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Michael Bevis, professor of Geodynamics

Posted on | April 6, 2011 | 1,000 views | Comments Off

What are tsunamis and how are they formed?
Most deadly tsunamis are caused when a giant earthquake lifts up a very large area of the seafloor. The ocean water is lifted up with the seafloor, so the overlying portion of the ocean’s surface is uplifted too.

This low but very long and wide hill of water then “slides downhill” or floods outward as would water in a reservoir if the dam walls suddenly disappeared. (It is more complicated than that, but this is a first approximation).

What are the dangers of tsunamis? Why are they so deadly?
Tsunamis are dangerous because they travel so fast in the deep ocean (more than 500 miles per hour) that they can arrive at a shoreline before anyone has managed to get a warning to the local population.

If you have not evacuated by the time the tsunami arrives, it is often too late to save your life as the huge flood of water rushes inland. This flood is relentless, and quite unlike a normal sea wave. It is more like a wall of water, and behind the wall the water level is the same height as the wall itself. The water rushes inland for 5, 10 or even 15 minutes before it starts to ebb, sweeping almost everything before it. The water is so full of debris even a superb swimmer can drown before reaching safety.

What are precautions people should take when there is a threat of tsunami?

Individually: If you ever feel a very powerful earthquake while near a coastline — one where the ground is shaking violently and this shaking lasts a minute or more — run for your life. Run to high areas, even strong buildings such as parking structures. Try to get at least 60 feet above sea level. When in a safe place, stay there for many hours. There can be as many as a dozen tsunami waves and in some cases the later waves are the biggest ones.

Don’t move from safety until you hear from well-informed officials. Also, if you ever see the ocean rapidly recede from the beach — run, because the imminent arrival of a tsunami is quite often signaled by a big withdrawal of shallow water before the giant flood arrives.

Governmentally:  It is essential to plan for great earthquakes and tsunamis before they occur, and to train the local population to act in a pre-planned way to official warnings.

One key need is to build systems that sound emergency klaxons and trigger radio announcements, cell phone broadcasts, etc., within minutes of an earthquake that has a significant potential to create a tsunami, and thereby kick off a well understood emergency response.

At least some of these redundant warning systems should be robust, as you are likely to suffer electrical power failures, collapses of communication systems, etc., as a result of the earthquake. You cannot plan an emergency response in real time, nor communicate directly with most of the population to explain to them what to do.

Everything has to run automatically.

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