EARTH’s hard outer shell, known as the lithosphere, is not continuous across the surface of the planet. Instead, it consists of 12 rigid plates between 60km and 200km thick that are composed of continental crust, oceanic crust, and the upper part of the mantle.
These plates ìfloatî on the underlying and more flexible layer known as the aesthenosphere. When two plates converge, pressure forces sections of crust to pile up, and mountain systems and volcanoes form on the overriding plate. Meanwhile, the subducting plate plunges deep into the mantle. Large earthquakes with epicentres located at undersea subduction zones are the most frequent causes of powerful ocean waves known as tsunamis.
Tsunamis carry the energy produced by earthquakes or, less frequently, other earth disturbances such as volcanoes, landslides, and meteorites. When tsunami waves break on land, this energy is released and can cause catastrophic damage. Certain physical characteristics of tsunami waves along with the waves’ behaviour upon reaching land explain a tsunami’s enormous capacity for destruction.
The deeper the water, the faster tsunami waves travel. Despite their high speed, in the open ocean tsunami waves have a low amplitude (height) and long wavelength (distance between crests). Because waves lose energy at a rate inversely related to their wavelength, tsunami waves typically experience very little energy loss as they travel across the ocean.
When a tsunami wave nears land, its wave profile changes drastically. In the open ocean, the wave energy extends from the surface to the bottom, but in shallower water, the wave gets compressed. As the wave’s leading edge interacts with the rising seafloor, it slows. The water moving in behind piles up, so that when the wave finally reaches the shore, it may have risen to tens of metres in height.
If the trough (or low part) of the wave is first to reach the shore, water at the shore gets drawn seaward before the peak (or high part) arrives. To help understand why this happens, imagine a string arranged in a wave pattern on a table. If you shorten the wavelength, the amplitude increases and the ends of the string pull inward. Likewise, as a tsunami wave approaches shore, its wavelength shortens, pulling water from all directions, including the shoreline.
Because water is very heavy ñ a cubic metre weighs a metric tonne ñ a tsunami wave is capable of inflicting immense destruction on land. The presence of reefs and steep coastal shelves, however, can act as a breakwater to lessen the force of the wave.
Tsunamis are the result of processes that shape the Earth’s surface: earthquakes, volcanic events, and landslides. People cannot control the dynamic forces that drive tectonic events. Nor can they prevent most of the physical devastation they impart. However, people can reduce the number of lives lost to tsunamis by using technology to assess hazards, develop warning systems, and educate the public.
The undersea earthquake that struck 160km west of Sumatra, Indonesia in December 2004 was more powerful than all of the world’s earthquakes over the last five years combined. The magnitude 9.0 to 9.3 event, which shifted the entire planet, spawned a tsunami that killed as many as 300,000 people and displaced more than 1.5 million people living near the Indian Ocean.
Many of those who were killed, especially in places far from the earthquake’s epicentre, might have been saved if a comprehensive tsunami warning system had been operating in the Indian Ocean. With 600 million or more people across the globe predicted to be living within 100km of coastlines by 2025, setting up effective tsunami warning systems in all the world’s oceans is a critical task. Still, there are challenges, including technological limitations, financial constraints, and the bureaucratic red-tape that hinders international cooperation and communication.
A warning system comprising a seismic detection system, a tide gauge system, and a communications system has been in place in the Pacific Ocean since 1948, and it provides a model for an Indian Ocean system. The Pacific system has accurately detected every major tsunami since its installation. New sea-floor sensors, designed to monitor pressure changes in the water above them, were installed in 2002 to help reduce the high false-alarm rate (75%). The cost of implementing a system with similar instrumentation in the Indian Ocean would be between $US250 million and US$400 million (K714 million to K1.14 billion). – www.teachersdomain.org