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Geophysics

Tsunami – Effect at a distance

The energy released by the Asian earthquake may well set off other strong tremors throughout the planet

The earth trembled, the sea revolted and in minutes small waves formed that ran at the speed of an airplane. Close to the beach, their amplitude expanded, reaching twenty meters in height, and they gained the power to advance five kilometers into the coastline of eleven countries in the south of Asia and on the east coast of Africa, on Boxing Day of last year. By the end of January the body count was more than 230,000 persons, mainly in Indonesia, Sri Lanka, India and Thailand, and there were millions of wounded and homeless, as well as towns destroyed, now without water, electricity and roads.

After the initial impact of the catastrophe that devastated, in perhaps an irreparable manner, the lives of the of the poor inhabitants of the region, the geophysicists have become intrigued as to the peculiarities and possible unfolding information on the largest earthquakes that have occurred over the past forty years – this one was the fourth largest registered since the invention of the first seismographs in 1900. This is the first time that such a strong earthquake was registered – of magnitude 9 – outside of the so-called Pacific Ring of Fire, the sinuous belt that follows the coastline of the oceanic countries of the east of Asia, and the coasts of North and South America, inside of which 80% of the planet’s earthquake are concentrated.

The energy released by the devastating earthquake of the 26th of December, was equivalent to 30,000 atomic bombs, such as the one used to destroy the Japanese city of Hiroshima in 1945, when summing up the vibrations that persist in the region and which can still cause lots of damage. “Theoretically”, says Marcelo Assumpção, a geophysicist at the University of Sao Paulo (USP), “major earthquakes can affect the distribution of tensions in all of the other tectonic plates that form the earth’s crust and can precipitate tremors equally as strong in other regions”.

Vasile Marza, a geophysicist at the University of Brasilia (UnB), believes that this earthquake may well represent the start of another cycle of very intense tremors. Since 1900, when the measuring of the magnitude of earthquakes, also called seisms, began, there have only been five occasions with a magnitude equal to or greater than 9 on the Richter Scale. Excluding the one of 2004, the other four were concentrated during a period of twelve years, between 1952 and 1964: there was one in Russia, two in Alaska and one in Chile. Even before this shock in Asia, based on statistics, specialists had been waiting for another magnitude 9 earthquake in the region of the north of Chile or the south of Peru, since there you are dealing with one of the geologically most unstable areas on the planet. It was in the south of Chile, during 1960, that the largest earthquake of the 20th century occurred, of magnitude 9.5, which was followed by huge waves – or tsunamis – which reached Hawaii, the Philippines and even Japan, devastating all that they found in their path.

“In any place where there has been an earthquake”, says Marza, “it is highly probable that others will follow, the so called post-quakes or replicas”. It is practically impossible to calculate the effect of super earthquakes upon distant unstable regions: the vibrations that echo through the planet could work like a drop of water added to a cup ready to overflow. Therefore, nobody knows what is going to happen, but it is of low probability that something as destructive would be repeated so soon in the south of Asia, since the earthquake that generated the tsunamis alleviated the accumulated tensions under the seabed in that area. Perhaps for that location an episode of that dimension will take at least two hundred years, since a similar one occurred in the south of Asia in 1833, in an indication, by the UnB researcher, that the cycle of repetition could well be around two centuries. “When the area and the accumulated time of pressure build within the planet is larger, the greater is the energy release”, he says.

The chance of the Brazilian coastline also being devastated by tsunamis is extremely remote – perhaps one case in every thousand years – and there is not even any historical record that one has occurred before. The earthquakes that occur in Brazil are relatively small and do not cause tsunamis, since the country is seated upon a relatively stable region, in the center of the South America plate. Around here, the strongest earth tremor was born in the depths of the Tombador ridge, in  the state of Mato Grosso, exactly fifty years ago, on the 31st of January 1955, and reached the magnitude of 6.2, a modest value when compared to other parts of the world. Even at that, we are not entirely free. Although much more common in the Pacific ocean, tsunamis sometimes form in the Atlantic: the destruction of the city of Lisbon some two hundred and fifty (250) years ago in November of 1755, is an incontestable and sad example of this rare possibility.

The news over the last few weeks, through which one can get a notion of the drama of the families affected, has demonstrated some of the immediate effects of this earthquake upon the planet itself. The descent of the Indo-Australian plate under the smaller Burma plate, which set off the tremor and the gigantic waves, left a scar of around 1,200 kilometers on the ocean floor. Up until that moment, for almost 200 years, the Indian plate had been pressing in a slow manner against the other fragment of the crust, which carries Sumatra and other Indonesian Islands, at a rate of approximately six (6) centimeters per year, close to the rate at which a fingernail grows.

The energy was accumulating until it was finally liberated at 7:59 a.m. on the last Sunday of the year (In Brasilia, it was eleven minutes to midnight on the Saturday), and was registered, to a greater or lesser intensity, by all of the seismographs throughout the world: those at the UnB detected the quake nineteen (19) minutes after it had struck. Its effect could be associated to an uncommon elevation of the level of the sea in Guanabara Bay, registered by engineers from the Federal University of Rio de Janeiro (UFRJ) one day afterwards, or the eruption of a volcano on one of the islands in southern Asia, detected two days after the catastrophe by the Geology Service of the United States.

The movement of the two tectonic plates has redesigned the map of southern Asia. The Indian plate moved close to twenty meters in the direction of Indonesia, elevating some islands and lowering others: it is believed that islands such as Andaman and Nicobar, in Indonesia will now be a few more meters above sea level, while the surface level of the Indonesian town of Banda Aceh seems to be lower. The devastating earthquake must as well have made the earth’s axis tilt a further 2.5 centimeters and shortened the period of rotation by around three millionths of a second. There were changes caused by the dislocation of the mass in the direction of the center of the planet, since one of the heavy plates descended under another, making the Earth turn more quickly.

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 Even though they are impressive, these effects do not change anything of the way of life of those who have survived the torment. “These small variations are within the earth’s normal oscillation”, says Marza, “and have more theoretical importance than practical”. The axis of rotation and the duration of a day normally alter with the passage of a comet close to the Earth; through the action of the Moon or even by tremors such as that of 1960 in Chile, when the Nazca plate submerged below the South American plate, creating an imbalance in level of some meters with an extension of one thousand kilometers along the coast.

According to the UnB researcher, it can now be perceived that there were precursor signals to the south Asian earthquake. “Since 1995”, says, “the yearly number of seisms of magnitude above 7.0 gradually has gone down, in an indication that there was an accumulation of energy”. Furthermore, last year there were only thirteen tremors with magnitude of 7.0 throughout the world – the annual average is twenty 0). Twelve of these quakes occurred in the western portion of the Pacific Ring and not in the Mediterranean-Himalayan Belt, which is in southern Asia, “Suggesting”, according to Marza, “a concentration of tectonic forces in the respective edges of the plates”.

By examining the map of the earthquakes of last year, the geophysicist from Brasilia realized that there were also two major earthquakes that could well have been considered precursors to that which occurred in the Indian ocean: one of them, in November 2002, sprung forth in the same area as that of December and reached a magnitude of 7.4, while  the other, in July of last year, with a magnitude of 7.3, came about more to the south of Sumatra. Finally, three days before, on the 23rd of December, there was a strong tremor – of magnitude 8.1 – in the southeast of Australia, in another indication that the Indo-Australian plate was showing itself to be in a critical state of instability. “This behavior makes up part of the preparatory process of the mega-earthquake of December”, he underlines. Even at that, the statistics of the occurrence of earthquakes helps little, because earth tremors are natural phenomena that occur by chance – those that are apparently lacking in one year could well not appear in the next, but only two or three years later.

At the end of January, one of the themes forecast for the World Conference on Disaster Reduction, in Kobe, Japan, was the installation of an early warning system against tsunamis in the Indian Ocean. It would involve apparatus similar to that of the Pacific ocean and could be put into operation in less than a year – if well used, and with a good slice of luck, it would allow the population of the areas at risk to seek safer shelter before the arrival of the gigantic waves. In 1975, on forecasting a tremor that would reach the magnitude of 7.3, China managed to avoid the deaths of 200,000 people, but the system of alert failed the following year when an even more intense earthquake almost entirely destroyed the city of Tangsham. Around 1 million people died.

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