducting) plate drags against the upper plate, causing flexing. Continued stress on the plate boundary results in the upper plate rebounding to its original position, displacing the sea water above.
The Questacon Tsunami show includes a demonstration of this process (between the 8 and 16 minute mark).
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Sumatra earthquake and tsunami, 26 December 2004
In the case of the December 2004 earthquake off Indonesia, the tsunami was generated 10 minutes after the event when the displaced sea surface spread outward from the epicentre as a tsunami. In this illustration, the red arrows indicate the direction in which the upper plate is deformed due to drag and release of the lower plate. (Copyright Geoscience Australia)
A Tsunami can really move!
In the deep water of the ocean, the waves that are created have a large wavelength but are not usually bigger than one metre tall. A tsunami wave may be hundreds of kilometres in length and it moves at a high speed across great distances without losing much of its energy.
Tsunami waves move out from where a large body of water has been disturbed, similar to the way ripples move away from the place where a rock has been dropped into a pond.
In the open ocean, a tsunami can travel as fast as 950 kilometres per hour, which can be represented by the speed of a passenger jet. It loses speed as it approaches land, but it does not lose much of its energy. As it slows down, the height of the waves build.
How big is a tsunami?
In the open ocean it can be difficult to notice a tsunami wave. However, as a tsunami wave approaches land and moves into more shallow water, the leading edge of the wave slows down but the back of the wave is still travelling at its original speed. This causes the water to bunch up and increase the wave height. This is known as ‘shoaling’. When it reaches land, it may behave like a series of breaking waves or a large, powerful wave.
The tremendous energy of the wave can cause great quantities of water to surge inland, far beyond where even the highest of tides would commonly reach.
Some of the largest tsunami waves have been generated by the volcanic eruption of Krakatoa in 1883. This tsunami reached a height of 37 m. In 1737, a tsunami was estimated to be 64 m high as it struck Cape Lopatka in northeast Russia
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Tsunami waves are different from normal waves
Tsunami are different from normal waves. Normal waves are generated by the wind and it is only the water near the surface that is moving. In a tsunami all the water from surface to sea bed is moving and the movement has been generated by something (usually an earthquake) that has displaced water. In the open ocean, tsunami create little movement and little threat to shipping.
When a tsunami wave reaches the shore, its wavelength might be more than 100 km. Tsunami can last for several hours or even days depending on location. This is much different to the waves that we are used to seeing at the beach. Typical ocean waves usually last for less than a minute and have a wavelength of 100 m.
The energy behind a tsunami can strip sand off beaches, undermine trees, and crush buildings.
People and boats are powerless against the force of a tsunami. The amount of water carried inland is capable of inundating vast areas of normally dry land.
Case studies
Chile tsunami 27 Feb 2010
This tsunami followed an earthquake measuring 8.8 on the Richter Scale. The epicentre of the earthquake was 115 km NNE of Concepcion. The epicentre of this quake was 230 km north of the largest earthquake ever recorded (9.5 in 1960). This quake was the result of movement between the Nazca Plate on the eastern side of the Pacific Ocean and the South American Plate. The first waves hit about 34 minutes after the earthquake. Properties and businesses were damaged and over 200 lives were lost.
Samoa tsunami 29 September 2009
At 6.49am an earthquake measuring 8.0 on the Richter Scale triggered this tsunami. The earthquake epicentre was located on the Pacific tectonic plate near its boundary with the Australian Plate approximately 200 km south of Samoa. Dozens of magnitude 4 and 5 aftershocks followed the initial earthquake, continuing through the next day. This area, near the Tonga Trench, is one of the most active earthquake regions in the world. The tsunami struck the islands of Savi’i, Upola, and Tutuila minutes later with waves that were approximately 3 m high. Smaller tsunami hit other Pacific islands farther away from the earthquake’s epicentre. The tsunami caused a great amount of damage to property and the natural environment and caused the death of over 100 people.
Solomon Islands tsunami 2 April 2007
On 2 April 2007 an earthquake measuring 8.1 on the Richter Scale had its epicentre 350 km NW of Honiara. The earthquake occurred in shallow water in the early morning and was quickly followed by a tsunami. The waves were up to 10 m high. Over 50 deaths were reported and thousands were left homeless. A tsunami warning was issued for Australia and Alaska 15 minutes after the earthquake.
Indian Ocean tsunami, 26 December 2004
This event was one of the most devastating caused by a natural hazard in recent years. The earthquake that triggered the tsunami occurred west of the Indonesian island of Sumatra and measured 9.0 on the Richter Scale, making it the largest earthquake worldwide in 40 years. The death toll in March 2005 was over 273 000 people, with many still missing.
Papua New Guinea tsunami 17 July 1998
An earthquake measuring 7.0 on the Richter Scale just off the northern coast generated a destructive tsunami. Waves up to 10 m high hit the villages in the Aitape region extremely quickly. More than 2000 people were killed and there was great damage to buildings and farmland.
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