One of the worst effects of offshore earthquakes — giant tsunami waves sweeping everything in its path. Until now, scientists could not answer a simple question: why in some cases they arise, and in the other — no? If it failed to install, then it would be a tragedy like the one that recently occurred in Indonesia, is unlikely to be repeated.
An earthquake measuring 7.5 points, which occurred late in the evening of Monday, October 25, in Indonesia, caused a tsunami, which affected Mentawai Islands in the western part of the country. Interestingly, the epicenter of the quake earthquake was 78 km west of the South Island Pag Mentawai archipelago, at a depth of 20 km below the bottom of the Pacific Ocean.
Perhaps that is why in the first minutes after the earthquake, the Indonesian government said the threat of a tsunami, but then canceled the alarm.
Specialists from the Tsunami Warning Center in the Pacific issued a statement in which he said that the threat of a tsunami occurred for some time, but it passed. However, as subsequent events showed, their optimism was somewhat premature. In the morning of October 26 a huge wave, whose height was about three meters, with roaring attacked poor Mentawai islands, whose inhabitants, trusting scientists project that has not had time to prepare.
Most of the Mentawai islands suffered the aforementioned South Pagano, where the element has destroyed 80% of all buildings. Many people died in the disaster, more were reported missing. Thus, in the village of Betu Monga of 200 residents of the disaster lost 160, with most of the missing — women and children.
In South Pagano tsunami penetrated up to 600 m deep into coastal villages on the island of North Pagano water to get close to homes. Hit the island of Sipura. Only about three thousand people are in special centers for the victims.
According to preliminary data, the huge wave of attack killed 270 people and about 500 are considered lost.
Most people learn from the media about the latest "oceanic hooliganism" and its victims have expressed outrage at the decision of the national authorities to take no action after the earthquake, tsunami because, they say, is no more. Some also blame and scientists — why not give an accurate forecast. In this case, few people think that, in fact, predict the occurrence of tsunami with 100% accuracy so far is simply unrealistic.
Where did these come from the huge ocean waves? It is believed that the tsunami (from Japanese "tsu" — the port, the bay — and "us" — the wave) occur for a variety of reasons. They are likely to generate underwater earthquakes (as in the case of the Mentawai islands) and oceanic volcanic eruption (here recall tsunami formed after the eruption of Krakatoa in 1883, which destroyed more than five thousand ships, and to destroy 36,000 people).
Tsunamis may also be caused by underwater landslides (the New Guinea tsunami in 1998), and because of rapid changes in atmospheric pressure over the ocean (a phenomenon often occurs in the Balearic Islands, where it is called "rissaga"). However, the main cause of the giant waves are underwater earthquakes that occur around oceanic islands, with the epicenter, located on the surface of the shallow bottom.
What exactly causes the water to "rear up"? As we recall, when an undersea earthquake areas of the earth's crust are deformed: some compressed, others crawl on the "neighbors" — that is, changing the surface area and shape of a particular area of the seabed. Because of the small compressibility of water and the speed of the deformation of the above sections of the column are based on the water also moves before it can spread, resulting in the formation of a certain ocean surface water elevation or depression.
Next formed outrage goes into the vibrational motion of water masses, and it already has a well-known tsunami waves that start to spread with great speed (50 to 1000 km / h). It is known that one such earthquake could generate several waves between their appearance sometimes takes more than an hour.
It turns out that, logically, every coastal underwater earthquake should generate these "sea giants," so scientists need to make forecasts on the basis solely of the fact of registration in "dangerous place" tectonic vibrations. But not all so simple. Observations show that often enough even the most powerful earthquake did not generate a tsunami. Conversely, sometimes a very small perturbation of the Earth's crust to the surface of the ocean is a giant wave.
Frankly, the problem of recognizing "tsunamigenic" earthquake has not been solved completely, so the warning service oriented solely on the magnitude of the earthquake.
It is possible that before the recent disaster that occurred in Indonesia, the Centre Tsunami Warning in the Pacific decided that the disturbance was too weak and tsunami will be. Declare a false alarm nobody wanted — the costs incurred by it were so not rich state in the evacuation of people in the "non-existent" catastrophe would have been hit hard by the national budget and, as a result, the income of the population.
But is the emergence of a tsunami really no way to predict? Actually, you can, if based on the model proposed by our scientists at the beginning of this century. Its essence lies in the fact that tsunamis occur only in very specific areas shake the ocean floor, called "seismic gap".
As you know, plate crust (including ocean), which is composed of the surface of our planet, are in constant motion. With one plate often creeps to the other, or "crawling" under it. As a result of such "podpolzany" ocean floor plates under the continental shelf at the contact points, a so-called "peak" — is the front edge of the upper plate, which, as it is sharpened by friction on the bottom.
Scientists have found that all the coastal earthquakes just happen in such a "peak." In this case, a seismologist Sergey Fedotov in the 70's of last century, observed that different parts of the "hood" behave differently. Some shake constantly and others — rarely, about once every hundred years. They are something scientist and described as "seismic gap".
Followers Fedotova found that these "seismic gaps" are much more dangerous than standing "quaking grass." The longer they are "silent", the greater the threat to the "explosion" because for many years they accumulate sufficiently strong material stress. But all the creeps lower plate continues to press the "peak" — and as a result, at some point seismic gaps "blast" that is shifted at high speed towards the ocean surface, "davilschika."
Since the mass of the material is given site, and the length of the shift are large enough, the vibrations transmitted to the huge water column, resulting in a wave is born with an enormous charge of energy. This is the infamous tsunami. Curiously, all the forecasts Fedotov, the time and place of the "sea of giants" in the Kuril Islands were confirmed — this is one of the best proofs that the proposed model of local scientists is really close to the truth.
So, as you can see, even predict the occurrence of tsunamis is difficult, but still possible. Obviously, this is necessary to make a detailed map of the locations of all seismic gaps oceans and closely monitor their behavior. When this is done, all the messages that seismologists again missed another tsunami, will no longer appear in the newspapers.