|
Tsunami are surges of water that can be generated by earthquakes, volcanic eruptions, landslides or meteorite impacts. Tsunamis, unlike normal cresting waves, hit the shoreline with a large plateau of water behind them.
The impact of a tsunami on land has been likened to a ‘violent onrushing tide’. Large objects such as ships can be carried several kilometers inland before a tsunami subsides. The enormous surge of a tsunami can also lift giant boulders, flip vehicles, and demolish houses.
Predictions
According to predictions by Geoscience Australia, earthquakes generated in the Indonesian Arc are most likely to be the cause of tsunamis to hit WA’s coast. It is thought that the coast from Shark Bay to the Kimberley would have significantly higher tsunamis than the rest of WA’s coast. In particular, the area between North West Cape (Exmouth) and Dampier is most likely to face the highest tsunami wave generated by an Indonesian earthquake.
However, the severity of any tsunami impact will depend on:
- the nature, intensity and location of the earthquake and
- the physical characteristics of the coast in its path. Apart from beaches, bay mouths, tidal flats and the shores of large coastal rivers are especially vulnerable to tsunami.
History of tsunamis in WA
Several large tsunami have hit Australia’s north-west coast. The largest, at Cape Leveque on August, 1977, ran onto land at a height of six metres. And in June, 1994, a tsunami struck the coastline in the north of the state, which included Broome, King Bay, Onslow and Carnarvon. In some places fish and other ocean creatures were carried inland for 300 metres. The same tsunami had killed 200 people four hours earlier in Java.
Minor tsunami which present little or no threat to our coastal communities have been recorded about once every two years.
How is a tsunami created?
Significant areas of ocean floor must shift vertically over a wide area to create a tsunami. A vertical shift in the sea floor displaces the water above it, creating a series of surges that spread out on the surface like ripples caused by a stone dropped in a still pond.
The speed and height of these surges are controlled by water depth. In deep water, tsunamis reach speeds of up to 1000 kilometres per hour and a height of less than 30 centimetres. However, they can go unnoticed by anyone in a boat.
As they reach shallower water, tsunamis slow down and gain height. In some cases, the face of a tsunami have been up to 50 metres high and caused significant damage to coastal areas.
Multiple waves are a characteristic of tsunamis and the distance between tsunami surges can be minutes or hours. During the tsunamis that decimated Asia on December 26th 2004, many people lost their lives by returning to the shoreline after the first wave had receded.
Australian Tsunami Warning Service
Tsunami warnings are being provided by the Australian Tsunami Warning Service, which is made up of a number of Commonwealth and State government agencies.
Geoscience Australia constantly monitors for tsunami waves in open ocean using a network of special stations, called DARTs buoys (Deep-ocean Assessment and Reporting of Tsunami). The stations consist of a wave sensor anchored to the seabed and a buoy on the ocean's surface to relay data to ground stations via satellite.
The Bureau of Meteorology (BoM) uses this information and data from coastal tide gauges and seismometers to determine if a tsunami has been generated. If there is a positive identification, BoM is also responsible for issuing alerts to emergency agencies, media and the public.
Upon advice from BoM, FESA activates the state arrangements so the emergency services will response to assist the Western Australian community to reduce the impact of a possible tsunami.
Links
Geoscience Australia: www.ga.gov.au
Bureau of Meterology: www.bom.gov.au |
| Tsunami threat map: This image shows a prediction for minimum off-shore tsunami heights off Western Australia. In any year there is a one-in-500 chance of these heights being exceeded. Click here for larger image. | |