FEATURE ARTICLE: TSUNAMI RISK TO AUSTRALIA

Tsunami is a Japanese word - tsu meaning 'harbour' and nami meaning 'wave'. It was coined several hundred years ago by fishermen who came back from sea to discover the harbour devastated by waves, even though there had been no wind and no unusual wave action observed in the open ocean.

Unlike wind-generated waves which cause movement of water near the sea surface, tsunami are waves generated by sudden movement of the sea floor, mostly by undersea earthquakes which cause the entire overlying water column to move vertically. The resulting wave spreads outwards from the source and has wavelengths of 100 kilometres or more. Submarine landslides and, less frequently, a volcano or meteorite impact also can cause tsunami. Although such impacts are rare, it has been suggested that tsunami generated by meteorites or comets may have reached Australian shores in prehistoric times (Bryant 2001).

The impact of a tsunami along a given section of coast is controlled by the depth of the water between the source of the earthquake and the coast as well as the geography of the coastline and the density of population and infrastructure near the coast.

In the deep ocean, tsunami rarely have a peak-to-trough wave height greater than two metres. Typically, they are as little as four to five centimetres. However, as the wave approaches shallow water, it slows down and the deeper, faster moving water catches up: this can dramatically increase the height of the wave. The Indian Ocean tsunami on 26 December 2004 followed an earthquake of magnitude 9.2 which occurred off the west coast of northern Sumatra (Indonesia).

The earthquake generated waves of less than two metres in the open ocean but reached heights of up to ten metres above sea level along many coasts, even on those thousands of kilometres from the earthquake. The maximum height on land (relative to mean sea level) which is reached by a tsunami is termed the run-up height and should not be confused with the peak-to-trough tsunami wave height which often is smaller. A relatively small one to two metre high tsunami at the coast can have much more power than a normal beach wave and inundate low lying coastal areas.

Despite the size and force of the 2004 earthquake, the tsunami hit with little or no warning, inundating coastlines and resulting in the deaths of more than 280,000 people and catastrophic destruction in 11 countries in and around the Indian Ocean. In the aftermath, the Australian Government committed $68.9 million over four years to establish the Australian Tsunami Warning System.

A joint Geoscience Australia, Bureau of Meteorology, Emergency Management Australia and AusAID project, which is due for completion in June 2009, is aimed at helping to protect Australia from the threat of tsunami and providing support to earthquake and tsunami monitoring in the Indian and the south-west Pacific Oceans. Overall, its role is to detect and warn of approaching tsunami generated by major earthquakes either from along one of the plate boundaries surrounding Australia or from further away.

Although a Pacific Tsunami Warning and Mitigation System has been in place for around 40 years, none has existed for the Indian Ocean. A number of nations in the Indian Ocean basin currently are establishing national systems and governments are working with support from the Intergovernmental Oceanographic Commission of the United Nations to weave the systems into an integrated Indian Ocean Tsunami Warning and Mitigation System by the end of 2008. In the meantime, an interim system has been established, with the Pacific Tsunami Warning Centre in Hawaii and the Japanese Meteorological Agency in Tokyo providing Indian Ocean countries with advice on tsunami threat.

In Australia, public advice on any tsunami threat is being provided by the Bureau of Meteorology and Geoscience Australia. The agencies continue to establish extensive networks of seismic and sea level monitoring instruments which are being integrated with sophisticated computer systems designed to detect and forecast the arrival of tsunami. However, because of the proximity of Australia to known earthquake zones off Indonesia and between New Guinea and New Zealand, the travel time for a tsunami from the nearest danger zones is around three to four hours. This provides some limited time to analyse the potential threat and issue warnings. For countries closer to an earthquake source, a tsunami may arrive in less than 15 minutes, allowing limited time for warnings to be issued. On 26 December 2004, strong currents and sea level variations were observed along Australia's west and southern coasts with around 35 people being washed out to sea and subsequently rescued.

While the overall risk from tsunami to the Australian population is lower than it is for many parts of the world, some preliminary assessments by the Australian Government indicate that the north-west and east coast have the potential to be affected by a damaging tsunami resulting from a large earthquake. A recent relatively significant event was the 17 July 2006 Java tsunami, which achieved a run-up height of eight metres above sea level on isolated sections of the Western Australian coast.

Historically, tsunami which have created the most significant run-up on shore in Australia have resulted from earthquakes off the south coast of Indonesia and inundated parts of the Western Australian coast. Although there have been 44 tsunami recorded along Australia's east coast, few of these have produced significant on shore tsunami run-ups (Dominey-Howes 2007). Despite this the east coast of Australia, especially Tasmania, is exposed to tsunami which could be generated by the Puysegur Trench to the south-west of New Zealand. Tsunami also are generated by events further afield as happened in 1960 when an earthquake with a magnitude of 9.5 in Chile produced a tsunami along the New South Wales coast which caused some damage to marine infrastructure. With the increased population and cost of infrastructure, the threat to life and potential damage to property may be considered to be much greater today.

Although earthquakes are seen as the most likely source for tsunami in the region, there are at least five active volcanic source regions capable of generating tsunami which could affect Australia (Ryan and Davidson 1999). However, the only documented eruption to affect Australia, the Krakatau eruption of 26-27 August 1883, generated a tsunami which, according to eyewitness reports, reached several locations along the Western Australian coast (Hunt 1929).

Because tsunami events are so infrequent and few have occurred in recorded history it is difficult to determine the probability of tsunami events and the resulting impact. Currently, a combination of geophysical, geological and historical research is used to try to determine the probability and characteristics of an earthquake with the capability to produce a tsunami.

In the meantime, the objective of the Joint Australian Tsunami Warning Centre, operated by the Bureau of Meteorology and Geoscience Australia, is to issue warnings to vulnerable areas at least 90 minutes before a tsunami reaches Australian shores.


REFERENCES

Bryant E, (2001) Tsunami: The Underrated Hazard, Cambridge University Press, Cambridge

Hunt HA, (1929) Results of Rainfall Observations Made in Western Australia, Bureau of Meteorology Report

Dominey-Howes D, Marine Geology, 239 (1), pp. 99-123, April 2007

Rynn J and Davidson J, (1999) ‘Contemporary assessment of tsunami risk and implications for early warnings for Australia and its island territories’, Science of Tsunami Hazards, 17(2):107-125