Are Tsunamis Dangerous?



How does tsunami energy travel across the ocean and how far can tsunamis waves reach?

Once a tsunami has been generated, its energy is distributed throughout the water column, regardless of the ocean's depth.  A tsunami is made up of a series of very long waves.  The waves will travel outward on the surface of the ocean in all directions away from the source area, much like the ripples caused by throwing a rock into a pond.   The wavelength of the tsunami waves and their period will depend on the generating mechanism and the dimensions of the source event.   If the tsunami is generated from a large earthquake over a large area, its initial wavelength and period will be greater.   If the tsunami is caused by a local landslide, both its initial wavelength and period will be shorter.  The period of the tsunami waves may range from 5 to 90 minutes.  The wave crests of a tsunami range from a few to a hundred kilometers or more apart as they travel across the ocean.  As the waves approach the coast, their wavelength decreases.

On the open ocean, the wavelength of a tsunami may be as much as two hundred kilometers, many times greater than the ocean depth, which is on the order of a few kilometers.   In the deep ocean, the height of the tsunami from trough to crest may be only a few centimeters to a meter or more - again depending on the generating source.  Tsunami waves in the deep ocean can travel at high speeds for long periods of time for distances of thousands of kilometers and lose very little energy in the process.  The deeper the water, the greater the speed of tsunami waves will be.  

For example, at the deepest ocean depths the tsunami wave speed will be as much as 800 km/h, about the same as that of a jet aircraft.   Since the average depth of the Pacific ocean is 4000 m (14,000 feet) , tsunami wave speed will average about 200 m/s or over 700 km/h (500 mph).   At such high speeds, a tsunami generated in Aleutian Islands may reach Hawaii in less than four and a half hours.  In 1960, great tsunami waves generated in Chile reached Japan, more than 16,800 km away in less than 24 hours, killing hundreds of people.

 

When tsunamis approach shore, they slow down but grow in size.

What are the factors of destruction from tsunamis?

There are three factors of destructions from tsunamis: inundation, wave impact on structures, and erosion.  Strong, tsunami-induced currents lead to the erosion of foundations and the collapse of bridges and seawalls.  Flotation and drag forces move houses and overturn railroad cars.  Considerable damage is caused by the resultant floating debris, including boats and cars that become dangerous projectiles that may crash into buildings, break power lines, and may start fires.  Fires from damaged ships in ports or from ruptured coastal oil storage tanks and refinery facilities, can cause damage greater than that inflicted directly by the tsunami.  Of increasing concern is the potential effect of tsunami draw down, when receding waters uncover cooling water intakes of nuclear power plants.

Destruction of Hilo, Hawaii harbor pier during 1946 Aleutians Islands tsunami.

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What determines how destructive a tsunami will be near the origin and at a distant shore?

Tsunamis arrive at a coastline as a series of successive crests (high water levels) and troughs (low water levels) - usually occurring 10 to 45 minutes apart.  As they enter the shallow waters of coastlines, bays, or harbors, their speed decreases to about 50-60 km/h.   For example, in 15 m of water the speed of a tsunami will be only 45 km/h.  However 100 or more kilometers away, another tsunami wave travels in deep water towards the same shore at a much greater speed, and still behind it there is another wave, traveling at even greater speed.  

As the tsunami waves become compressed near the coast, the wavelength is shortened and the wave energy is directed upward - thus increasing their heights considerably.  Just as with ordinary surf, the energy of the tsunami waves must be contained in a smaller volume of water, so the waves grow in height.  Even though the wavelength shortens near the coast, a tsunami will typically have a wavelength in excess of ten kilometers when it comes ashore.   Depending on the water depth and the coastal configuration, the waves may undergo extensive refraction - another process that may converge their energy to particular areas on the shore and thus increase the heights even more.  Even if a tsunami wave may have been 1 meter of less in the deep ocean, it may grow into a huge 30-35 meter wave when it sweeps over the shore.  

Thus, tsunami waves may smash into the shore like a wall of water or move in as a fast moving flood or tide - carrying everything on their path.  Either way, the waves become a significant threat to life and property.   If the tsunami waves arrive at high tide, or if there are concurrent storm waves in the area, the effects will be cumulative and the inundation and destruction even greater.   The historic record shows that there have been many tsunamis that have struck the shores with devastating force, sometimes reaching heights of more than 30-50 meters.  For example, the 1946 tsunami generated by an earthquake off Unimak island in Alaska's Aleutian Islands, reached heights of more than 35 meters, which destroyed a reinforced concrete lighthouse and killed its occupants.  

Finally, the maximum height a tsunami reaches on shore is called the runup.  It is the vertical distance between the maximum height reached by the water on shore and the mean sea level surface.  Any tsunami runup over a meter is dangerous.  The flooding by individual waves will typically last from ten minutes to a half-hour, so the danger period can last for hours.  Tsunami runup at the point of impact will depend on how the energy is focused, the travel path of the tsunami waves, the coastal configuration, and the offshore topography.  

Effects on Islands
Small islands with steep slopes usually experience little runup - wave heights there are only slightly greater than on the open ocean.  This is the reason that islands with steep-sided fringing or barrier reefs are only at moderate risk from tsunamis.  

However, this is not the case for islands such as the Hawaiian or the Marquesas.  Both of these island chains do not have extensive barrier reefs and have broad bays exposed to the open ocean.  For example, Hilo Bay at the island of Hawaii and Tahauku Bay at Hiva Oa in the Marquesas are especially vulnerable.  The 1946 Aleutian tsunami resulted in runup, which exceeded 8 m at Hilo and 10 m at Tahauku; 59 people were killed in Hilo and two in Tahauku.  Similarly, any gap in a reef puts the adjacent shoreline at risk.  The local tsunami from the Suva earthquake of 1953 did little damage because of Fiji's extensive offshore reefs.  However, two villages on the island of Viti Levu, located on opposite gaps in the reef, were extensively damaged and five people were drowned.

Where and how frequently are tsunamis generated?

Tsunamis are disasters that can be generated in all of the world's oceans, inland seas, and in any large body of water.  Each region of the world appears to have its own cycle of frequency and pattern in generating tsunamis that range in size from small to the large and highly destructive events.  Most tsunamis occur in the Pacific Ocean and its marginal seas.  The reason is that the Pacific covers more than one-third of the earth's surface and is surrounded by a series of mountain chains, deep-ocean trenches and island arcs called the "ring of fire" - where most earthquakes occur (off the coasts of Kamchatka, Japan, the Kuril Islands, Alaska and South America).  Many tsunamis have also been generated in the seas which border the Pacific Ocean.  Tsunamis are generated, by shallow earthquakes all around the Pacific, but those from earthquakes in the tropical Pacific tend to be modest in size.  While such tsunamis in these areas may be devastating locally, their energy decays rapidly with distance.   Usually, they are not destructive a few hundred kilometers away from their sources.

That is not the case with tsunamis generated by great earthquakes in the North Pacific or along the Pacific coast of South America.  On the average of about half-a-dozen times per century, a tsunami from one of these regions sweeps across the entire Pacific, is reflected from distant shores, and sets the entire ocean in motion for days.   For example, the 1960 Chilean tsunami caused death and destruction throughout the Pacific.   Hawaii, Samoa, and Easter Island all recorded runups exceeding 4 m; 61 people were killed in Hawaii.  In Japan 200 people died.  A similar tsunami in 1868 from northern Chile caused extensive damage in the Austral Islands, Hawaii, Samoa and New Zealand.

Although not as frequent, destructive tsunamis have been also been generated in the Atlantic and the Indian Oceans, the Mediterranean Sea and even within smaller bodies of water, like the Sea of Marmara, in Turkey.  In 1999, a large earthquake along the North Anatolian Fault zone, generated a local tsunami, which was particularly damaging in the Bay of Izmit.

In the last decade alone, deadly tsunamis have occurred in Chile (2007, 2010), Haiti (2010), Indonesia (2004, 2005, 2006, 2010), Japan (2011), Peru (2001), Samoa - American Samoa - Tonga (2009), Solomons (2007).  Of these, only Indonesia (2004) and Japan (2011) caused deaths at distant shores.

 

Why are locally generated tsunamis so dangerous?

A locally generated tsunami may reach a nearby shore in less than ten minutes.  There may not be sufficient time for tsunami warning centers, such as the Pacific Tsunami Warning Center, or for local authorities to issue a warning.  For people living near the coast, the shaking of the ground from an earthquake is nature's natural warning that a tsunami may be imminent. Observation of unusual ocean changes, or the hearing of loud ocean roars are also nature's natural tsunami warning signs. If you sense of these and are near the occean, evacuate immediately inland and to higher ground.  

For tsunamis from more distant sources, however, tsunami warning centers play an important role in ensuring public safety. Accurate warnings of when a tsunami might arrive are possible because tsunamis travel at a known speed. Warning centers will work with local authorities to advise them on whether a tsunami will impact their coasts.

 

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Why aren't tsunamis seen at sea or from the air?

In the deep ocean, tsunami wave amplitude is usually less than 1 m (3.3 feet).  The crests of tsunami waves may be more than a hundred kilometers or more away from each other.  Therefore, passengers on boats at sea, far away from shore where the water is deep, will not feel nor see the tsunami waves as they pass by underneath at high speeds.  The tsunami may be perceived as nothing more than a gentle rise and fall of the sea surface.  

The Great Sanriku tsunami, which struck Honshu, Japan, on June 15, 1896, was completely undetected by fishermen twenty miles out to sea.  The deep-water height of this tsunami was only about 40 centimeters when it passed them and yet, when it arrived on the shore, it had transformed into huge waves that killed 28,000 people, destroyed the port of Sanriku and villages along 275 km of coastline.  For the same reason of low amplitude and very long periods in the deep ocean, tsunami waves cannot be seen nor detected from the air.   From the sky, tsunami waves cannot be distinguished from ordinary ocean waves.

 

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