Tsunami

A roar like the sound of an approaching airplane resonates ominously through the air. Then thundering masses of water from huge waves grab at everything in their path. Trees collapse, people scream and try to escape from the floods, others disappear in the waves. Finally, the tsunami has run riot and is taking everything that is not nailed down with it back into the sea.

These or similar were the descriptions of the survivors after the tsunami catastrophes mostly. And there were quite a few of them. The Hawaiian archipelago in the middle of the Pacific Ocean alone has been rocked by major tsunamis 13 times this century.

What actually are tsunamis? What causes can they have? Can you predict tsunamis? We will try to answer these and other questions on the following pages.

Dieter Lohmann
Status: 06.08.2000

What is a tsunami??

Tsunami, the "long harbor wave", the euphonious term for the notorious giant waves originates from Japanese and goes back to early observations by Japanese fishermen. While working on the high seas, they usually did not notice any major waves, but when they returned to their home port, they found their villages and fields devastated by giant waves.

The Japanese name for giant waves has now also become established among scientists. Since an international scientific conference in 1963 it is known and accepted worldwide.

Perhaps the earliest description of a tsunami dates back to 479 B.C., when a natural disaster in the northern part of the Aegean Sea caused enormous damage. Today, however, it is known for certain that giant waves had already occurred earlier. Two million years ago, for example, a meteorite impact off the coast of South America triggered a huge tsunami that devastated the coast of Chile, among other places. Some researchers even suspect that the Flood may have had its cause in a tsunami event…

Coastal dwellers in particular fear tsunamis, as they often lead to the total destruction of low-lying areas. In this century alone, tens of thousands of people have already perished in giant waves.

Meanwhile, tsunamis are the subject of intensive international research. The goal of marine researchers around the world is to discover the nature of these giant waves and thus minimize the damage caused by tsunamis in the future.

For a long time tsunamis were mistaken for tidal waves. Today we know that their origin has nothing to do with the diurnal change of high and low tide..

Seaquakes as tsunami triggers

When the earth shakes somewhere along the plate boundaries in the sea or hot spots, it is not uncommon that tsunami danger is imminent. Then if the earthquake still has a magnitude of 7 or more on the Richter scale, it becomes critical. Especially along the Pacific Ring of Fire in the Pacific Ocean, such major earthquakes that lead to a tsunami occur again and again. Here, the denser oceanic plates submerge under the continental plates, a process geoscientists call subduction. During subduction, plates become entangled with each other and stresses build up in the rocks. Due to a sudden jerky movement of the fracture floes, these tensions dissolve – the earth shakes.

Map showing earthquake distribution in the Pacific region

The bottom of the sea is shaken violently and lowered or bounced up sharply in the process. The huge masses of water overlying the geological fault are thrown off balance by the quake and set in vertical motion. In an attempt to restore balance, waves of greater or lesser strength are created under the influence of gravity. When very large areas of the seabed are deformed, the effect is sometimes so strong that a tsunami is formed.

The resulting elongated ocean waves then spread out at great speed in a circle from the earthquake’s source. A similar phenomenon can be observed in everyday life when you throw a stone into a pond or simply into a bathtub.

Although most tsunamis are triggered by seaquakes, there are other events on the seabed or near the oceans that can lead to the gigantic surges..

Events that can lead to giant waves

In principle, tsunamis can occur with any "disturbance" in the sea that brings large masses of water out of their equilibrium – including, of course, volcanic eruptions. A violent undersea explosion, for example, releases enough energy to push the water column upward. But even with eruptions above sea level, the momentum can be strong enough for tsunami waves to spread from their point of origin at least at TGV speed…

Tsunami

One of the biggest tsunami disasters in human history also occurred after such an eruption. Fire Mountain Krakatau exploded in 1883 in the Sunda Strait between Java and Sumatra. Not only a large part of the summit was blown off, but also the submarine caldera fell into the empty magma chamber.

At an average speed of 100 kilometers per hour, the resulting tsunami raced toward the coastal regions of the neighboring islands. On the open sea, the shallow waves were hardly noticeable at first. Only when approaching the coast, the tsunami slowed down its speed and the waves piled up to enormous heights. Within two hours of the volcanic eruption on Krakatau, large areas in the coastal regions of the islands of Sumatra and Java were caught in the tsunami.

Almost 36.000 people died within a radius of 80 kilometers around Krakatau in ocean waves up to 40 meters high, 295 places were completely destroyed in the course of the catastrophe.

Landslides

Violent undersea landslides, which often occur in conjunction with earthquakes or volcanic eruptions, can also result in dreaded tsunamis. Pressure is exerted on the water column from above, which can lead to dangerous wave formation.

However, tsunamis generated by volcanic eruptions or landslides usually carry much less energy than tsunamis generated by submarine earth tremors. Therefore, they usually dissipate much faster and rarely affect coastal regions far from the point of origin.

Danger from outer space

Tsunami

Deep Impact and Armageddon say hello – another, rather rare variant of tsunami generation comes from space.

For example, if a meteorite plunges into the sea at high speed, U.S. scientists calculate that it could give birth to a massive tsunami that would rush toward coasts with wave heights of 50 to 100 meters, bringing death and devastation with it.

Two million years ago there was already such a catastrophe. The distant ancestors of man living at that time, the australopithecines, above all the world-famous "Lucy", will probably have had a good scare when Eltanin crashed into the Pacific Ocean off the coast of Chile.

As the scientist Dr. C. As Mader noted, the huge celestial body – between one and four kilometers in diameter – did not cause a crater in the ocean floor, but it did cause an oversized tsunami that struck the coast of South America, among other places. However, meteorite impacts of this magnitude on Earth are very rare after all.

Nuclear explosions

More realistic is – unfortunately – the possibility that a nuclear explosion could trigger a tsunami. Whether the people potentially affected by the tsunami will then have time to fear the wave, however, remains to be seen…

Where do tsunamis occur most frequently?

From the observations of scientists and historical sources we know today that tsunamis can occur in all major oceans of the world. Nevertheless, they are not equally frequent everywhere. Just as the mutated giant lizard Godzilla prefers to wreak havoc in Japanese cities, tsunamis have chosen a special "playground" for themselves.

The Pacific Ocean is particularly threatened by these catastrophic events because tsunami-generating events, such as earthquakes, are more common along the Pacific Ring of Fire. Ocean-wide tsunamis that threaten large parts of space are, however, extremely rare. They occur on average only every ten to twelve years.

But that doesn’t mean Pacific riparians will have peace from death waves in the meantime. Once or twice a year, dangerous local tsunamis race through the ocean somewhere in this area, causing major damage. Weaker giant waves, which have no or only minor consequences, are even more common.

Scientists in the various tsunami research institutes do not quite agree, but supposedly the last Pacific-wide giant wave occurred more than 30 years ago. In 1964, after the earthquake catastrophe in the Aleutian Trench off the coast of Alaska, this one claimed many hundreds of lives. Its effects were still felt in Antarctica.

If the calculations of the scientists are correct, a new death wave is long "overdue". Due to the strong population growth and the dense settlement of the mostly fertile coastal regions in the Pacific region, many more people would be threatened by the catastrophe this time than in 1964..

Giant waves in the other oceans

In the geologically less active oceans such as the Atlantic, the Indian Ocean or the Mediterranean, however, severe tsunamis are much rarer. That one should not rely too much on such basic rules is confirmed by a look back into the history of Europe. Many of the 70.000 dead after the great 1755 earthquake in Lisbon perished less as a direct result of the earthquake than as a result of the water masses carried to the coast by a tsunami.

Tsunami

Historical records also contain numerous references to tsunamis in the Mediterranean region. For example, the volcanic eruption on Santorini in 1650 is said to have produced a giant wave that caused great devastation on the shores of the surrounding islands.

According to recent findings by U.S. scientists at the Woods Hole Oceanographic Institute, Columbia University and the University of Texas at Austin, the situation in some areas of the Atlantic Ocean could change fundamentally in the near future. During their investigations, researchers discovered cracks on the continental shelf off Cheskapeake Bay. As the seabed splits along the east coast of the USA, landslides leading to tsunamis could occur more frequently in the future. The main crisis areas would then be North Carolina and Virginia, but the U.S. capital Washington would also be threatened by the towering waves.

How do tsunami waves differ from "normal" ones? Ocean waves?

Once tsunami waves are on their way, they "travel" at jet speed through the oceans towards the coasts and flood the lower-lying regions there.

But how do tsunami waves differ from "normal" ocean waves?? The "maker" of ocean waves is the wind. The height and length of the waves depends on the strength with which it blows and on the size of the sea area it covers.

Tsunami

Even in storm-lashed seas, the wind-generated surges barely reach a height of ten meters and a maximum length of 150 meters. However, most of these wind-generated waves are much smaller and shorter. No matter how much it storms, the effect of the wind is limited to the upper layers of the sea. From a depth of about 200 meters, everything is calm; the energy of the wind is no longer noticeable here.

Tsunami waves, on the other hand, which are generated by geophysical processes, behave quite differently. Wavelength and height depend critically on the strength of the event that triggered the tsunami. Normally, the wavelengths of tsunamis on the open sea are huge. 100 to 300 kilometers, in extreme cases even 500 kilometers, lie between the individual wave crests.

Such tsunamis can travel huge distances. The longer the wavelength, the less energy a wave loses as it travels through the ocean. At least this is what the physicists claim. No wonder, then, that some tsunamis are still more than 10.000 kilometers from the point of origin can show their enormous forces. After the volcanic eruption on Krakatau, for example, changes in sea level could even be measured at European tide gauges.

The speed at which tsunamis chase through the world’s oceans depends on the depth of the sea in question. Marine researchers at the Pacific Marine Environmental Laboratory have found that in the Pacific Ocean, with an average water depth of 4.200 meters, top speeds of more than 750 kilometers per hour are not uncommon – you can’t get to your destination much faster, even with an Airbus. In the deepest ocean regions of the world, these records are even topped. The tsunami can travel almost 1000 kilometers – more than the distance from Flensburg to Munich – before an hour has passed.

Unlike normal wind-generated waves, in a tsunami the wave energy is transmitted from the entire water column down to the sea floor. So tsunamis have ground contact virtually everywhere in the oceans.

While even a simple paddle boat need not fear tsunamis on the open sea – the waves here are extremely tame and only reach heights of about 50 to 100 centimeters – real wave giants arise near the coast..

A tsunami on "shore leave

Similar to the dramatic change in appearance of a tadpole as it metamorphoses into an adult frog, a tsunami also transforms as it leaves the deep ocean areas and approaches land. The elongated wave, barely a meter thick, becomes a towering giant, sometimes up to 30 meters high, which threatens to engulf entire coastal regions. The reason for this is that the growing ground friction abruptly slows down the speed of the wave. The wavelength of the tsunami shrinks dramatically without any significant reduction in the energy carried by the waves.

Many people think that a tsunami always comes ashore in the form of a roaring, breaking wave – well known from the surf beaches of Hawaii or elsewhere – and destroys everything in a gigantic whirlpool of water. But this is rarely the case.

Much more frequently, however, a lightning-fast rising and falling "tide" is observed, flooding the lower-lying areas with great ferocity. How the giant wave behaves in detail depends on various factors on the ground. Reefs, estuaries, undersea mountains, offshore islands, the slope of a beach – all these topographical elements can influence the appearance of the tsunami as it "makes landfall".

According to observations by various U.S. scientists, wave heights rise particularly sharply during a tsunami when it enters bays that have a wide opening and become narrower and narrower toward the shore. Normal waves, triggered by the wind, can additionally ride on the crests and further increase the tsunami wave. An offshore coral reef, on the other hand, usually takes away much of the tsunami’s energy and reduces its impact on coastal regions.

Just like the radio operator on board a ship in danger S.O.S.-As a tsunami sends out clear warning signals before it reaches the coast, it also sends out clear warning signals before it reaches the coast. Before the arrival of the tsunami, tsunami researchers first register a rapid rise and fall of the sea level by one or two meters several times. At the latest, when a longer, pronounced low tide follows, during which rocks and reefs otherwise covered by water are exposed, the observer should hurry to find a safe spot.

The chroniclers of such natural disasters, however, repeatedly report of daredevils who perished while trying to take advantage of this low tide for a short walk. They were suddenly confronted with a tsunami wave that had built up in front of them. There was then no time left for escape.

But there is another reason why caution is advised when it comes to tsunamis. A tsunami usually consists not of a single wave but of several successive surges. Many people have lost their lives because they returned to their homes after the first tsunami wave and were surprised there by another giant wave, some of which were even stronger.

A balance of giant waves

On average, a tsunami wave contains one million tons of water in motion – this is the estimate of oceanographers. The amount of energy that a tsunami carries with it when it reaches the mainland is correspondingly large. No wonder that the damage caused by a tsunami is usually enormous.

More than 50.000 people have drowned in the seething waters of the waves during the various tsunami catastrophes in the last 100 years alone, entire stretches of coastline have been devastated, ships and their crews have been thrown ashore, buildings have been destroyed, and crops have been destroyed. Severe erosion damage is also repeatedly observed after tsunamis. Beaches that nature has created over decades or centuries can be eroded by a tsunami in 30 seconds.

The floods and devastation usually affect only the shallow coastal areas; only rarely do the waves eat their way up to a kilometer inland. But at least up to 30 meters of altitude can be overcome in the process.

Much of the worst damage is not even caused by the actual flooding itself, but by the powerful suction that forms when the tsunami recedes after the attack. Whole houses or even sports halls, including their foundations, are swept back into the sea by the "killer waves".

The damage amounts to billions of dollars in the meantime. The word tsunami therefore has a bitter ring to it, even among insurance companies. They count the giant waves among the most dangerous and expensive natural disasters.

The Pacific Tsunami Warning Center

Once again raging water masses. Palm trees snap off like matchsticks. Ships are hurled ashore. People running for their lives. The death wave tsunami struck again in 1946. This time in Hawaii. And once again out of the blue and without any warning. 159 people could not escape the floods, 96 of them in Hilo alone, the main town of the Big Island. Hilo is considered a "wave trap" by oceanographers anyway. Time and again, tsunamis are brought here by the special relief of the seafloor with underwater mountains around the Hawaii archipelago.

Tsunami

But this time, the natural catastrophe also had something good to offer. Finally, scientists and politicians began to think about how to protect people and coastal regions from the worst consequences of the giant waves. It was clear that tsunamis cannot be prevented, so strategies had to be developed to warn the possibly affected regions of an impending tsunami. Good advice was expensive at first. Finally, in 1948, the U.S. established the Pacific Tsunami Warning Center (PTWC), based in Hawaii.

What began as a protection for the U.S. population has now become a multinational effort. Well over 20 Pacific riparian states now participate in the Pacific Tsunami Warning System and work hand in hand with the American institutes and researchers.

Today there is not only the Tsunami Warning System in the area of the Pacific Ocean. Researchers in other parts of the world have also learned from natural disasters and installed technical aids to warn the population. To protect their coasts from smaller, local tsunamis, particularly endangered countries, such as Japan, even have their own warning centers. But how does such a tsunami warning system work??

How the warning system works?

Since the most important cause of tsunamis – seaquakes – cannot be predicted in time and with the necessary certainty, it is also impossible to know in advance when tsunamis will occur. PTWC researchers must therefore limit themselves to detecting and localizing new earthquakes as quickly as possible with the help of well-monitored seismographs distributed throughout the Pacific region. Computers quickly calculate the strength and position of the quake and the direction of movement of the rupture floes from the data obtained.

In the case of an earthquake with a magnitude of 7 or more on the Richter scale, special urgency is required in the evaluation process. As long as it was not yet clear exactly how strong the quake was and whether a tsunami wave had really been triggered, a "tsunami watch" message was first forwarded to the relevant agencies in the various countries.

Tsunami

The PTWC only issues a tsunami warning when significant changes in sea level are detected at several measuring stations in the Pacific region, i.e. when it is relatively likely that a tsunami wave is imminent. First of all, of course, the measuring stations that are in the immediate vicinity of the epicenter of the earthquake are surveyed.

Once it is clear which path the tsunami will take, the authorities inform the general public in the affected areas via radio and television stations. In parallel, any necessary evacuation measures will be initiated.

Since the relief of the sea floor is relatively well known to the oceanographers and the speed of a tsunami depends essentially on the water depth, the scientists can determine the travel times and travel directions of a tsunami quite accurately.

In the meantime, there are even world maps showing the areas that are considered to be at risk of tsunamis based on historical experience. And in Hawaii, public telephone directories include graphics showing the various evacuation zones on each island.

Everything is fine, one would think. But it is not quite like that..

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