Water And Oceanography

Misconceptions about Tsunamis

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A tsunami is a massive wave produced by the sudden displacement of a large volume of water. It increases in height where the sea becomes shallower or narrower, such as many harbors. They are most commonly associated with subduction earthquakes, but can also have other causes. If you live or vacation in a coastal region, knowing the truth about tsunami myths can save your life.

Tsunamis are not always preceded by an earthquake

Most major tsunamis are associated with subduction earthquakes along the Pacific Ring of Fire, and to a lesser extent with earthquakes along the edges of continental plates in the Indian Ocean. The Atlantic Ocean and Mediterranean Sea are also vulnerable to tsunamis, as are nearly all smaller bodies of water.

However, tsunamis can also be caused by meteor strikes or mountain-sized landslides into the ocean or other body of water. They can even be caused by volcanoes or the calving of glaciers. The important factor is that water must be displaced suddenly, and the resulting wave energy must be forced upwards by an upwards-sloping sea floor or a narrowing channel.

Even when tsunamis are caused by earthquakes, the earthquake may be too deep or too far away to be felt by anything other than seismographs. A tsunami can travel thousands of miles without losing much energy. Most regions are at greatest risk from tsunamis which are generated locally only because very little warning is possible in these cases.

Tsunamis do not arrive slowly

Most sea waves only travel at about 5 miles per hour. However, tsunamis travel at over 500 mph in deep water. They slow down as the water becomes shallower, but they still hit shore at close to highway speeds. It is only the extreme massiveness of the wave that makes it seem to travel in slow motion until it strikes land, the same way as many rivers look slow and peaceful on the surface, even if their current is very strong and fast, until the rushing water encounters shallows or other obstacles.

A person on foot cannot possibly outrun a tsunami over level ground, although he may be able to make it to high ground in time if he begins evacuating as soon as the tsunami warning is given. In a multifloor concrete building, the best option in an emergency may be to climb the stairs to the third floor or higher. That is sufficient for the height of most tsunamis, although a few can be even higher when the geography is just right. The second floor in a wood-frame house is not sufficient, because the force of the tsunami can break the timbers and sweep it away.

A deep area offshore will not break the force of a tsunami

This misconception comes from the fact that in deep water, a tsunami may only be a couple of feet high, and will often pass unnoticed except by a sensitive buoy. This is even the case in a deep-water harbor.

However, even in deep water, the wave energy remains constant. This energy originally came from the cause of the water displacement, and is carried by the motion of the entire water column from the top of the ocean to the bottom across a width of several miles. As soon as the ocean floor becomes more shallow, all that volume of water is forced upwards.

This is why tsunamis have often been described as the whole sea level suddenly having gotten much higher. Their extremely long wavelength means that a single tsunami wave crest can be sweeping onshore for miles, with only the leading edge temporarily breaking into surf the way other waves do. A good way to picture the effect is to imagine the land suddenly dropping below sea level, with any dikes giving way all at once.

Tsunamis can arrive more than once

A tsunami is a wave, which means that it has a wavelength. The trough and crest of the wave is what causes the characteristic ebbing before some tsunamis hit. Nearly all tsunamis have more than a single wave crest, although it may come as much as half an hour after the first crest hits. In many cases, the second crest can be even higher than the first.

Tsunamis are not rogue waves

Rogue waves are extremely high, isolated waves which seem to come out of nowhere. A few documented rogue waves have been up to 100 feet high, with all the surrounding waves much less than half that. Tsunamis never reach that height outside an enclosed or shallow area, but rogue waves always happen in the open sea. This is what makes documentation of tsunamis so easy and documentation of rogue waves so hard.

Their cause is also different. Tsunamis are always caused by sudden water displacement. Rogue waves are caused by mutual reinforcement of ocean waves, where the crests come together just right so that there is only reinforcement of crests and little or no cancelation by a wave trough. While a simultaneous tsunami may contribute to those crests, it is never the only cause of a rogue wave.

This makes rogue waves much more frequent than comparably sized tsunamis. As mentioned above, tsunamis which are not confined to a single bay generally need a meteor strike, mountain-sized landslide, or a strong undersea earthquake to trigger them. These are fairly rare events. However, rogue waves only need the freak combination of perfect reinforcement. Radar data from the North Sea's Goma oilfield has recorded an average of 39 rogue waves each year.

Tsunamis are not  tidal waves

Tsunamis were once called tidal waves because their long wavelength can make them appear as though the tide were sweeping onshore. However, the only difference the tide makes to a tsunami is that a tsunami which hits during low tide will be a little lower than a tsunami which hits during high tide, by the same amount as the local sea level changes.

A true tidal wave, also known as a tidal bore, has a completely different wave cause and composition from a tsunami. A tidal bore consists of short, low waves across the width of a tidal estuary. These waves are produced when the incoming tide meets a strong river current in an open-mouthed river channel which narrows further upstream.

Do animals sense that a tsunami is coming?

This tsunami myth is partly true. Elephants in Sri Lanka did move away from the shoreline shortly before the arrival of the 2004 Indian Ocean tsunami. Many other cases of this kind have been mentioned.

It is currently thought that some animals may be sensitive to infrasound or ground vibration, which would give hours of warning before a tsunami. However, no one knows for sure how some animals detect the coming danger, or how reliable that sense is.

More about this author: Michael Totten

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