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, tsunamis 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, tsunamis 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. Tsunamis 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 tsunamis generated?
The most common cause of tsunamis 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?
Tsunamis 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?
Tsunamis are different from normal waves
Tsunami 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.
Tsunamis have long wavelengths
In 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. Tsunamis 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 minutes up to an hour, compared to seconds for wind-driven waves.
As a tsunami approaches land, the size increases
The 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.
Tsunamis are fast
In the deep ocean, a tsunami can travel at more than 900 kilometres per hour, close to the speed of a jumbo jet, and in shallow water, it can be described as roughly the speed of a fast cyclist.
Tsunamis retain their energy
As well as travelling at high speeds, tsunamis can also travel large distances with limited energy losses. Tsunamis can therefore have sufficient energy to travel across entire oceans.
Tsunami waves move outwards, away from their source
The 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. Tsunamis 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 waves
A 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 over 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.
Tsunamis can vary in size and severity
The 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 meters in some places, killing over 600 people. Large tsunamis cause strong rips and currents in oceans around the world for up to a few days after the initiating earthquake.
Where and how frequently are tsunamis generated?
Most tsunamis 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 tsunamis. The frequency of tsunamis is variable across the globe and over time. In the two years after the event of 26 December 2004 the Pacific Tsunami Warning Centre (PTWC) issued 52 tsunami alerts for six tsunamia, two of which resulted in significant loss of life. The major Japan (Tohoku) tsunami event of 11 March 2011 killed over 20,000 people.
Australia's vulnerability to tsunamis
Australia is surrounded to the northwest and east by some 8,000 kilometres of active tectonic plate boundaries capable of generating tsunamis, 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 tsunamis detected?
Typically, earthquakes that may generate a tsunami are detected through a network of seismic monitoring stations. Any resulting tsunamis 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 tsunamis. The sea-level gauges and deep ocean tsunami detection buoys then measure any abnormal changes in sea level to verify if a tsunami has been generated.
Tsunami warnings for Australia
With the introduction of the Joint Australian Tsunami Warning Centre (JATWC) in July 2007, Australia now has the capacity to detect and verify potential tsunamis in our region.
The JATWC combines the operational roles of Geoscience Australia (GA) and the Bureau of Meteorology (Bureau). GA notifies the Bureau of any earthquakes that may cause a tsunami. The Bureau then uses its network of sea-level monitoring equipment to determine the existence of a tsunami and tsunami forecast models to confirm the existence of a tsunami and issue a tsunami warning if required.
The Attorney-General's Department (AGD) supports the system through its community awareness and preparedness campaigns. Through its Crisis Coordination Centre, it also has a role in coordinating a national response to a tsunami disaster if requested by the States or Territories.
The establishment of the Australian Tsunami Warning System (AWTS) is a four-year project funded by the Federal Government that was completed in June 2009. Australia now has 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 warnings
JATWC is a key element of the Indian Ocean Tsunami Warning and Mitigation System (IOTWS), providing detailed threat information to National Tsunami Warning Centres (NTWCs) in the Indian Ocean to help them determine the level of threat and warnings they will issue for their communities. Australia also provides observations from its seismic and sea-level monitoring networks in the Southwest Pacific to the Pacific Tsunami Warning Centre (PTWC) in Hawaii to support its warning service for the Pacific Ocean.
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.
- A shaking of the ground in coastal regions may reflect the occurence of a large undersea earthquake nearby that may generate a tsunami.
- 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.
- 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 stable building.
- Do not return to the coast until you receive official clearance.
- Continue to follow emergency services instructions.