Calculating the threat of tsunamiOn 26 December 2004, an earthquake measuring 9.3 on the Richter scale created a tsunami that led to the deaths of over 289,000 people living in coastal villages in Asia and Africa. Scientists have made important advances towards predicting tsunami by combining mathematics, geology and physics.
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Key textThe word 'tsunami' is Japanese, meaning 'harbour waves'. The Japanese know a lot about the destructive nature of these giant waves, having suffered from their effects for at least 1100 years. But the term 'harbour wave' is misleading, since tsunami don't just occur in harbours. Similarly, the popular term 'tidal wave' is inaccurate, since tsunami are not tidal (although the strength of their impact may be partly dependent on the tide level at the time of arrival). This scientific definition from the Department of Geophysics at the University of Washington, USA is perhaps more informative:a tsunami is a wave train, or series of waves, generated in a body of water by an impulsive disturbance that vertically displaces the water column. What causes a tsunami? Tsunami are most often triggered by earthquakes (Box 1: What caused the Indian Ocean tsunami on 26 December 2004?), but they can also be produced by landslides, volcanic eruptions, explosions and the impact of meteorites or asteroids. A tsunami is often invested with a great deal of energy by the earthquake or other disturbance that triggers it. This was illustrated graphically by the tsunami that hit Chile in 1868 and left ships stranded 3 kilometres inland. In southern New South Wales, tsunami of past ages are believed to have tossed boulders weighing up to 90 tonnes on to cliffs 30 metres high. Wave energy The energy in a wave is proportional to the length (the distance between two crests of the wave) and to the square of the height (the distance between the trough and the crest). This means that high-energy tsunami in the deep ocean may have a height of less than a metre but a wave length of up to 650 kilometres – in effect, its energy is spread out across the ocean. A tsunami with such a long wave length will lose energy quite slowly, so it is possible for it to travel vast distances and still wreak havoc when it hits a coast. For example, an off-shore earthquake near Chile in 1960 sent waves of water speeding in all directions across the Pacific Ocean. The height of the waves was no more than a metre or so – making them indistinguishable in deep water from the general swell. The waves were travelling extremely fast and, just 22 hours later, a wave 6 metres high struck the coast of Japan on the other side of the ocean, killing about 200 people. The tsunami continued to reverberate around the Pacific for days, causing damage whenever it struck land. When a tsunami hits shallow water The energy in a wave is one factor that helps determine the damage a tsunami causes. When the wave hits shallow water, other factors that influence the destructive effect of a tsunami come into play – the velocity of the wave (which itself is influenced by the shape and depth of the sea bed) and its period. Period (T) is defined as the time interval between the passage of two successive crests past a given point. A wave's velocity (v) is calculated by dividing the distance between two crests of the wave (L) by T:
A tsunami grows in height as it approaches land. As the depth of the water decreases so too do wave length and wave velocity, but the energy invested in the wave train remains nearly constant. Because the energy in a wave is proportional to the length and to the square of the height, wave height increases as the seabed becomes shallower. Many tsunami waves don't break as they hit land. They simply surge, flooding low-lying areas and rebounding off cliffs or hills - often causing as much or more damage as they recede back into the ocean. Tsunami calculations are complicated Calculating the velocity, wave height and destructive force of a tsunami for any stretch of coastline is complicated by several factors. For example, the shape of the sea bed can produce effects that might not be predicted by a simple wave equation. The presence of harbours and headlands also cause the waves to reflect, diffract and refract, changing their direction – indeed, some tsunami have been known to 'bend' around islands, eventually engulfing the coast on what was supposedly the protected side. Other complicating factors include the effect of backwash from one wave on the waves that follow and the exact nature of the disturbance that generated the tsunami in the first place. Using maths to solve tsunami mysteries In order to account for such complications in tsunami prediction, scientists use computer-based mathematical modelling techniques. One such technique has provided an answer to one of history's enduring mysteries – the demise of a civilisation on the island of Crete in the Aegean Sea (Box 2: The disappearance of an ancient civilisation). Elsewhere, mathematical modelling and an understanding of tsunami-causing events are used to predict the vulnerability of coastal areas to tsunami. In Australia, for example, models predict that the northwest coast is most susceptible to tsunami. Other parts of Australia are also vulnerable, including the south and central coasts of New South Wales. In Queensland, the Great Barrier Reef may not provide absolute protection from tsunami originating in the Pacific – scientists claim to have found evidence of tsunami having come through gaps in the reef to deposit huge coral boulders. Modelling tsunamis created by asteroids Using computer modelling techniques that were developed to simulate the blast effects of nuclear weapons, scientists calculated recently that an asteroid measuring 4.8 kilometres in diameter landing in the mid-Atlantic would create a tsunami high enough to swamp the entire upper east coast of the USA. The city of New York, for example, would disappear. The chance of such an event is quite remote – Earth is likely to be struck by an asteroid of that size every 10 million years or so. But even an asteroid of about 400 metres in diameter could spawn waves approaching 100 metres in height. Tsunami are rare events To accurately assess tsunami risk, scientists need information on the likely occurrence and location of a tsunami-generating event, the expected magnitude of the event, the shape of the sea bed and the topography of the affected coastal area (Box 3: The warning system in the Pacific). Rarely is all this information available, complicating the identification of potentially vulnerable areas. Nevertheless, as we collect more information, the identification of vulnerable regions using computer models should become increasingly accurate. This should assist town planning – for example, in vulnerable regions it may be necessary to restrict development on low-lying areas, or to build walls to protect dwellings from inundation. To some extent, the danger posed by tsunami is increased by the fact that they are relatively rare events. Efforts made now to predict and guard against tsunami could turn the tide against this killer.
The earthquake was caused by the movement of two of the earth's tectonic plates. The oceanic India plate is moving north at an average of 6 cm per year and is being forced under the continental Burma plate. However, on the 26 December the plates suddenly shifted 15 meters over a length of 1200 km on the ocean floor. The epicenter of the earthquake was off the west coast of northern Sumatra, Indonesia. The shift in the plates on the ocean floor created a tsunami, which spread across the Indian Ocean. Unfortunately, there is no early warning system for tsunamis in countries surrounding the Indian Ocean as there is in the Pacific. The earthquake was detected by many monitoring stations, but there are no tidal buoys or wave sensors in place to detect potential tsunamis that might follow a quake. Although the tremors were felt in neighboring countries, victims were unaware of the approaching tsunami. Apart from the immediate devastating effects of the tsunami, the long term effects on agricultural land due to salt water, sources of fresh water and the psychological impact on the survivors is not yet known. Related sites
One theory is that the Minoans could have been wiped out by falling ash, another is that they were destroyed by a tsunami caused by the volcano. This latter theory is generally held to be more plausible than the first, but until recently there has been little evidence to support it. Joe Monaghan, a mathematician at Monash University in Melbourne, Victoria, used a computer-based mathematical modelling technique called smoothed particle hydrodynamics to help clarify the mystery. He showed that lava spewed out by the volcano could indeed have generated a tsunami. He also calculated that, given the peculiar sea-bed topography between Santorini and Crete, the tsunami could have produced waves up to 40 metres high. By modelling the topography of Crete, Monaghan showed that waves of that height would have flooded large chunks of agricultural land on the island and destroyed dwellings. Scientists are now speculating that soil salinisation caused by the inundation, coupled with the destruction of fish stocks due to pollution from the volcano, may have caused food shortages on Crete that eventually led to a revolt by peasants and the ultimate demise of the civilisation. Further reading
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When high-risk areas are identified, planning procedures could be put in place to ensure that residential developments and major constructions such as power stations are restricted to higher ground, and sea walls built to deflect some of the energy of a tsunami. One such wall, standing about 15 metres high and made of reinforced concrete, was built on Japan's Okushiri Island after it was struck by a tsunami that killed 120 people in 1993. The second method involves monitoring and warning systems such as the one already in place around the Pacific. The Pacific Tsunami Warning System consists of a series of seismic monitoring stations and a network of gauges that measure sea levels. When a seismic disturbance is detected, its location and magnitude are computed. In some susceptible regions, warnings are issued if the magnitude is above a certain threshold. Then the gauging stations are monitored for abnormal changes in sea level. If a tsunami is detected, computer-based mathematical models are used to calculate its speed and direction – taking into account diffraction, refraction and reflection effects, as well as peculiarities in the shape of the sea bed. Coastlines lying in the predicted path of the tsunami are warned of the approaching wave train. This system, coupled with proposed innovations such as deep ocean sensors that can pick up changes in water pressure as a tsunami passes, represents a significant advance in tsunami warning. But it is not so effective against tsunami caused by local or regional events. In the recent tragedy in Papua New Guinea, about half an hour elapsed between the occurrence of the earthquake and its associated tsunami, insufficient time for a warning system to have been of any use. Australia is a participating member of the Pacific Tsunami Warning System, and scientists are considering the development of a tsunami prediction system for vulnerable coastlines around the continent. Related sites
Ausgeo News Issue 78, June 2005 Geoscience Australia’s role in the Australian Tsunami Warning System (by Phil Cummins) Looks at an initiative to provide a warning system for tsunamis.
March 2005, page 1 The Boxing Day tsunami – and the need for a warning system in the Indian Ocean Discusses the cause of the Boxing Day tsunami and the need for an early warning system in the Indian Ocean.
September 2004, pages 4-7 Small threat but warning sounded-tsunami research Geoscience Australia has been modeling open-ocean propagation of earthquake-related tsunamis that affect our western coastline.
Australasian Science May 2006, page 10 Mangroves don’t stop tsunami damage Reports that studies indicate mangrove vegetation does not protect against tsunamis.
Cosmos 27 September 2006 Tsunami less damaging to reefs than people Reports that the activities of humans on coral reefs cause more damage than the December 2004 tsunami.
Ecos No. 128, 2006, page 34 Washing the salt from Aceh’s wounds Covers the efforts of Australian researchers to assist farmers.
Focus Tsunami Special Edition 2005 This special edition contains a number of articles about the December 2005 tsunami.
Issues March 2007, pages 28-30 Past tsunami in Australia (by Ted Bryant) Describes evidence of past tsunami on the east coast of Australia.
March 2007, pages 23-27 The tsunami hazard in Australasia (by Phil Cummins) Suggests that studies of records of tsunami in historic and prehistoric times can indicate where future events may occur.
Nature 5 January 2005 Inadequate warning system left Asia at the mercy of tsunami (by Emma Marris) Scientists and governments were caught unprepared.
5 January 2005 Triple slip of tectonic plates caused seafloor surge (by Michael Hopkin) Biggest quake in 40 years redraws the map.
New Scientist A collection of New Scientist articles on the Asian tsunami is available.
5 September 2007, page 21 Tsunami threat hangs over Bay of Bengal Reports on the increased earthquake risk in northern Bay of Bengal.
1 September 2007, pages 40-43 Disaster machines: Simulating nature’s fury Looks at machines built to simulate tsunamis and other natural disasters.
24 February 2007, pages 52-53 The wave from nowhere (by Richard Lovett) Looks at the cause of an earthquake and tsunami in 1929.
29 July 2006, page 14 How a lullaby can warn of an approaching tsunami (by Rachel Nowak) Suggests that indigenous knowledge saved some coastal communities from the 2004 tsunami.
15 April 2006, page 14 Is replanting coasts the way to protect against tsunamis? (by Emma Young) Asks whether coastal vegetation provides protection against tsunamis.
22 October 2005, pages 38-42 Swept away (by Bill McGuire) Summarises Atlantic Ocean tsunami in the past and the possibility of further Atlantic tsunami.
28 May 2005, page 10 Echoes of the tsunami quake (by David Chandler) Provides analysis of the earthquake that caused the Indian Ocean tsunami.
29 January 2005, page 4 Asian earthquake risk suggested before tsunami disaster
15 January 2005, page 14 Tsunami: The impact will last for decades (by Fred Pearce and Bob Holmes) Discusses some of the long-term effects of tsunami.
15 January 2005, page 16 Tsunami: Reconstructing a most deadly wave (by Rachel Nowak) Researchers are reconstructing the details of the tsunami from the accounts of witnesses.
15 January 2005, page 17 Anatomy of an earthquake (by Katharine Davis) Provides analysis of the size and timing of the earthquake that caused the Indian Ocean tsunami.
11 September 2004, page 14 Hawaiian tsunami left a gift at foot of volcano Research shows that marine fossils at the base of a volcano were deposited by a huge tsunami.
14 September 2002, page 15 Get ready for the killer wave (by Nicola Jones) Describes Australian research linking tsunamis to meteor impacts.
1 September 2001, page 7 Wave goodbye (by Eugenie Samuel) Describes a model that shows tsunamis caused by landslides can turn corners.
Scientific American January 2006 Tsunami: Wave of change (by Eric Geist, Vasily Titov and Costas Synolakis) Describes how data from the December 2004 Indian Ocean tsunami is being used to forecast and model tsunamis.
May 1999, pages 44-55 Tsunami! (by Frank I. González)
Savage Earth (Public Broadcasting Service, USA) Uses animated diagrams to explain how tsunamis occur.
How tsunamis work (How Stuff Works)
Current information on how tsunamis are created, monitoring tsunamis, early warning systems and footage of the 26 December tsunami off the coast of Indonesia.
Tsunamis (Geoscience Australia)
An overview of tsunamis including how they occur and how tsunami warnings are issued.
Welcome to Tsunami! (Department of Geophysics, University of Washington, USA)
A comprehensive site for the non-specialist.
Australian Broadcasting Corporation
Introduction to tsunami (University of Wollongong, Australia)
Discusses the occurrences of tsunamis in Australia in historic and prehistoric times.
Tsunami from asteroid/comet impacts (Australian Spaceguard Survey)
Includes mathematical methods of estimating the risk to coastal regions from tsunami generated by asteroid/comet impacts.
More information can be found at Asteroids data sheet (SPACE.com, USA). meteorite. A fragment of an asteroid or a planet that has been broken off by a collision and eventually falls on the Earth. It consists of solid matter which survives the descent and lands on the Earth's surface. Richter scale. A scale for measuring the magnitude or size of an earthquake. The scale relates to the energy released by an earthquake and is determined from the logarithm of the amplitudes (heights) of the seismic waves recorded at seismograph stations on the Earth's surface. For more information see The Richter magnitude scale, The severity of an earthquake, Measuring the size of an earthquake and Magnitude/intensity comparison (United States Geological Survey). salinisation. The accumulation of soluble salts in soil or water so that they become unfit for their normal uses, such as growing plants or providing drinking water. The main salt is sodium chloride (common table salt) but potassium chloride and magnesium sulfate can also accumulate. tectonic plates. The Earth's surface is made up of huge tectonic plates that have moved very slowly during geological history, and continue to move, thus changing the position of continent and oceans. The plates are about 100 kilometres thick and move at a rate of about 1-12 centimetres per year. For more information see Plate tectonics (Nova: Science in the news, Australian Academy of Science). topography. Surface features of a region (eg, mountains, valleys).
External sites are not endorsed by the Australian Academy of Science. Posted May 1999. The Australian Foundation for Science is also a supporter of Nova.
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