Science » Planet Earth
Author`s name Dmitry Sudakov

Mysteries of unpredictable tsunami waves

One of the most terrible consequences of offshore quakes is giant tsunami waves, sweeping everything on their way. Until now, scientists cannot answer a simple question: why in some cases they happen, and in others they do not? If it was established, then a tragedy like the one that has recently occurred in Indonesia could have been avoided.

An earthquake measuring 7.5 points, which occurred late on Monday, October 25 in Indonesia, caused a tsunami, which affected Mentawai islands in the western part of the country. Interestingly, the epicenter of the aftershocks of this earthquake was located 78 km west of the South Island Pagai of the Mentawai archipelago, at a depth of 20 km below the bottom of the Pacific Ocean. Apparently, this is why the first few minutes after the earthquake the Indonesian government reported the tsunami threat, but later canceled the alert.

Specialists from the Tsunami Warning Center in the Pacific issued a statement declaring that the threat of a tsunami was acute for some time, but then has passed. However, as subsequent events showed, their optimism was somewhat premature. In the morning of 26 October a huge wave, whose height was nearly three meters, with a roaring noise attacked Mentawai islands, whose inhabitants, trusting scientists' predictions, did not have time to prepare.

Among the islands that suffered the most is aforementioned Southern Pagai, where the tsunami destroyed 80% of all buildings. Many people died in the disaster, even more have been declared missing. In the village of Betu Monga of 200 local residents 160 were lost in the disaster; most of the missing are women and children. In Southern Pagai the tsunami penetrated up to 600 meters deep into the coastal villages, on the island of North Pagai the water crept up to the homes. The island of Sipura was also hit. Approximately three thousand people are placed in specially established centers for the victims.

According to preliminary data, as a result of a gigantic wave, 270 people are killed and nearly 500 are missing.

Most people have learned from the media about the tsunami and its victims, and expressed outrage at the decision of the authorities of the country not to take any action after the earthquake, since the tsunami, as they said, was no longer a threat. Some also blame scientists for not giving an accurate prediction. In this case, few people know that, strictly speaking, so far it is simply unrealistic to predict the occurrence of a tsunami with 100% accuracy.

Where do these huge ocean waves come from? It is believed that the tsunami (from Japanese port and wave) occur for several reasons. They are likely to be generated by undersea earthquake (as in the case with Mentawai islands) and the eruption of oceanic volcanoes (remember the tsunami that has formed after the eruption of Krakatoa in 1883, which destroyed over five thousand ships and killed 36,000 people).

Also, tsunamis may occur due to underwater landslides (New Guinea tsunami in 1998), as well as due to sudden changes in atmospheric pressure over the ocean (a phenomenon that often occurs in the Balearic Islands, where it is called ‘rissaga'). However, the main reason for the appearance of gigantic waves is undersea earthquakes that occur around oceanic islands, with the epicenter located close to the bottom.

What exactly causes the water to "stand up"? As we remember, during underwater earthquakes parts of the earth's crust are deformed: some are compressed, the other creep on their "neighbors", that is, changing size and shape of the surface of an area of the seabed. Due to small compressibility of water and the speed of the process of deformation of the aforementioned parts of the bottom, the water located above them is displaced, and having no time to spread, results in ocean surface water rise or fall.

Further it causes into the vibrational motion of water masses, which is nothing else but the well-known tsunami waves, which begin to spread with a great speed (50 to 1000 km / h). It is known that one such earthquake could create a few waves that appear with the interval of over an hour of each other.

It turns out that, logically, every coastal underwater earthquake should generate these "ocean giants," so scientists should make forecasts based solely on the fact of registration of tectonic fluctuations in dangerous areas. However, not everything is that easy. Observations show that quite often even the most powerful earthquake do not generate a tsunami. Conversely, sometimes a very small perturbation of the Earth's crust causes gigantic waves on the ocean surface.

Frankly, the issue of recognizing tsunami potential of an earthquake has not yet been solved completely, therefore preventing services focused exclusively on the earthquake magnitude.

It is possible that before the recent disaster that occurred in Indonesia, the officials of the Tsunami Warning Center in the Pacific have agreed that the disturbance was too weak and the tsunami was not to happen. No one wanted to issue false alarm. The costs incurred by the poor nation in case of evacuation of people with non-existent tsunami would deal a great blow to the budget of the country and, ultimately, the income of the population.

But is the appearance of the tsunami really impossible to predict? In fact, it is possible, if based on the model proposed by Soviet scientists in the beginning of this century. Its essence lies in the fact that tsunamis occur only in very specific areas of the ocean floor, called seismic gaps.

As you know, the plates of the Earth's crust (including the ocean), which compose the surface of our planet, are in constant motion. In this case, one plate is often creeps onto the other, or creeps under it. As a result of such creeping, the plates of the ocean floor under the continental shelf form the so-called shield in the areas of contact - a cutting edge of the upper plate, which sharpens by friction on the bottom.
Researchers found that all coastal earthquakes happen in these areas. In this case, seismologist Sergey Fedotov in the 1970's observed that different parts of this area behave differently. Some shake constantly, and others - rarely, once every hundred years. They were called seismic gaps by the scientist.

Followers of Fedotov found that the seismic gap is far more dangerous than the areas that are constantly shaking. The longer they are silent, the greater the risk of explosion, because for many years, they accumulated a sufficiently strong voltage material. But creeping of lower plate continues to put pressure on the area, and as a result, at some point seismic gap explodes, i.e. moves with a great speed towards the ocean surface.

Since the mass of the material that makes up the area and the length of the displacements are large enough, the vibrations are transmitted to a huge water column, resulting in a wave born with an enormous charge of energy. This is the infamous tsunami. Interestingly, all Fedotov's forecasts about the time and place of tsunami in the Kuril Islands were confirmed - this is one of the best evidence that Soviet scientists proposed a model indeed close to the truth.

As you can see, predicting the occurrence of a tsunami is difficult, but still possible. It is true that a detailed map of the locations of all seismic gaps in the oceans needs to be compiled and their behavior should be closely monitored. When this is done, no messages about seismologists who have again missed a tsunami will appear in the media.

Anton Evseev

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