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What is a Tsunami?

What causes a tsunami?

The most common cause is a seafloor earthquake. Other triggers are undersea landslides, undersea volcanic eruptions, and meteorite impact. Sudden changes to the seafloor cause the ocean to flow away from the disturbance, creating waves.

How fast do tsunami waves travel?

In the open ocean, tsunami waves travel at 600kph to 700kph. In the deep ocean, waves from a large tsunami may be as little as 60cm high. They pass ships unnoticed. As they encounter shallow water, they slow down to about 30kph and increase in height.

How far inland can a tsunami go?

In low-lying coastal areas they can travel a long way inland. In parts of Thailand, the tsunami went 3 km inland destroying almost everything in its path. In Banda Aceh, the tsunami surged 6km inland.

Why does the seawater recede a long way out, sometimes hundreds of metres, before coming back in as a tsunami?

Tsunamis are not just moving lumps on top of the ocean surface, they also include hollows, and sometimes the hollow reaches the coast first. When this happens, the ocean first draws down and sucks water away from coastlines. It then rushes back in with enormous speed and force as the lumps arrive. People who notice the receding water have as little as five minutes to flee inland to higher ground.

How does seawater destroy buildings?

Easily! A tsunami is not just seawater. It picks up a huge amount of debris which gives it added destructive power. People don't die just from drowning, many are killed by being hit by debris and heavy objects in the surging torrent. A retreating tsunami wave can cause as much damage as the initial forward surge. A tsunami can exert huge forces against the side of a building. In Thailand, engineers calculated that modern reinforced concrete walls of hotels were blown out by tsunami waves generating as much pressure as 3000kg/sqm. This is far greater than earthquake design loadings.

Is a tsunami a dream come true for extreme surfers?

No. A tsunami is not a wave in the classical sense, but a raging torrent of water that surges inland with enormous power - much more power than a surfer can handle. Besides the waves do not stop at the shore, so surfers may find that they crash into buildings or all the other debris caught up in the surging water. Invariably a tsunami consists of successive surges or torrents and equally violent return flow to the sea. The first wave or torrent of a tsunami is not necessarily the biggest. The second or third or even much later waves may be bigger. Intervals between successive waves can vary. It may just be minutes, or it could be more than an hour.