Detectors on the seabed will warn of the giant waves before they hit the shore, and will allow residents to be evacuated from the danger zone
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Strong waves caused by earthquakes or underwater landslides - often known by their Japanese name tsunami - pose a serious threat to many residents living on the Pacific coast. When a 15-meter-high wave hit the northern shores of Papua New Guinea in July 1998, residents were surprised. The tsunami, the most destructive of its kind in recent years, killed more than 2,200 villagers at the time, but it was just one in a string of deadly waves that have hit the western Pacific in recent years. Since 1990, ten large tsunami waves have claimed the lives of more than 4,000 people. The possibility of predicting the tsunami ahead of time will allow the evacuation of the residents of the beaches.
It is not an easy thing. Seismometers (which measure earthquakes) and coastal tide gauges - devices already used for other purposes - can provide some warning. The seismometer measures earthquakes that can cause tsunamis, but it cannot predict the formation of the waves themselves. It also cannot predict landslides caused by earthquakes, but these landslides often cause the most destructive tsunami waves.
Tide gauges can locate a tsunami with great accuracy based on its effect on sea level, at least when it approaches the coast. But since the tsunami moved at a speed of more than 450 km/h, it is difficult to define this as a warning. One solution to the early detection problem is to place pressure detectors on the seabed, which detect the increased pressure created by the tsunami.
Japan was the first to place such detectors on the seabed, and today it uses 14 detectors. They are more efficient than tide gauges, but the problem is that they are connected to the land via underwater cables, so they can actually only be used at a distance of 50 km from the land. To get around the problem, the US National Oceanic and Atmospheric Administration (NOAA) tries to track the waves in the open sea. The "tsunometers", as the director of the laboratory for the marine environment of the Pacific Ocean, Eddie Barnard, calls them, transmit alerts to floats on the surface of the sea, and these transmit the information, via satellites, to the Maritime and Atmospheric Administration.
Five sonometers were placed on the seabed in the North Pacific Ocean and one was placed in the South. A seventh tsunamimeter will be launched near Chile in November this year, to intercept tsunami waves that form off the coast of South America. Although the system has yet to prove itself in detecting a large tsunami (no such wave has yet formed since the detectors were placed), it has successfully detected several small tsunami waves.
At the same time, the Japanese are trying a different approach. A group of researchers at the Institute for Earthquake Research at the University of Tokyo is developing a warning system that relies on GPS navigation satellites that monitors the movement of buoys placed in the open sea. By placing a GPS antenna on the beach and on the buoys, the researchers can compare the "height" of the buoy with the height of a stable point on the beach. With this method, says Teriyuki Kato, who heads the project, it is possible to measure the vertical movement of the buoy with an accuracy of a few centimeters, thus detecting dangerous tsunami waves in the open sea, while they are still small waves. The waves only reach their deadly height when they reach shallow water.
Another research direction is the analysis of a sound wave known as a T-phase wave. Rocks deteriorating on a slope create such sound waves, which the sea carries both to the distant shores and to the nearby shores. Emil Okal of Northwestern University in Evanston, Illinois noticed that these waves, created by earthquakes, can be picked up by hydrophones (underwater microphones) of the type used to locate submarines.
However, the early detection of the tsunami is not enough, but the wave must be classified according to the degree of danger inherent in it. One way to do this is through computer simulation. Models developed by Vasily Titov of the Oceanic and Atmospheric Administration and Costas Sinoulakis of the University of Southern California can predict the shape and size of the waves that will be generated by a particular tsunami as well as the rate of coastal inundation that the wave will cause. This information can assist in the evacuation of disaster-prone areas.
Idan Earth - the stalwarts of nature
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