questions on Tsunami

What is a tsunami?

The name tsunami, from the Japanese words tsu meaning harbour and nami meaning wave, is now used internationally to describe a series of waves travelling across the ocean. These waves have extremely long wavelengths, up to hundreds of kilometres between wave crests in the deep ocean. In the past, tsunami have been referred to as ‘tidal waves’ or ‘seismic sea waves’. The term ‘tidal wave’ is misleading. Even though a tsunami’s impact upon a coastline is dependent on the tidal level at the time a tsunami strikes, tsunami are unrelated to the tides. Tides result from the gravitational influences of the moon, sun and planets. The term ‘seismic sea wave’ is also misleading. Seismic implies an earthquake-related generation mechanism. Earthquakes are only one of several ways that a tsunami can be generated. Tsunami can also be caused by events such as underwater landslides, volcanic eruptions, land slumping into the ocean, meteorite impacts, or even the weather when the atmospheric pressure changes very rapidly.

How are tsunami generated?

The most common cause of tsunami is an undersea earthquake that results in a sudden rise or fall of a section of the earth’s crust under or near the ocean. This earthquake creates an explosive vertical motion that can displace the overlying water column, creating a rise or fall in the level of the ocean above. This rise or fall in sea level is the initial impulse that generates a tsunami wave.

What type of earthquake generates a tsunami?

Tsunami are typically generated by earthquakes that occur along subduction zones. A subduction zone is an area on the earth where two tectonic plates meet and move towards one another, with one sliding underneath the other and moving down into the earth at rates typically measured in centimetres per year.

What are the characteristics of a tsunami?

Tsunami are different from normal wavesTsunami waves involve the movement of water all the way to the sea floor. The effects of wind-driven ocean waves are seen only near the surface of the ocean. Tsunami have long wavelengthsIn the deep ocean tsunami waves have extremely long wavelengths. In comparison to wind-driven waves, tsunami waves may have wavelengths up to hundreds of kilometres between wave crests. Tsunami are therefore much more destructive than normal waves because the huge flooding body of water can continue to rush onto land for an extended period of time. This may be anything from a few minutesup to an hour, compared with seconds for winddriven waves. As a tsunami approaches land, the size increasesThe speed and size of a tsunami is controlled by water depth. In the deep ocean tsunami waves may be unnoticed by ships or from the air. As the wave approaches land it reaches shallow water and slows down. Relative to the front of the wave, the rear is still in slightly deeper water (so it is going slightly faster) and catches up. The result is that the wave quickly ‘bunches up’, the wavelength becomes shorter and the body of water becomes much higher. This is called shoaling.Tsunami are fastIn the deep ocean, a tsunami can travel at more than 900 kilometres per hour, close to the speed of a jumbo jet, and in shallowwater, it can be described as roughly the speed of a fast cyclist.

Tsunami retain their energyAs well as travelling at high speeds, tsunami can also travel large distances with limited energy losses. Tsunami can therefore have sufficient energy to travel across entire oceans.Tsunami waves move outwards, away from their sourceThe path of a tsunami is never symmetrical and is determined by a number of factors including the bathymetry of the sea floor. Bathymetry is the measurement of the depth of the ocean floor from the water surface and is the oceanic equivalent of topography.

Tsunami move outwards at right angles to the subduction trench where the earthquake has occurred. A tsunami travels faster through deep water and slower through shallow water. This directs the wave along undersea valleys. The magnitude of the earthquake, the shape of the earthquake and the orientation of the subduction zone that ruptures are also influencing factors.A tsunami is a ‘series’ of wavesA tsunami generally consists of a series of waves. The amount of time between successive waves is known as the wave period.

Waves can be a few minutes or more than two hours apart. In most cases, the first tsunami wave is not the largest. Subsequent waves, sometimes the fifth or sixth, can be many times larger.Tsunami can vary in size and severityThe impact of a tsunami can vary widely. A small tsunami may result in unusual tides or currents that can be dangerous to swimmers or cause damage to berthed boats. A large tsunami can cause widespread flooding and destruction such as that seen off the west coast of Northern Sumatra on 26 December 2004. The south Java tsunami (17 July 2006) was caused by a relatively small earthquake (magnitude 7.7) that generated a 0.5 metre tsunami. This tsunami inundated the coast with run-up heights of four metres in some places, killing more than 600 people. Large tsunami cause strong rips and currents in oceans around the world for up to a few days after the initiating earthquake. Subduction zones along tectonic plate boundaries (shown in red) around Australia that have the potential to generate a tsunami that may impact on Australia’s coast

.Where and how frequently are tsunami generated?

Most tsunami occur in the Pacific and Indian Oceans. The boundary of the Pacific Ocean, known as the Ring of Fire, experiences frequent earthquakes. There are two major subduction zones in the Indian Ocean that can also generate tsunami. The frequency of tsunami is variable across the globe and over time. In the two years after the event of 26 December 2004 the Pacific Tsunami Warning Centre issued 52 tsunami alerts for six tsunami, two of which resulted in significant loss of life.Australia’s vulnerability to tsunami

Australia is bounded to the northwest, northeast and east by some 8,000 kilometres of active tectonic plate boundaries capable of generating tsunami, which could reach our coastline within two to four hours. One-third of earthquakes worldwide occur along these boundaries. The impact of a tsunami hitting vulnerable, low-lying areas on the Australian coast could be significant.

How are tsunami detected?

Typically, earthquakes that may generate a tsunami are detected through a network of seismic monitoring stations. Any resulting tsunami are then verified by sea-level monitoring stations and deep ocean tsunami detection buoys. The seismic monitoring stations can determine the location and depth of earthquakes that have the potential to cause tsunami. The sea-level gauges and deep ocean tsunami detection buoys then measure any abnormal changes in sea level to verify whether a tsunami has been generated.Tsunami warnings for AustraliaWith the introduction of the Joint Australian Tsunami Warning Centre (JATWC), Australia has the independent capacity to detect and verify potential tsunami in our region. The JATWC combines the operational roles of Geoscience Australia (GA) and the Bureau of Meteorology (the Bureau). GA notifies the Bureau of any earthquakes that may cause a tsunami. The Bureau then uses its network of sea-level monitoring equipment and tsunami forecast models to confirm the existence of a tsunami and issue a tsunami warning to emergency services, media outlets and the public if required.Emergency Management Australia (EMA) supports the system through community awareness and preparedness campaigns. EMA also has a role in coordinating a national response to a tsunami disaster if requested by the States or Territories.The establishment of the fully functional Australian Tsunami Warning System (ATWS) is a four-year project funded by the Federal Government and is due to be completed in June 2009. At the completion of the project Australia will have considerably improved tsunami detection equipment in Australia and around the region, enhanced scientific modelling of tsunami, a responsive warning system, and increased public awareness and community preparedness. International tsunami warningsAustralia is participating in the development of a tsunami warning system for the Indian Ocean through the Intergovernmental Oceanographic Commission, an agency of UNESCO. Australia is also enhancing seismic and sea-level monitoring networks in the Pacific Ocean, with observations being provided to the Pacific Tsunami Warning Centre ( PTWC) in Hawaii, in support of its warning service for the Pacific Ocean.ocean oor tsunami waves wave lengthwave heightsea levelCross-section of the coast during a tsunami demonstrating that tsunami are flooding waves that can inundate low-lying foreshore areas.

What are the warning signs of a tsunami?

The number one warning sign of a tsunami in Australia is the advice you may receive from the media (on radio or television) or from police and other emergency services. Follow their instructions immediately.The following are natural signs of a tsunami that you may, but not always, experience when you are near the coast in Australia or overseas. If you notice any of these three warning signs take action.

1. A shaking of the ground in coastal regions may reflect the occurrence of a large undersea eathquake nearby that may

generate a tsunami.

2. As a tsunami approaches shorelines, the sea may, but not always, withdraw from the beach (like a very low and fast tide) before returning as a fast-moving tsunami.

3. A roaring sound may precede the arrival of a tsunami.

What should I do if I notice the warning signs or hear a warning from my local emergency services?

• If you are at the beach, immediately move inland or to higher ground.

• If your boat is in deep water and offshore, maintain your position.

• If your boat is berthed or in shallow water, secure your vessel and move inland or to higher ground.

• If you are on the coast and cannot move inland, seek shelter in the upper levels of a st




Frequently Asked Questions About Tsunami



What generates tsunami?
Tsunami(s), also called seismic sea waves, are a series of waves generated by large, violent earthquakes occuring near the ocean. Not all earthquakes generate tsunamis.By far, the most destructive tsunamis are generated from large shallow earthquakes with an epicenter or fault line near or on the ocean floor. Vertical displacements of the earth's crust along the rupture, resulting from such earthquakes, can generate destructive tsunami waves which can travel across an ocean spreading destruction across their path. Similar displacements of the ocean floor can also be produced by volcanic eruptions and submarine avalanches, or submarine landslides. However, these sources are considered as point sources and , although destructive locally, the energy of the waves is rapidly dissipated as they travel across the ocean.

The wave crests of a tsunami may be a hundred kilometers or more apart as they travel across the ocean. The height from trough to crest may be only a few centimeters or meters in the open ocean. A tsunami cannot be felt aboard ships in deep water.

Why are tsunami so destructive?

As the tsunami enters the shoaling water near the coast, its velocity decreases and its height increases. It is in these shallow waters that tsunamis become a threat to life and property, for they can crest to heights of of more than 30-50 meters and strike with devastating force.

Finally, terminal height or run-up of the tsunami at the point of impact will depend on how the energy is focused, the travel path of the waves, the coastal configuration, and the offshore topography. Tsunami run-up is the vertical distance between the maximum height reached by the water on shore and the mean sea level surface. Tsunamis are among the most terrifying natural hazards known to man. They have been responsible for tremendous loss of life and property throughout history.