Deep-Ocean Tsunami Detection Buoys

What is a deep-ocean tsunami detection buoy?

Deep-ocean tsunami detection buoys (DART™) are one of two types of instrument used by the Bureau of Meteorology (Bureau) to confirm the existence of tsunami generated by undersea earthquakes. These buoys observe and record changes in sea level out in the deep ocean. This enhances the capability for early detection and real-time reporting of tsunami before they reach land.

How does a deep-ocean tsunami detection buoy work?

A typical tsunami buoy system comprises two components; the pressure sensor anchored to the sea floor and the surface buoy. The sensor on the sea floor measures the change in height of the water column above by measuring associated changes in the water pressure. This water column height is communicated to the surface buoy by acoustic telemetry and then relayed via satellite to the tsunami warning centre.

The system has two modes - 'standard' and 'event'. The system generally operates in 'standard' mode, where it routinely collects sea level information and reports via satellite at relatively low frequency transmission intervals (i.e. every 15 minutes). This helps to conserve battery life and hence extend the deployment life. The tsunami buoy is triggered into 'event' mode when the pressure sensor first detects the faster moving seismic wave moving through the sea floor. It then commences reporting sea level information at one minute intervals to enable rapid verification of the possible existence of a tsunami. The system returns to standard mode after 4 hours if no further seismic events are detected.

Data from Australia's tsunami detection buoys are made freely available to the international community and the tsunami warning centres of other countries in real-time using the World Meteorological Organization's dedicated Global Telecommunication System (WMO GTS).

How is the location for deployment of a tsunami buoy determined?

The best location for deployment of a tsunami buoy is determined by careful consideration of a number of factors. The tsunami buoy needs to be far enough away from any potential earthquake epicentre to ensure there is no interference between the earthquake signal at the buoy and the sea-level signal from the tsunami. On the other hand, the tsunami buoy needs to be close enough to the epicentre to enable timely detection of any tsunami and maximise the lead time of tsunami forecasts for coastal areas. In addition, tsunami buoys must ideally be placed in water deeper than 3000m to ensure the observed signal is not contaminated by other types of waves that have shallower effects (e.g. surface wind-waves). International maritime boundaries must also be considered when deploying tsunami buoy systems.

Where are Australia's tsunami buoys being deployed?

Australia is potentially vulnerable to tsunami generated by undersea earthquakes along subduction zones (where the earth's tectonic plates are moving under each other) to the northwest, northeast, east and southeast of Australia. Deep-ocean tsunami detection buoys (and coastal sea level stations) are used to monitor the oceans in each of these regions. There are also spare buoys on hand to ensure maintenance regimes and emergency replacements can be carried out if necessary.

A schematic representation of a deep-ocean tsunami detection buoy (DART™).

Australia's first tsunami detection buoy was deployed on 15 April 2007 in the South East Tasman Sea, some 1200 km from Tasmania. The deployment was carried out in collaboration with NOAA. This buoy captures critical tsunami data from the oceans near the Puysegur fault line southwest of New Zealand.

How do these buoys contribute to tsunami warnings in Australia?

Due to the complexity and uncertainty as to whether an undersea earthquake has the potential to generate a tsunami, the observation of sea levels is a critical factor in verifying whether a tsunami has actually been generated. The use of actual sea level observations, as compared with reliance on seismic observations alone, therefore helps to significantly reduce the risk of false tsunami warnings being issued. All Australian-owned buoys, as well as deep-ocean buoys operated by other countries in the Australian region, provide critical data to Australia's tsunami warning system.

The buoys are just one part of Australia's sea-level observing system, which also includes a number of new and long standing coastal sea level stations that now have the ability to report sea level variations in real-time to monitor for tsunami. Combined, these technologies provide a constant stream of sea level data for the Joint Australian Tsunami Warning Centre (JATWC) operated by the Bureau of Meteorology and Geoscience Australia, enhancing tsunami warnings for the Australian public.

What is the in-water life of a tsunami detection buoy?

The life cycle of a deployed tsunami buoy is approximately 2 to 4 years. The Bureau's maintenance regime will involve the replacement of the surface buoy and the sea-floor pressure sensor every one to two years. The devices retrieved during regular maintenance are refurbished and made ready for the next redeployment.

The deployment of a DART™ buoy in the Tasman Sea.

Interesting Facts

Deep-ocean tsunami detection buoy technology was initially developed in the United States of America by the Pacific Marine Environmental Laboratory (PMEL) of the National Oceanic and Atmospheric Administration (NOAA).

The deep-ocean tsunami detection buoys (DART™) II systems contain two independent and redundant communications systems as back-up

These systems are capable of measuring sea-level changes of less than a millimetre in the deep ocean.

Two-way communication between the tsunami buoy and the tsunami warning centre means that the buoy can be controlled remotely. This two-way communication allows for troubleshooting of the system and also allows people to put the systems into 'event' mode in case of a possible tsunami or for research purposes.

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Tsunami Warnings

Tsunami are recorded in Australia about once every two years, and most are small and present little threat of land inundation to our coastal communities. However the abnormal waves, tides and currents caused by even relatively small tsunami can be dangerous to swimmers and mariners.

As a result of the Australian Tsunami Warning System (ATWS) Project, jointly undertaken by the Bureau of Meteorology (Bureau), Geoscience Australia (GA) and Emergency Management Australia (EMA) and funded by the Australian Government, Australia now has a comprehensive, independent tsunami warning service to advise the media, public and emergency authorities of any tsunami threat to Australia and its offshore territories.

Tsunami Watches and Warnings for Australia

The Joint Australian Tsunami Warning Centre (JATWC), jointly operated 24 hours a day by the Bureau and GA detects, monitors, verifies and warns of any tsunami threat to the coastline of Australia and its offshore territories.

The first stage of the tsunami warning process is a 'tsunami watch' phase, where seismologists have determined that there is the potential for an identified undersea earthquake to cause a tsunami threat to Australia. If there is a threat, the Bureau would issue a National Tsunami Watch. If there is no threat, a National Tsunami No Threat Bulletin would be issued.

 The National Tsunami Watch may be reissued if a tsunami remains unconfirmed by sea level observations and any potential first point of impact on Australia is more than 90 minutes away (the minimum time criteria for warning).

If sea level observations and further data confirm the tsunami threat, or if any potential first point of impact is less than 90 minutes away, the Bureau, on behalf of the JATWC, will use its severe weather communication infrastructure to issue Tsunami Warnings for the States and Territories under threat. The Bureau's Regional Forecasting Centres (RFCs) in capital cities are the main liaison point for emergency services during tsunami events.

Once separate tsunami watches or warnings are being issued for individual States and Territories, a National Tsunami Warning Summary will also be issued listing all the watches, warnings and cancellations that are in effect for the current tsunami event. The JATWC website www.bom.gov.au will provide a complementary coastal threat graphic showing the regions currently under threat.

Cancellation of a Tsunami Warning

When the main threat has passed or if a tsunami didn't eventuate, the Bureau will issue a Tsunami Watch Cancellation or Tsunami Warning Cancellation. The relevant State or Territory emergency authority will inform the public when it is safe to return to the affected area. A National Tsunami Event Summary is then issued after the event.

Distribution of Tsunami Bulletins and Warnings to the Media and Public

Tsunami Bulletins, Watches, Warnings, Cancellations and Event Summaries are part of a suite of warnings for severe weather events and hazards issued by the Bureau of Meteorology.

Media organisations across Australia work with the Bureau to also inform the public in the case of a tsunami event. Tsunami Warning distribution lists are maintained at each of the Bureau's State and Territory RFCs. These distribution lists are used for both National JATWC Bulletins and Regional Warnings. In addition to the media, key agencies such as the State and Territory emergency services, local councils, port authorities, and police are included on these dissemination lists. The bulletin and warning messages are also automatically uploaded to the Bureau's website.

Tsunami Warning Messages Summary

Messages: Types and Purpose

National No Threat Bulletin:

To advise people that the earthquake has been assessed and that no tsunami threat exists

National or State/Territory Watch:

To advise people that a tsunami threat may exist and that they should look out for further updates

State/Territory Warning:

To advise people that a tsunami threat does exist and to advise them of the level of threat and action they should take marine = blue, land = red

National Warning Summary:

To provide the public, media and emergency authorities with the status of tsunami warnings nationally

Event Summary:

To provide the public, media, emergency authorities and government with summary information that can be used in post-event analysis

Messages: Layout & ContentMessages

Product Identifier:

Identify type product/auto notifier

Media Instructions:

How urgently message/s should be broadcast. Use of Standard Emergency Warning Signal (SEWS) or not

Message Title and Issue:

Time Type, date/time and number sequence of message

Headline Message:

Key message; eg. No Threat, Potential Threat, Threat

Summary:

What, where and when the threat is

Threat Information:

Level of threat , coastal areas affected, time of Arrival

Community Response Advice:

What action people should take

Next Update Time:

When the next update will be issued:

Where the Public can get Further Information:

Web and telephone details for further/latest information

Tsunami watches and warnings indicating the level of threat are issued for coastal zones around Australia and its offshore territories.

Categories of Threat Levels within Tsunami Warnings

In order to assist the community, tsunami threat levels in Regional Tsunami warnings are categorised into three levels with required community responses; these have been determined in consultation with emergency management authorities. These Threat Levels will be specified for the same coastal areas that are used for routine Bureau Coastal Waters Forecasts, already known to marine users.

1. No threat An undersea earthquake has been detected, however it has not generated a tsunami, or the tsunami poses no threat to Australia and its offshore territories.
2. Marine and immediate foreshore threat Warning of potentially dangerous waves, strong ocean currents in the marine environment and the possibility of only some localised overflow onto the immediate foreshore.
3. Land inundation threatWarning for low-lying coastal areas of major land inundation, flooding, dangerous waves and strong ocean currents.

Sample - Summary

Tsunami warning for parts of Western Australia
LAND Threat

• For low-lying coastal areas from Kalbarri to Cape Preston, including Denham, Carnarvon, Coral Bay, Exmouth and Onslow, there is a threat of MAJOR LAND INUNDATION, FLOODING, DANGEROUS WAVES AND STRONG OCEAN CURRENTS for several hours from 7:45 PM WST on Sunday
• People in affected areas are strongly advised by FESA to go to higher ground or at least one kilometre inland

MARINE Threat

• For the marine environment from Cape Preston to Wallal, including Dampier and Port Hedland, there is the possibility of DANGEROUS WAVES, STRONG OCEAN CURRENTS, AND SOME LOCALISED OVERFLOW ONTO THE IMMEDIATE FORESHORE for several hours from 8:00 pm WST on Sunday
• For the marine environment from Jurien Bay to Kalbarri, including Geraldton, there is the possibility of DANGEROUS WAVES, STRONG OCEAN CURRENTS, AND SOME LOCALISED OVERFLOW ONTO THE IMMEDIATE FORESHORE for several hours from 8:15 pm WST on Sunday
• For all threatened areas, people are advised to get out of the water and move away from the immediate water's edge
• For latest and further information call 1300 TSUNAMI (1300 878 6264) or visit www.bom.gov.au
• Next update will be issued by 6:45pm WST on Sunday 30 September 2007

Sample - Community Response Advice

COMMUNITY RESPONSE ADVICE FROM EMERGENCY AUTHORITY FOR AREAS UNDER THREAT OF MAJOR LAND INUNDATION AND FLOODING:

• The local emergency authority has ordered the evacuation of low-lying parts of coastal towns and villages including [insert names]
• People are strongly advised to go to higher ground, at least ten metres above sea level, or if possible move at least one kilometre away from all beaches and the water's edge of harbours and coastal estuaries.
• Take only essential items that you can carry including important papers, family photographs and medical needs.
• It may be in your own interests to walk to safety if possible to avoid traffic jams.
• If you cannot leave the area take shelter in the upper storey of a sturdy brick or concrete multi-storey building.

COMMUNITY RESPONSE ADVICE FOR THREATENED MARINE ENVIRONMENT AREAS:

• The local emergency authority advises people in all threatened areas to get out of the water and move away from the immediate water's edge of harbours, coastal estuaries, rock platforms, and beaches.
• Boats in harbours, estuaries and in shallow coastal water should return to shore. Secure your boat and move away from the waterfront.
• Vessels already at sea should stay offshore in deep water until further advised.
• Do not go to the coast to watch the tsunami, as there is the possibility of dangerous, localised land inundation of the immediate foreshore.
• Check that your neighbours have received this advice.

CAUTION:

• Tsunami waves are more powerful than the same size beach waves, with the first wave not necessarily being the largest.
• Low-level effects may be observed in neighbouring coastal areas. People are advised to take care.

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