Tsunami: natural warnings and going high at once

Most tsunamis are caused by undersea earthquakes: the seabed suddenly rises or sinks and displaces the entire column of water above it. Less often, underwater landslides, volcanic eruptions or rockfalls into the sea trigger a tsunami. The word comes from Japanese and roughly means “harbour wave”.

On the open sea a tsunami is low and inconspicuous, but very fast – ships often hardly notice it out there. Only when the water becomes shallower towards the coast does the wave slow down and pile up. Unlike a wind wave, it does not simply break on the beach, but pushes far inland like a rising flood, carrying debris, cars and trees with it. Afterwards the water flows back out with great force.

Sometimes a trough reaches the coast before the first wave. Then the sea retreats unusually far and exposes the seabed. This looks strange and fascinating – but it is one of the clearest warning signs. With an earthquake close to the coast, the time until the first wave can be very short, so natural signs are often the only warning.

If you live on or holiday at a coast at risk, look at the shortest way up or inland on the first day. In many areas at risk there are tsunami signs showing evacuation routes and safe zones. These few minutes of preparation can decide everything in a real event.

The natural warning signs are: a strong or long-lasting earthquake at the coast, the sea retreating unusually far or suddenly rising, and a loud roar from the ocean. At any of these signs, do not wait for an official warning. During an earthquake, first protect yourself with “Drop, Cover, Hold On” – as soon as the shaking stops, leave at once.

Go without luggage, on foot if the roads are full, and take others along and warn them. Height and distance from the sea protect you. If no higher ground can be reached, go to the upper floors of a stable, tall reinforced-concrete building. Even if the authorities warn after a distant earthquake and you felt nothing yourself, follow their instructions.

The most dangerous mistake is curiosity: going to the beach to look at or photograph the retreating sea. When the sea pulls back, the water comes back – often faster than you can run. Just as dangerous is gathering luggage or sitting in a traffic jam in the car instead of walking.

A tsunami usually consists of several waves, and the first is not always the largest. Between the waves the water can drain away and everything can seem calm – that is no sign that it is over. Further waves can come hours later. Return only when the authorities give the official all-clear, and meanwhile follow the radio and official announcements.

If possible, call the local emergency number (112 in the EU) only once you yourself are safe. The first minutes belong to the way up. And a false alarm is no reason to stay next time.

The Indian Ocean tsunami of December 2004 showed the world how important it is to know the natural warning signs. Many people did not recognise at the time what the retreating sea meant. Since then, warning systems have been expanded and public education strengthened – but for nearby earthquakes, your own knowledge remains the fastest warning.

Tsunamis are rare on European coasts but possible in the Mediterranean, because there are earthquake-prone regions there. For travel to coasts in earthquake-prone regions worldwide, knowing the warning signs is therefore part of basic preparation, just like knowing where the nearest higher place is.

How high a tsunami gets, when it arrives and which height is safe depends on the individual event and on the terrain. That is why there is no universal figure for it. The simple rule remains: as high and as far from the sea as possible – and back only after the official all-clear.