5.3 — Tsunami

One rule governs this entire chapter.

If you are near the coast and you feel a strong earthquake, or you see the sea withdraw, or you hear a loud roar from the sea — go inland and uphill immediately, on foot, without waiting for any official warning.

That single rule is what saves people, and it is the whole of the actionable content.

The natural warnings

Three, any one of which is enough.

A strong or long earthquake near the coast

The most important.

If the shaking is strong enough that it is difficult to stand, or if it lasts more than about twenty seconds, treat it as a tsunami warning.

The wave from a nearby earthquake can arrive within minutes, which is less time than any official warning system can deliver.

The shaking is the warning. Do not wait for another one.

The sea withdrawing

The water pulls back, exposing the seabed, reefs and stranded fish, further than any tide.

This is one of the trough of the wave arriving first, and it means the crest is coming.

And the historical pattern is grim and worth stating so it is not repeated: people go down onto the exposed seabed to look.

If the sea goes out unusually far, run.

A loud roar

Frequently described as sounding like a train or aircraft.

What to do

Five things.

Go inland and uphill. The standard guidance is to aim for at least 30 metres of elevation or 3 kilometres inland, and where that is not achievable, get as high and as far as you can.

Go on foot if the distance is short. Roads jam immediately, and a stationary vehicle in a tsunami is far worse than being on foot.

If there is no high ground: vertical evacuation. **The upper floors of a strong, tall, reinforced concrete building. Not a low or wooden structure. The roof if necessary.

Take nothing that slows you down.

And keep going. **Do not stop at the first slightly higher ground. Tsunami run-up can extend far further inland than people expect, particularly up river channels — and rivers are a specific danger because the wave travels up them much further than it does over open land.

The thing that kills people who survived the first wave

It is not one wave.

A tsunami is a series, and the first is frequently not the largest.

The waves can arrive over a period of hours, and the interval between them can be from several minutes to over an hour.

Which means: do not return.

Not to check the house, not to look for anybody, not because it seems to be over. Stay on high ground until officials say the threat has passed — which may be many hours.

The pattern of people returning between waves is one of the most consistent causes of death in tsunami events.

The currents

Worth knowing because it explains the danger.

The destructive force is not a breaking wave like a surf wave.

It is a fast-moving wall of water that keeps coming — and it carries debris: vehicles, timber, concrete, sheet metal.

Even a modest depth of moving water is unsurvivable on foot (5.4), and the water is full of things travelling at speed.

And the outflow is as dangerous as the inflow, dragging people and debris out to sea.

If you are caught

Genuinely difficult, and there are things that help.

Grab anything that floats — a door, a tree, debris. Survivors of the 2004 event routinely describe having held onto something.

Try to get on top of debris rather than in the water among it.

Protect your head.

And do not fight the current. Move across it if you can, towards anything fixed and high.

If you are on a boat

Two opposite answers depending on where you are.

At sea in deep water: stay there. **A tsunami in deep water is a low, long wave that is barely noticeable — it becomes destructive only in shallow water. Head further out to sea if you have warning and time.

In harbour: leave the boat and go inland and uphill. Harbours are where the water piles up and where the currents are most violent.

Preparation

Four things, if you live or holiday on a coast.

Know your elevation and know where the high ground is. **Look, once, in good weather. Not on the day.

Know the evacuation route on foot and walk it once. Time it.

Learn the local warning signal — sirens, mobile alerts, and the local convention.

And on holiday, spend two minutes on arrival asking which way is high ground. A very large number of tsunami deaths in tourist areas are visitors who had no idea where to go.

What to do with this page

Strong or long shaking near the coast is the warning. Go, without waiting for anything official.

If the sea goes out, run. Do not go and look.

Inland and uphill, on foot, and further than feels necessary.

It is a series of waves and the first is frequently not the largest. Do not return for hours.

And ask where the high ground is when you arrive somewhere coastal. Two minutes, and it is the whole of the preparation available to a visitor.

Next: 5.4 — flood and days of continuous rain, where the danger is almost entirely misjudged.