The term tsunami comes from two Japanese words that mean "harbor" and "wave."
A tsunami is a series of large waves generated by an abrupt movement on the ocean floor that can result from an earthquake, an underwater landslide, a volcanic eruption or - very rarely - a large meteorite strike.
However, powerful undersea earthquakes are responsible for most tsunamis. Seismologists say only earthquakes measuring greater than 7.0 on the Richter scale can produce a major tsunami.
Most earthquakes that generate tsunamis - including Friday's jolt off Japan's eastern coast - occur in areas called subduction zones, where pieces of the Earth's crust press against each other. Subduction means that one tectonic plate slides beneath another and sinks deep into the Earth's mantle.
The friction between two slow-moving plates of the Earth's crust creates vast amounts of seismic energy which is released in the form of an earthquake. When a strong undersea earthquake strikes a relatively short distance below the sea floor, it abruptly pushes up one of the immense plates of the Earth's crust. That suddenly displaces an enormous amount of ocean water which becomes a tsunami, spreading outward in every direction from the epicenter of an earthquake - like ripples on a pond, only on a much larger scale.
Most tsunamis, about 80 percent, happen within the Pacific Ocean’s “Ring of Fire,” a geologically active area where tectonic shifts make volcanoes and earthquakes common.
Tsunamis may also be caused by underwater landslides or volcanic eruptions. They may even be launched, as they frequently were in Earth’s ancient past, by the impact of a large meteorite plunging into an ocean.
In deep ocean, tsunami waves may appear only a foot or so high. But as they approach shoreline and enter shallower water they slow down and begin to grow in energy and height. The tops of the waves move faster than their bottoms do, which causes them to rise precipitously.
A tsunami is a series of large waves generated by an abrupt movement on the ocean floor that can result from an earthquake, an underwater landslide, a volcanic eruption or - very rarely - a large meteorite strike.
However, powerful undersea earthquakes are responsible for most tsunamis. Seismologists say only earthquakes measuring greater than 7.0 on the Richter scale can produce a major tsunami.
Most earthquakes that generate tsunamis - including Friday's jolt off Japan's eastern coast - occur in areas called subduction zones, where pieces of the Earth's crust press against each other. Subduction means that one tectonic plate slides beneath another and sinks deep into the Earth's mantle.
The friction between two slow-moving plates of the Earth's crust creates vast amounts of seismic energy which is released in the form of an earthquake. When a strong undersea earthquake strikes a relatively short distance below the sea floor, it abruptly pushes up one of the immense plates of the Earth's crust. That suddenly displaces an enormous amount of ocean water which becomes a tsunami, spreading outward in every direction from the epicenter of an earthquake - like ripples on a pond, only on a much larger scale.
Most tsunamis, about 80 percent, happen within the Pacific Ocean’s “Ring of Fire,” a geologically active area where tectonic shifts make volcanoes and earthquakes common.
Tsunamis may also be caused by underwater landslides or volcanic eruptions. They may even be launched, as they frequently were in Earth’s ancient past, by the impact of a large meteorite plunging into an ocean.
In deep ocean, tsunami waves may appear only a foot or so high. But as they approach shoreline and enter shallower water they slow down and begin to grow in energy and height. The tops of the waves move faster than their bottoms do, which causes them to rise precipitously.
A tsunami’s trough, the low point beneath the wave’s crest, often reaches shore first. When it does, it produces a vacuum effect that sucks coastal water seaward and exposes harbor and sea floors. This retreating of sea water is an important warning sign of a tsunami, because the wave’s crest and its enormous volume of water typically hit shore five minutes or so later. Recognizing this phenomenon can save lives.
Damage is usually worst in areas closest to the undersea quake, often because the fast-moving waves will hit land so quickly. Tsunamis radiate out from the site of an undersea quake nearly as fast as a jet airliner can travel, but because the expanse of the Pacific is so wide, alert systems are in place to sound a warning to those in the path of an advancing wave.
A tsunami is usually composed of a series of waves, called a wave train, so its destructive force may be compounded as successive waves reach shore. People experiencing a tsunami should remember that the danger may not have passed with the first wave and should await official word that it is safe to return to vulnerable locations.
Some tsunamis do not appear on shore as massive breaking waves but instead resemble a quickly surging tide that inundates coastal areas
Signs of a Tsunami along the shoreline
Fortunately, a few natural warning signs can help you understand that a tsunami is approaching and they can be easily recognized. They include:
strong ground shaking from an earthquake: if you are on the coast and there is an earthquake, it may have caused a tsunami, so escape immediately to higher ground or inland, avoiding river valleys;unusual sea-level fluctuations: a noticeable rapid rise or fall in coastal waters is a sign that there may be a tsunami approaching. If you see the water recede quickly and unexpectedly from a beach exposing the ocean floor (the so-called drawback) or a sudden rise of the sea level, escape immediately to higher ground or inland, avoiding river valleys;abnormally huge wave: the first wave in a tsunami wave train is usually not the largest, so if you see an abnormally huge wave, even bigger waves could be coming soon. Run immediately to higher ground or inland, avoiding river valleys;loud ocean roar: if you hear a roaring sound offshore, similar to that of a train or jet aircraft, a tsunami may be approaching, so escape immediately to higher ground or inland, avoiding river valleys.
If you experience any of the phenomena described above, do not wait for official evacuation orders. Immediately leave low-lying coastal areas and move to higher ground or inland, avoiding river valleys. Do not return to low-lying land after a tsunami has hit the coast because other waves are likely to follow the first one and each wave can last up to 30 minutes. The danger may last for many hours and, in some cases, more than 24 hours.
Man-made Tsunami Warning signs and sirens have been placed along the roads of many beaches worldwide to inform the public about the potential risk; these signs should not be ignored and immediate action should be taken if any of the natural signs described above occurs. Response timeliness may save up many lives.
Tsunamis: warning signs in the deep sea
As we have seen in the previous pages, in the deep sea the height of a tsunami from trough to crest can be only about 3 feet tall (1 meter) because the bulk of a tsunami lies beneath the sea surface. As a consequence, tsunamis are hard to detect when they travel across the open ocean and they can arrive completely unexpectedly.
A network of deep-sea sensors would be needed worldwide to detect tsunami movement within the ocean, but that is not easy to implement. Creating the monitoring and communications infrastructure needed to issue timely warnings is a huge task presenting considerable problems, particularly for the poorest areas of the world.
But even with the latest technology and despite a lot of seismic data available, it is equally hard to predict with accuracy when and where a tsunami will hit land and how big it will be. All that tsunami warning centers worldwide can do is to send out an alert to all communities concerned along the coast within few minutes after the detection of a magnitude 7.0 or greater earthquake near or under the ocean.
After receiving a tsunami warning, local authorities must decide what action has to be taken and provide a clear guidance on evacuation procedures. Warning for local tsunamis gives very little time for evacuation.
Modern warning systems are precise but unfortunately cannot be fast enough for warning of a quake close to the shoreline.
