Sun, Jan 02, 2005 - Page 8 News List

Preparation is the key if a tsunami strikes here

By Yeh Hsueh-wen 葉學文

The massive tidal waves of last Sunday, triggered by the most powerful earthquake in 40 years off the west coast of Sumatra, have caused tremendous damage and killed an untold number of people. This article is intended to help people better understand these phenomena so that in the event that a tsunami strikes here, the resulting destruction and loss of life can be kept to a minimum.

The word "tsunami" is Japanese and means "harbor wave," based on the devastating effects these waves have had on Japanese coastal communities. The generation of a series of waves is not associated with wind or tides, however. Instead, this is produced by a displacement of the sea floor. Tsunamis can be caused by underwater volcanic eruptions and landslides, and have specific wavelengths and cycles.

The tsunamis that most people talk about are those with long wavelengths associated with shallow earthquakes -- at the base of the continental slope -- on the sea floor.

The speed of such tsunamis can reach between 700kph and 800kph, though at these speeds the waves are of almost neglible height out in the deeper ocean.

The cause of tsunamis is primarily the large-scale displacement of the sea floor, which in turn generates huge pressure on the ocean surface.

In other words, the formation of a tsunami is generally believed to be the result of an earthquake-induced horizontal and vertical plate slip on a major underwater belt.

The most conducive environment for such tectonic movement is the so-called destructive plate boundary -- where plates moving towards one another result in one being pushed under the other.

This boundary can be found along the belt surrounding the Pacific Ocean -- thus it also passes along the east coast of Taiwan.

The latest disaster is one example of the Indo-Australian and Eurasian plates lodging on top of each other, deep under the ocean near Sumatra. There is, consequently, a permanent risk of tsunamis occurring in Taiwan.

The risk of a tsunami occurring is greater in areas nearer such faults than in those further away. For people living further away, there may be more time to evacuate; for Taiwanese people situated so near the earthquake belt, evacuation time is quite limited if there is any at all.

Taiwanese are not only unprepared for tsunamis generated locally, they would not even be able to cope with a tsunami generated further away. This is of course a matter of considerable concern.

In order to correct this problem, we need to encourage basic, practical research projects and change the way people think about the problem. At the level of government, earthquake information should include tsunami awareness.

The government should also begin to develop infrastructure that would reduce the damage and loss of life caused by tsunamis.

The public should learn more about the risk of tsunamis and be familiar with mechanisms for evacuation.

As with the risk of landslide, evacuation would normally be the only response.

The public should also pay more attention to the possibility of long-distance tsunamis inflicting damage.

It is hoped that Taiwan will not have to suffer the kind of damage that we have witnessed over the last week across South and Southeast Asia.

But if it does, we need to be well-prepared.

Yeh Hsueh-wen is a senior researcher at the Institute of Earth Sciences at the Academia Sinica.

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