QUARTERLY FOCUS
July 2009

Tsunami warning system passes its first test

Predicted maximum wave heights after an earthquake near New Zealand, from Bureau of Meteorology computer modelling.


Sea level at Port Kembla, NSW, during the tsunami on 15 July 2009.

The Bureau of Meteorology is a member of the Joint Australian Tsunami Warning Centre and is responsible for maintaining a network of deep-ocean buoys, wave measurement technologies and sophisticated computer models of tsunami behaviour that enable it to issue accurate tsunami watches and warnings. Geoscience Australia is another member of the warning centre and is responsible for seismic information.

A Tsunami Watch is first issued by the bureau when there is the potential risk of a tsunami in the region and will remain in place while the level of threat is assessed. Soon after, the watch will be cancelled or one of two warnings will be issued for specified parts of the coastline; either
a Tsunami Warning for the Marine Environment or
a Tsunami Warning of Land Inundation.
A warning will remain in place for some hours while the waves and dangerous current associated with the tsunami threaten the identified areas.

On Wednesday 15 July 2009, a significant earthquake of magnitude 7.9 occurred near the New Zealand south coast. The bureau identified the quake as having the potential to generate a tsunami that would threaten many parts of eastern Australia and New Zealand, and issued a Tsunami Watch.

Soon after the quake, sea-level gauges and deep-ocean instruments detected that a tsunami had indeed been generated, and a Tsunami Warning for the Marine Environment (on the beach and just offshore) was issued for the coasts of mid to southern NSW, eastern Victoria, eastern and southern Tasmania, and Norfolk Island. A more serious Tsunami Warning of Land Inundation for low-lying areas was issued for Lord Howe Island, however this was later downgraded to a marine threat.

Small tsunami were observed at several Australian sea-level gauges, including Port Kembla, NSW, and no damage was reported. The largest tsunami was at Southport in southern Tasmania which recorded a 55-centimetre wave above the expected tidal level.

Timeline (Eastern Standard Time)

More information
Joint Australian Tsunami Warning Centre
Geoscience Australia, tsunami and Geoscience Australia, earthquakes
Emergency Management Australia
Pacific Tsunami Warning Centre (US)
National Geophysical Data Centre (US)

FAQs

What is the difference between marine and land tsunami warnings?

Some members of the media and public were unfamiliar with the two-level warning system, which the Joint Australian Tsunami Warning Centre uses. If the presence of a tsunami has been confirmed by sea-level gauges, the centre will issue either a:
Tsunami Warning for the Marine Environment
A warning of potentially dangerous waves, strong ocean currents in the marine environment and the possibility of only some localised overflow on to the immediate foreshore. No significant inundation of land areas is expected. OR
Tsunami Warning of Land Inundation
A warning for low-lying coastal areas of major land inundation, flooding, dangerous waves and strong ocean currents. Local emergency authorities may order evacuations of high-risk areas.

Were the warnings an overreaction?

No. The new warning system worked well and allowed the earthquake to be detected quickly, the presence of a tsunami to be confirmed, and warnings to be issued to emergency authorities, the media and the public. Deep ocean buoys detected tsunami waves for some hours, as did tide gauges along the coast. The level of threat for south-east Australia was confined to the marine interface and this was communicated well in warnings. For this event, the warning centre only ever issued a marine warning for eastern parts of Australia. That warning was justified. A small tsunami was experienced on the coast with waves up to 55 centimetres larger than the normal tide level. Although this may seem a small wave, tsunami are different to wind-driven waves and have much greater energy because they involve water all the way to the sea floor and for long distances out to sea. So far there have been no reports of damage, but there were unusual currents and waves close to shore, as predicted in the warnings.

What about Lord Howe Island?

Initial indications from the Joint Australian Tsunami Warning Centre ’s tsunami model were that the impact of the tsunami might be worse for Lord Howe Island, and so a warning indicating a threat of land inundation was issued for that location only. Thankfully the island was spared the maximum impact from the tsunami but would still have experienced dangerous currents. The Lord Howe Island warning was downgraded to a marine threat when sea-level data indicated it was safe to do so.

Why did the Pacific Tsunami Warning Centre in Hawaii cancel so much earlier?

The Joint Australian Tsunami Warning Centre is the official warning centre for Australia. The bureau and Geoscience Australia work closely together to provide levels of detail for Australia not provided by Pacific Tsunami Warning Centre. The system in Hawaii does not identify the marine threat levels that the Australian system does. The cancellation by the Hawaiian agency meant that it no longer saw any threat of destructive waves affecting land. In Australia, destructive waves were never expected for the mainland. The bureau warned only of marine threats which the Hawaiian agency is not concerned with. The Australian warnings continued to be updated and at about midnight flagged that the risk would likely continue to 1am, when warnings were cancelled.

What is the bureau’s response to reports that members of the community went down to the ocean to see the tsunami?

The community is strongly advised to take heed of tsunami warnings. As was seen in 2004, tsunami waves have the potential to cause significant destruction. The bureau strongly recommends that members of the community take heed of advice provided by local emergency services in their area.

Are the warnings being improved?

The Joint Australian Tsunami Warning System is based on the very best science that is currently available. It has developed over the past four years to a level of capability greater than most overseas systems. It involves feeding earthquake information into sophisticated computer models of tsunami wave behaviours, and sophisticated wave measurement technologies including that from deep-ocean buoys in some of the most inhospitable parts of the oceans that surround Australia. An upgraded model of tsunami behaviour will be made operational later this year and is expected to reduce the chance of false alarms. There is still a lot to learn about tsunami. Australia is a world leader in this field. If anything, the tsunami event demonstrates that the warning system is working well.

How did emergency services manage during the event?

Local emergency services have the responsibility to order and manage evacuations. In the event on July 15, the bureau did not issue a land inundation warning for any part of mainland Australia or Tasmania, so land evacuations were not necessary. If the threat level had been higher, local emergency authorities would have put into action their plans to alert and evacuate people at risk. At the only location for which a warning of possible land inundation was issued (Lord Howe Island), emergency authorities did efficiently carry out evacuations.

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